Brian F Kuhn

Lazarus Kgasi

Dominique Gommery

Fossil rarities from the Bolt's Farm Palaeokarst System, Gauteng, South Africa

Keywords: Panthera, Clavicle, Haemal Arch, Plio-Pleistocene

Certain skeletal elements of mammals are rare in both modern and fossil collections. These include clavicle, baculum, haemal arches; basically, skeletal elements with little or no articulation with the rest of the skeleton. The reason for this is complex. In cases of the modern specimens the above-mentioned elements could be easily disposed together with the muscle mass prior to skeletonisation, while in cases of fossilised specimens this phenomenon might be due to misidentification or the lack of preservation. Whatever the reason, here we have recovered multiple haemal arches and two clavicles from two Plio-Pleistocene fossil deposits at the Bolt's Farm Palaeokarst System, South Africa. All the preserved material appears to be that of Panthera Oken, 1816.

INTRODUCTION

For various reasons, from preservation to simply being overlooked or misidentified, certain anatomical structures are rare in the fossil record. These include clavicles (in particular nonprimate) [START_REF] Hartstone-Rose | The clavicles of Smilodon fatalis and Panthera atrox (Mammalia: Felidae) from Rancho La Brea, Los Angeles, California[END_REF] and haemal arches (to date no references have been found regarding mammalian haemal arches in the Plio-Pleistocene fossil record). Even in modern anatomical collections in some of the largest museums around the world these bones are rare (pers. obs.). The major exception is the Rancho La Brea Tar Pit collections at the George C. Page Museum in Los Angeles, California [START_REF] Hartstone-Rose | The clavicles of Smilodon fatalis and Panthera atrox (Mammalia: Felidae) from Rancho La Brea, Los Angeles, California[END_REF]. HartstoneRose et al. suggest the reason why modern collections lack clavicles is that the bone was simply tossed aside with the muscle mass during skeletonisation. The morphology of haemal arches are widely published in species of fish, reptiles and dinosaurs, but aside from veterinary texts and some comparative anatomy texts, little is published from the mammalian groups, and even less in the fossil literature [START_REF] Kardong | Vertebrates: comparative anatomy, function, evolution[END_REF][START_REF] Otero | Sauropod haemal arches:morphotypes, new classification and phylogenetic aspects[END_REF][START_REF] Evans | Miller's anatomy of the dog, 4th edn[END_REF][START_REF] Gatesy | Caudofemoral musculature and the evolution of theropod locomotion[END_REF]Romer, 1956 and references therein). In reptiles and sauropods, the haemal arches start at the 2nd or 3rd caudal vertebrae and continue down the tail, with distinct morphological changes as the tail thins [START_REF] Otero | Sauropod haemal arches:morphotypes, new classification and phylogenetic aspects[END_REF]. In mammals the haemal arch is located on the ventral surface of some of the caudal vertebrae and slopes caudally. In dogs it is only found in association with the fourth, fifth and sixth caudal vertebrae [START_REF] Evans | Miller's anatomy of the dog, 4th edn[END_REF]. One consistency between these elements in various groups of vertebrates is that they do not maintain a strong articulation with the rest of the skeleton [START_REF] Evans | Miller's anatomy of the dog, 4th edn[END_REF]. For whatever reason, these skeletal elements are rarely recovered, or are misidentified in the fossil record.

Recent excavations at the site of Bolt's Farm, South Africa (Fig. 1), have yielded three examples of haemal arches and two non-primate clavicles. These rare elements come from two different fossil deposits within the Bolt's Farm Palaeokarst System (Fig. 2). The haemal arches and one clavicle were all recovered from the site known as Bridge Cave (BC) and the second clavicle was recovered from Aves Cave I (AC). Bolt's Farm has a long history of exploration and excavations. Dr Broom and Mr Bolt did their initial reconnaissance of the area back in 1936 [START_REF] Cooke | Dinofelis barlowi (Mammalia, Carnivora, Felidae) cranial material from Bolt's Farm, collected by the University of California African Expedition[END_REF] and subsequent descriptions of a few specimens were completed by [START_REF] Broom | On some new Pleistocene mammals from limestone caves of the Transvaal[END_REF][START_REF] Broom | A preliminary account of the Pleistocene carnivores of the Transvaal caves[END_REF][START_REF] Broom | Some South African Pliocene and Pleistocene mammals[END_REF]. The University of California African Expedition run by Dr Charles Camp excavated numerous localities at Bolt's Farm in April of 1948 [START_REF] Camp | Camp field notes[END_REF]. Sporadic publications have trickled out from this expedition over the years, for instance Peabody's report on Taung (1954), and specific fauna descriptions by [START_REF] Cooke | Ictonyx bolti, a new mustelid from cave breccias at Bolt's Farm, Sterkfontein area, South Africa[END_REF][START_REF] Cooke | Dinofelis barlowi (Mammalia, Carnivora, Felidae) cranial material from Bolt's Farm, collected by the University of California African Expedition[END_REF]Cooke ( , 1993a,b),b). 51 bulk of specimens from the expedition remain unpublished at the University of California Berkeley Museum of Paleontology. In 2001 the combined team of the DITSONG: National Museum of Natural History (formerly the Transvaal Museum) Pretoria, South Africa, the CNRS in France, HOPE (Human Origins and Past Environments) and HRU (HOPE Research Unit) began to relook the Bolt's Farm complex [START_REF] Gommery | Bolt's Farm Cave System dans le Cradle of Humankind (Afrique du Sud): un exemple d'approche multidisciplinaire dans l'étude des sites à primates fossils. [Bolt's Farm Cave System in the Cradle of Humankind (South Africa): an example of multidisciplinary approach to the study of fossil primate sites[END_REF][START_REF] Gommery | Plio-Pleistocene fossils from Femur Dump, Bolt's Farm, Cradle of Humankind World Heritage Site[END_REF]Thackeray et al., 2008). Of the approximately 30 loci excavated by Camp and Peabody, the latest excavations have recovered over 2000 fossils from 16 distinct fossil deposits (Fig. 2). Five rare fossils were recovered from two of the aforementioned fossil localities, clavicles and haemal arches. Here we discuss the morphology and species identification of these elements. In addition, we illustrate that through careful excavations and identification processes even these rare skeletal elements can be recovered.

MATERIAL AND METHODS

RESULTS

AC 384 and BC 512 are partial left clavicles recovered from Aves Cave I and Bridge Cave, respectively (Fig. 3). Both specimens are missing the sternal ends, thus measurements A, D, E, F and G (from Hartstone-Rose et al., 2012) were unattainable. For AC 384 measurement B (acromial width) is 9.03 mm and measurement C (acromial thickness) is 3.18 mm. Visually AC 384 is distinct from the two modern lion (Panthera leo Linnaeus, 1758) clavicles used for comparative purposes and most resembles LACM HC 30082 ('C'), Panthera atrox (Leidy, 1853) from Hartstone-Rose et al., (2012: 983). In a comparison of measurements (Table. 1) AC 384 has an acromial width much less than P. atrox (Leidy, 1853) and noticeably less than P. leo.

(Linnaeus, 1758). The acromial width of AC 384 falls between P. tigris (Linnaeus, 1758) and P. onca (Linnaeus, 1758). The acromial thickness of AC 384 is slightly greater than those reported for both P. leo and P. tigris and less than that recorded for P. atrox -the large morph from Rancho La Brea (see HartstoneRose et al., 2012). BC 512 has an acromial width (B) of 8.94 mm and acromial thickness (C) of 2.86 mm. The acromial width is less than the measured width of AC 384, as well as the reported widths for lion and tiger and the left lion clavicle measured here. The acromial thickness falls within the range of measured and reported lion and tiger and is noticeably less than AC 384. All three haemal arches come from the Bridge Cave deposits, and in particular are directly associated with fossils of an extremely large Pantherinae (Kuhn et al., in prep.). Two of the three specimens are complete (Fig. 4). Aside from some sauropod descriptions [START_REF] Otero | Sauropod haemal arches:morphotypes, new classification and phylogenetic aspects[END_REF] there is little in the way of published descriptive analyses for mammalian haemal arches. We have chosen to measure the breadth of the specimen (A), the height of the specimen (B) and the length of the haemal spine (C) (Fig. 4, Table . 2). Unlike the aforementioned sauropod descriptions which illustrate morphological change as one examines the length of the tail, these three specimens appear to have similar morphology. That being said, one can observe differences between the three specimens, suggesting that they could come from different caudal vertebrae. BC276, while damaged, is more robust than the other two specimens and has a distinctly broader convex shape than the other two specimens. In addition, BC276 has a groove where the other specimens have a ridge running down the ventral midline. While BC255 and BC369 appear more alike than BC276, there are subtle differences between them as well. BC369 has a more gracile, slender appearance when compared to BC255. These differences suggest that BC276 is more cranial in anatomical position, BC255 would follow with BC369 being the most caudal of the three vertebrae.

DISCUSSION AND CONCLUSION

When examining the clavicle referred here as AC 384 the initial identification pointed towards Dinofelis (Zdansky, 1924). After reviewing the work of [START_REF] Hartstone-Rose | The clavicles of Smilodon fatalis and Panthera atrox (Mammalia: Felidae) from Rancho La Brea, Los Angeles, California[END_REF], it was concluded that this specimen is that of a large Panthera (Oken, 1816), and not Machairodont. Size alone eliminates P. pardus as a possibility (see Table . 2). Considering the variation reported by [START_REF] Hartstone-Rose | The clavicles of Smilodon fatalis and Panthera atrox (Mammalia: Felidae) from Rancho La Brea, Los Angeles, California[END_REF], and the visual difference in the pair of clavicles taken from an individual male lion, to go beyond Panthera (Oken, 1816) is difficult. While P. atrox is not a likely fit, the records for fossil pantherine cats in Africa are sparse, with likely candidates being P. leo, or a potentially new species (Kuhn et al., in prep.). BC 512 is morphologically indistinct from modern lion clavicles that have been reported or examined. This specimen is definitively Panthera, and due to association with other pantherine fossils we are confident in identifying it as a new species of Panthera (Kuhn et al., in prep.). Of note is the identification of two suid species from Aves Cave, Potamochoeroides hypsodon (Dale, 1948) and Notochoerus capensis (Broom, 1925), which date the locality to c. 3.0-2.6 Ma (Pickford andGommery, 2016, 2020). Bridge Cave has also yielded suid remains, Phacochoerus modestus (Van Hoepen and Van Hoepen, 1932) and Metridiochoerus andrewsi (Hopwood, 1926), both of which give an approximate age of 1.8 Ma [START_REF] Pickford | Fossil suids from Bolt's Farm Palaeokarst System, South Africa: implications for the taxonomy of Potamochoeroides and Notochoerus and for biochronology[END_REF]. With a sparce published literature regarding haemal arches we have little in the way of comparative, documented, material with which to draw conclusions. We can conclude with confidence that we have seen haemal arches in two separate fossil collections, one of Machairodus (Kaup, 1832) specimens and the other from an extensive collection of P. spelaea (Goldfuss, 1810). The three specimens from Bridge Cave are morphologically indistinguishable from either Machairodus (Kaup, 1832) or P. spelaea (Goldfuss, 1810). This comes as no surprise given Machairodontinae (Gill, 1872) and Felidae (G. Fisher, 1817) are sister taxa [START_REF] Christiansen | Phylogeny of the great cats (Felidae: Pantherinae), and the influence of fossil taxa and missing characters[END_REF][START_REF] Christiansen | Phylogeny of the sabertoothed felids (Carnivora: Felidae: Machairodontinae)[END_REF] and share many similarities in their postcranial anatomy (pers. obs).

Here we have identified five specimens of two skeletal elements (BC 255,BC 276,BC 369,BC 512 and AC 384) which are known to be rare in both modern and fossil collections around the world, clavicle (non-primate) and haemal arch. Worth noting is that both of these elements come from one of the rarer genera identified in the South African fossil record, that being Panthera (Oken, 1816). Of note is that when combined with the identified Suidae (Gray, 1821), we can say with confidence that Pantherinae (Pocock, 1917) existed at Bolt's Farm as far back as 2.6-3.0 Ma.

  All specimens were recovered from decalcified breccia. Bridge Cave specimens BC 255 and BC 276 were excavated in 2014. Aves Cave I specimen AC 384 and Bridge Cave specimen BC 369 were excavated in 2015. BC 512 was excavated in 2016. AC 384 is a partial clavicle recovered from Aves Cave I. BC 255, BC 276 and BC 369 are all haemal arches recovered from Bridge Cave. BC512 is a partial clavicle recovered from Bridge Cave. All measurements were taken with PROFiTEXX digital calipers to the nearest 0.01 mm. All of the fossils are curated in the Plio-Pleistocene Palaeontology Section, Vertebrate Department, DITSONG: National Museum of Natural History, Pretoria, South Africa. Digital photos were taken with a Pentax K 20 D digital SLR camera. Modern leopard (Panthera pardus Linnaeus, 1758) clavicle is part of the University of the Witwatersrand comparative lab. In addition, fossil comparative material from Machairodus is housed at the Departamento de Paleobiología, Museo Nacional de Ciencias Naturales-CSIC (Madrid, Spain). Additional comparative material from Panthera spelaea at Château (Château, France) and Panthera spelaea and Homotherium crenatidens material at Université Claude Bernard Lyon 1 (Villeurbanne, France) were also used during this study. Additional comparisons were made with a pair of modern Panthera leo clavicles (see Fig. 3) and published results of Hartstone-Rose et al. (2012: 985).

Fig. 1

 1 Fig. 1 Map showing location of Bolt's Farm.

Fig. 2

 2 Fig. 2Map showing fossil localities within Bolt's Farm Palaeokarst System (adapted from[START_REF] Kuhn | The carnivore guild of Bolt's Farm, South Africa: carnivore biodiversity in the Plio-Pleistocene[END_REF].

Fig. 3

 3 Fig. 3Clavicles from Panthera, BC512 and AC384.

Table 1

 1 Acromial width and thickness of AC 384 compared to reported large Panthera species and modern specimens. All measurements are in mm.

	Species Acromial	Acromial width (B)	Acromial thickness (C)
	AC 384	9.03	3.18
	Panthera leo*	11.6	2.9
	Panthera onca*	8.1	1.7
	Panthera tigris*	10.1	2.6
	Rancho La Brea	15.1	3.9
	large morph (P. atrox)*
	Panthera pardus BPI 12/75	4.81	1.55
	Panthera leo (R)	6.52	2.41
	Panthera leo (L)	9.18	2.49
	*Hartstone-Rose et al., 2012

Table 2

 2 Measurements of haemal arches BC255, BC276 and BC369; A is the width of the cranial aspect, B is the height dorsal-ventral and C is the length of the spine.

	Specimen	A	B	C
	BC255	17.81 mm	11.41 mm	25.11 mm
	BC276	18.86 mm	10.01 mm	Damaged
	BC369	16.12 mm	11.35 mm	25.06 mm

ACKNOWLEDGEMENTS

Excavations were conducted under SAHRA permits ID 866 and ID 2672. Excavations were funded by the LIA 1041, CNRS-NRF and the MPFSA (Mission Paléoanthropologique Franco-Sud-Africaine) of Pôle Sciences humaines et sociales, Archéologie et Patrimoine (sousdirection de l'Enseignement supérieur et de la Recherche) of the French Ministry of Europe and Foreign Affairs. Permission to access sites was given by Mrs C. Klinkert. We are grateful to Dr Manual Salesa for access to Machairodus material housed at Departamento de Paleobiología, Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain, as well as Drs Alain and Jacqueline Argant for access to Fig. 4 Ventral aspect of haemal arches BC255, BC276 and BC369 and measurements taken of haemal arches; A is the width of the cranial aspect, B is the height dorsal-ventral and C is the length of the spine. Panthera spelaea material housed from and at Château, France, and Emmanuel Robert for the access to Collections de Géologie (Panthera spelaea and Homotherium crenatidens), Université Claude Bernard Lyon 1, Villeurbanne, France. Dr Bernhard Zipfel granted permission to access leopard clavicle at the University of the Witwatersrand and Mr Sifelani Jurah provided measurements.


W Dunn

G Berland

E Roussos

G Clark

P Kollmann

D Turner

C Feldman

T Stallard

G Branduardi-Raymont

E E Woodfield

I J Rae

L C Ray

J A Carter

S T Lindsay

Z Yao

R Marshall

A N Jaynes

Y Ezoe

M Numazawa

G B Hospodarsky

X Wu

D M Weigt

C M Jackman

K Mori

Q Nénon

R T Desai

L W Blum

T A Nordheim

J U Ness

D Bodewits

T Kimura

W Li

H T Smith

D Millas

A D Wibisono

N Achilleos

D Koutroumpa

S C Mcentee

H Collier

A Bhardwaj

A Martindale

S J Wolk

S V Badman

R P Kraft

What is the nature of the global structure and time-variability of the most intense radiation belts in the solar system, and what processes govern this? 2. How do losses of material to the atmosphere, moons, ring materials and neutral tori balance and ultimately limit radiation belt intensities? 3. How do X-ray signatures characterize astrophysical plasmas of different species? 4. Do rapidly-rotating giant planets, such as Jupiter, have a magnetospheric cusp and what is the extent and time-variability of this structure? From Earth's orbit, Jovian magnetosphere observations are limited by X-ray photon fluxes and spatial resolution. Soft X-ray (0.2-10 keV) imaging instruments have never flown to the realms of the gas giants. New developments like miniaturized X-ray optics and radiation-tolerant detectors, provide compact, lightweight (~10 kg), wide-field X-ray instruments perfectly suited to the Jupiter system, enabling this exciting new possibility. There is a fast growing heritage of such instruments on current/upcoming NASA/ESA/JAXA/CAS missions (AXIS on AEPEX; MIXS on BepiColombo; the SXI on SMILE; MXT on SVOM; WXT on Einstein Probe; LEXT on Gamow). Here, we argue that such instrumentation is necessary to address fundamental heliophysics questions on radiation belt physics and relativistic particle behavior, thus enabling heliophysics to provide a unique and irreplaceable stepping-stone to astrophysical systems beyond the Solar System.

Synopsis: Jupiter's magnetosphere is considered to be the most powerful particle accelerator in the Solar System, accelerating electrons from eV to >70 MeV and ions to GeV energies. How electromagnetic processes drive energy and particle flows, producing and removing energetic particles, is at the heart of Heliophysics. Particularly, the 2013 Decadal Strategy for Solar and Space Physics was to "Discover and characterize fundamental processes that occur both within the heliosphere and throughout the universe". The Jovian system offers an ideal natural laboratory to investigate all of the universal processes highlighted in the previous Decadal.

The X-ray waveband has been widely used to remotely study plasma across astrophysical systems. The Chandra and XMM-Newton X-ray Observatories alone have produced >14,000 refereed publications (>120,000 citations) to date. However, for <1% of these works the systems explored are within reach of in-situ measurements. The majority of astrophysical emissions can be grouped into 5 X-ray processes: fluorescence, thermal/coronal, scattering, charge exchange and particle acceleration. The Jovian system offers perhaps the only system that presents a rich catalog of all of these X-ray emission processes and can also be visited in-situ, affording the special possibility to directly link fundamental plasma processes with their resulting X-ray signatures. This offers invaluable ground-truths for astrophysical objects beyond the reach of insitu exploration (e.g., brown dwarfs, magnetars or galaxy clusters that map the cosmos).

Here, we show how coupling in-situ measurements with in-orbit X-ray observations of Jupiter's radiation belts, Galilean satellites, Io Torus, and atmosphere addresses fundamental heliophysics questions with wide-reaching impact across helio-and astrophysics, including:

Introduction:

In companion white papers submitted to the Decadal [START_REF] Clark | Comprehensive Observations of Magnetospheric Particle Acceleration, Sources, and Sinks (COMPASS): A Mission Concept to Jupiter's Extreme Magnetosphere to Address Fundamental Mysteries in Heliophysics[END_REF][START_REF] Kollmann | Jupiter's radiation belts as a target for NASA's Heliophysics Division[END_REF][START_REF] Turner | Cross-scale physics and the acceleration of particles in collisionless plasmas throughout the Heliosphere and beyond: III. Radiation belts[END_REF], we present the case for exploration of the fundamental electromagnetic processes that govern particle acceleration and loss in heliophysics, by exploring the natural laboratories presented within the Jovian radiation belts, in particular through the NASA-funded mission concept Comprehensive Observations of Magnetospheric Particle Acceleration, Sources, and Sinks (COMPASS -Clark+, 2022). Here, we focus specifically on the use of X-rays to provide global context to single-point in-situ particle and field measurements, to offer new insights into the dynamics that govern such systems and to address fundamental questions in heliophysics.

Hitherto, all (0.2 -20 keV) X-ray observations of the Jovian system were acquired by observatories in Earth-orbit. Since 1999, these were most commonly conducted by NASA's Chandra and NuSTAR X-ray observatories, ESA's XMM-Newton X-ray observatory and JAXA's Suzaku satellite (see Dunn, 2022 for review). These flagship X-ray observatories have provided revolutionary insights into the processes governing rapidly rotating magnetospheres, revealing: a variety of heavy ion aurorae (e.g., Branduardi-Raymont[B-R]+, 2008); magnetosphereionosphere (M-I) coupling through auroral hot spots (Gladstone+, 2002, Nature); clockwork-like electromagnetic pulsations (Dunn+, 2017, Nature Ast.); Jupiter's magnetar-like aurora (Mori+, 2022, Nature Ast.); X-ray fluorescence from plasma losses to moons, rings and tori (Bhardwaj+, 2005;Elsner+ 2002); radiation belt ultrarelativistic electron emissions [START_REF] Ezoe | Discovery of diffuse hard X-ray emission around Jupiter with Suzaku[END_REF] and the potential universality of Electromagnetic Ion Cyclotron (EMIC) waves as a driver of ion aurorae (Yao+, 2021, Science Adv.). These breakthroughs were achieved despite limited Jovian X-ray fluxes at Earth-orbit assets, low spatial resolution and no access to Jupiter's nightside. While Jupiter's magnetosphere glows in many wavelengths (visible, radio/synchrotron, UV, IR, visible and X-rays), what makes X-rays the standout waveband for radiation belt science is not only that their generation involves energetic particle interactions, but also that it is possible to distinguish the interactions of different particle species in different regions (moons, aurora, radiation belts, Io torus, jovian atmosphere), and map these to different energy ranges (from keV to relativistic energies). For example, where electrons can produce 0.2-20 keV (non-)thermal bremsstrahlung continuum emissions, colliding ion species each produce distinctive spectral lines (e.g., O 6+ ions are characterized by a triplet line at ~0.56 keV -Fig. 4b). In contrast, observations of e.g. Jupiter's synchrotron emissions only measure the low-altitude electron belt, offering little information on other species/regions. The X-ray spectrum depends both on the properties of the energetic particles (species and flux) and the scattering medium or neutrals. The X-ray spectra therefore provide unique insights into the energetic particle distributions (section 1 & 3), rates of collision and nature of the collisional material (section 2).

X-rays are thus the ideal waveband to globally observe the multiple components and processes of Jupiter's radiation belts. The archive of observations demonstrate that it is not only a feasible task, but that there is enormous potential for a step-change in understanding and for ground-breaking discoveries, through a dedicated Jupiter-orbiting X-ray imager (section 4).

Identifying Global Dynamics & Source Processes through Time-Varying X-ray Spectra & Imaging of Jupiter's Radiation Belts:

The radiation belts are magnetospheric regions possessing high intensities of trapped energetic particles. Earth, Jupiter, Saturn, Uranus, Neptune, and Ganymede all possess radiation belts, suggesting that these may be a universal property of bodies with robust magnetospheres. Of the Solar System radiation belts, Jupiter's are the most intense, with >2 orders of magnitude higher intensities of >10 MeV electrons than the other planets [START_REF] Mauk | Electron radiation belts of the solar system[END_REF] and >GeV ions [START_REF] Becker | High latitude zones of GeV heavy ions at the inner edge of Jupiter's relativistic electron belt[END_REF]. For Jupiter, the belts begin within 0.5 RJ (RJ=Jupiter radii) of the cloud tops and continue beyond the orbit of Europa (at 9.4 RJ), dwarfing the scale and intensity of the entire closed terrestrial magnetosphere. In such a vast system, single-point in-situ particle/field measurements are key to identify driver processes, but such measurements require global context to identify which processes govern the system. Here, global images contemporaneous with in-situ measurements are essential.

The high energy regime of the Jovian system makes it an ideal target for X-ray observations. The 3 Suzaku X-ray Imaging Spectrometer (XIS) observations of Jupiter taken from 2006 to 2014 each independently revealed diffuse 1-5 keV X-ray emission from Jupiter's radiation belts (Fig. 1;[START_REF] Ezoe | Discovery of diffuse hard X-ray emission around Jupiter with Suzaku[END_REF][START_REF] Numazawa | Suzaku observation of Jupiter's X-rays around solar maximum[END_REF]2021). The radiation belt X-ray spectrum shows that these emissions trace ultrarelativistic (>10 MeV) electron distributions -producing X-rays by Inverse Compton scattering of visible solar photons.

In tandem with in-situ measurements, this global imaging provides a powerful tool to test the sources for Jupiter's radiation belts. For a mission with approach-phase observations or a 30-90 RJ apojove, this offers quasi-continuous coverage of the radiation belts, correlating currently unconstrained relationships between the belts and e.g. measurements of solar wind compressions, Io's volcanic activity or tail reconnection, addressing long-standing questions of the sources and processes regulating Jovian energetic particles. Comparison with global views of Earth's belts (from e.g. Van Allen Probes/SAMPEX) will reveal which processes are universal.

Acquiring images and spectra on sufficiently short timescales to connect these processes requires a Jupiter-orbiting X-ray instrument. Low X-ray fluxes arriving at Earth-orbit meant that each Suzaku detection required a 40-hr exposure. At Jupiter orbit, Jovian X-ray fluxes are ~10 7 larger than fluxes reaching Earth, enabling short-timescale (~hr) exposures that can measure time-variability in longitude, local-time and L-shell of the plasma distributions.

As is the case for other solar system X-ray sources (Lisse+, 2017; Dunn+, 2021), Jupiter's radiation belts are brighter than models predict (Numazawa+, 2021). Addressing these X-ray brightness inconsistencies is also key for interpretation of X-ray signatures across astrophysics. Solving this for Jupiter's radiation belts requires suitable, in-situ relativistic particle distribution measurements (Kollmann+, 2022) in tandem with X-ray observations, as proposed with COMPASS (Clark+, 2022). Such measurements will enable needed Inverse Compton models applicable to bodies from pulsars (Lyutikov+, 2012) to galaxies [START_REF] Strong | Cosmic-ray propagation and interactions in the Galaxy[END_REF](Strong+, -1262 citations) citations). [START_REF] Numazawa | Suzaku observation of Jupiter's X-rays around solar maximum[END_REF]: in the energy range 0.2-1 keV (a) and 1-5 keV (b), showing electron emissions from Jupiter's radiation belts at 1-5 keV. The green circle indicates Jupiter's position and size (39") at the time. Grey dotted lines show magnetic field lines with equatorial crossings of 3, 6, 9, 12, 15, and 18 RJ. The black line is Io's orbital path.

Fig. 1: Suzaku XIS images of Jupiter and its Radiation Belts

Characterizing Radiation Belt Losses through X-ray maps of Jupiter's Moons, Tori and Rings:

While acceleration and source processes are a key focus in radiation belt physics, losses are also important because without them intensities would accumulate indefinitely. For Jupiter, the radiation belts are embedded deep in the inner magnetosphere (<15 RJ), far from the magnetopause standoff distance (60-100 RJ). Consequently, unlike for Earth, Jupiter's radiation belt particles cannot directly escape to the solar wind (SW). Instead, processes within the inner magnetosphere control the state and losses of the belt population. Across the solar system, moons, neutrals and rings modulate, erode and destroy radiation belts [START_REF] Kollmann | The evolution of Saturn's radiation belts modulated by changes in radial diffusion[END_REF][START_REF] Roussos | A radiation belt of energetic protons located between Saturn and its rings[END_REF]. Jupiter's inner magnetosphere is rich with moons, rings and gas tori that not only supply plasma, but also remove energetic particles from the system (Fig. 2). Why the balance of processes favors radiation build-up is not understood. X-ray emissions provide the key: remotely monitoring and diagnosing energetic particle collisions with neutrals, to identify, map and characterize global loss processes and their time-variability. In 1999, the first Chandra X-ray observations of Jupiter revealed the presence of X-ray emissions from Io (Fig. 3), Europa, and the Io Torus (Elsner+, 2002). X-ray emissions from the Galilean satellites are produced when energetic particles collide with the moons' surfaces (Nulsen+, 2020). The resulting X-ray emissions are therefore dependent on the composition of the surface and the contributions of bombardment by ions and electrons [START_REF] Paranicas | The ion environment near Europa and its role in surface energetics[END_REF]. Ion collisions produce Particle Induced X-ray Emission (PIXE -Johansson+, 1995), characterized by strong, discrete fluorescence lines. In contrast, electrons produce thick-target bremsstrahlung (TTB), which has continuum emissions, and can also produce PIXE lines (mostly for Z > 13 elements) [START_REF] Markowicz | Composition dependence of Bremsstrahlung background in electron-probe X-ray microanalysis[END_REF]. Precipitation of particles onto the moons' surfaces depends on their pitch angle and thus on the plasma population, magnetic field configuration and wave-particle interactions (Nénon+, 2017; 2018) so that X-ray emissions track the magnetospheric drivers of precipitation.

From Earth orbit, detectable X-ray fluxes from the Galilean moons are very low, so that it is impossible to explore variation with time/hemisphere. While studies from Earth-orbit assets have reached their ceiling, the ~10 7 X-ray flux increase available to a Jupiter-orbiting instrument transforms capabilities for the Galilean moons, enabling time variability and maps of energetic particle losses to be studied simultaneously with in-situ measurements of the belt conditions.

Unlike Saturn, where X-ray fluorescence from the rings is observed (Bhardwaj+, 2005), Xray emissions from radiation belt collisions with Jupiter's more tenuous rings, and moons Amalthea and Thebe (Fig. 2), are yet to be explored from Earth-orbit, due to signal limitations. Xray emissions from the Io Torus have also been observed from Earth, but their cause is unclear, seemingly due to a combination of electron-and ion-neutral collisions (Elsner+, 2002) that track the role of tori in loss processes. The low-signal at Earth-orbit makes interpretation of Io torus variation challenging. This is again solved through orbiting X-ray instrumentation.

Across the wider domain of astrophysics, X-rays are used to study dusty systems (e.g., inter/stellar objects and nebulae -see, Heinz & Corrales, 2016 for review), through the interaction of energetic particles with neutrals. For such systems, the observations proposed here provide critical ground-truths to enable interpretation of solely remote signatures. However, no Earth-orbiting X-ray instrument will provide improved spatial resolution for the foreseeable future. E.g., ESA's next-generation X-ray observatory, ATHENA+ (launch 2030s), has an order of magnitude lower spatial resolution than Chandra. Breakthroughs from mapping and characterizing particle losses to the moons require a Jupiter-orbiting X-ray instrument. Elsner+, 2002) and modeled Inverse Compton X-rays from Jupiter's radiation belts.

Fig 3. Schematic of observations by the COMPASS X-ray Imager of: charged particle precipitation into the Jovian atmosphere (with insert Chandra observation showing Jupiter's auroral and equatorial Xray emissions -Gladstone+, 2002); charged particle losses to the moons (with insert Chandra observation of Io from

Atmospheric Belt Losses, M-I Coupling & the Universality of Open Magnetospheric Systems:

Energetic particles precipitating into Jupiter's atmosphere produce an array of X-ray emissions. Depending on the precipitating particle, different X-ray spectral signatures are produced. Electron precipitation produces bremsstrahlung, diagnostic of the electron distribution (B-R+, 2004; Mori+, 2022, Nature Ast.), while sufficiently charged or >300 keV/amu ions produce CX lines (Fig. 4), identifying the precipitating ion species and energies (B-R+ 2007;[START_REF] Houston | Jovian auroral ion precipitation: X-ray production from oxygen and sulfur precipitation[END_REF].

Like Earth, Jupiter loses radiation belt particles via precipitation to the atmosphere (Kollmann+, 2021). While Jupiter's low-latitude X-ray emissions are dominated by solar photons that elastically scatter off atmospheric hydrogen [START_REF] Maurellis | Jovian X-ray emission from solar X-ray scattering[END_REF], there are also emission enhancements where the radiation belt loss cone is largest [START_REF] Bhardwaj | Low-to middle-latitude X-ray emission from Jupiter[END_REF]McEntee+, in review). From Earth-orbit, where Jupiter's sunlit face is the only observable hemisphere, the Xray spectrum from this radiation belt precipitation cannot be distinguished against the dominant scattered solar photon signal. However, for Jupiter-orbiting spacecraft, nightside observationsfree from scattered sunlight -can monitor spectral/spatial/temporal characteristics of electron and ion belt losses comparable with simultaneous measurements of e.g. EMIC/whistler waves [START_REF] Nénon | A new physical model of the electron radiation belts of Jupiter inside Europa's orbit[END_REF][START_REF] Masters | A more viscous-like solar wind interaction with all the giant planets[END_REF]; connecting source and loss processes.

Jupiter possesses the most powerful aurora in the Solar System, which dominates its high latitude X-ray emissions (Fig. 4). As with pulsars, Jupiter's aurorae are tilted to the rotation axis and fixed in the rotating frame so that they rotate in and out of view with planetary rotation. While many wavebands provide key insights into Jupiter's aurorae, the X-rays stand out by showing emission from the precipitating particles and not the atmospheric response (e.g. UV/IR aurora) and by distinguishing electron aurora (Bremsstrahlung) from heavy ion aurora (CX lines). This offers a holistic characterization of M-I coupling, tracking upward and downward currents. Particularly, Jupiter's heavy ion aurorae are a unique Solar System laboratory, providing insights into M-I coupling in magnetospheres with high heavy ion abundances. Observations hint at arcs, time-variable pulsations and boundaries, that change on timescales too rapid for low-signal (~1 count/min) Earth-orbit observations to resolve (Dunn+ 2016). The proposed COMPASS X-ray Imager (XRI) would observe a truly transformative planetary signal of ~10,000 counts/second, revealing the morphology of what are currently sparse clusters of dots. Small green dots show <2 keV photons -predominantly CX lines from precipitating heavy ions. Large green dots show >2 keV photons from electron bremsstrahlung. Bremsstrahlung is co-located with the UV main oval, while the heavy ion aurorae occur poleward of this. The 10° latitude-longitude spaced grid is fixed in Jovian System III coordinates with 180° toward the bottom and 270° to the left (B-R+, 2008). b) XMM-Newton high resolution spectra from Jupiter (blue crosses), combining the RGS 1+2 spectra. Overlaid in red is the best fit model, which includes a CX model, representative of Jupiter's soft X-ray aurorae, and scattered solar X-ray lines to represent the Jovian disk emission (B-R+, 2007)

c) Jupiter X-ray aurora Signal to Noise Ratio in and out of the radiation belts with the COMPASS X-ray Imager (XRI).

In heliophysics, it is critical to understand to what extent magnetospheres are universally magnetically open or closed, and the importance of the Dungey cycle for different systems. For rapid-rotating systems like Jupiter, the timescale over which open magnetic field lines convect into the tail is much longer than the planet's rotation, leading to discourse over whether a Dungey cycle is feasible at all for such systems [START_REF] Mccomas | Jupiter: A fundamentally different magnetospheric interaction with the solar wind[END_REF][START_REF] Cowley | Comment on" Jupiter: A fundamentally different magnetospheric interaction with the solar wind" by DJ McComas and F. Bagenal[END_REF]. With heliospheric distance, systems may transition from Dungey-dominated (e.g. Mercury) to viscousdominated SW-interactions (e.g. Jupiter) [START_REF] Delamere | Solar wind interaction with Jupiter's magnetosphere[END_REF][START_REF] Masters | A more viscous-like solar wind interaction with all the giant planets[END_REF].

X-ray observations may uniquely solve the longstanding mystery of the extent to which

Jupiter is open to the SW. Through SWCX emissions, in which SW ions collide with neutrals and produce CX lines, it is possible to identify intervals when SW precipitates into the atmosphere. On rare occasions, X-ray observatories do detect SWCX signatures from Jupiter (Dunn+, 2020b). However, it has been impossible to identify their source region and duration due to the low spatial resolution and signal at Earth-orbit. This problem vanishes for the photon-rich signal available for a Jupiter-orbiting instrument. By utilizing SWCX signatures, the ESA-CAS SMILE spacecraft will study Earth's cusps in this way [START_REF] Sibeck | Imaging plasma density structures in the soft X-rays generated by solar wind charge exchange with neutrals[END_REF]B-R+,2018). Similar searches for SWCX from Jupiter's cusp will test the presence, orientation and time variability of Jupiter's cusp and the extent to which rapidly-rotating and/or distant magnetospheres are open to the SW.

New Advances in X-ray Instrumentation that Enable Ground-Breaking Heliophysics Studies:

Recent step-changes in X-ray instrumentation such as Micro-Pore Optics and radiation tolerant detectors open the possibility of compact, lightweight (~10 kg), low power (~W), X-ray instruments perfectly-suited for Heliophysics science at Jupiter. To observe the sources discussed in this paper, an instrument requires: an energy range of 0.2-8 keV with resolution of 0.15 keV at 0.56 keV; Field of View (FoV) of 22+/-5° (global views of belts from 30-90RJ); angular resolution of 1° to access L-shell resolution on Jupiter during perijove[PJ] (4-6 RJ) and to resolve the moons at apojove, or ~10' resolution to resolve surface features on moons from <10 RJ.

Several instruments in flight or due for launch within 5 years carry lightweight X-ray instruments that meet the requirements outlined above and will achieve related science goals, including: MIXS on BepiColombo, to observe PIXE and TTB from Mercury (Bunce+, 2020); the SXI on the SMILE mission, to study SWCX from the terrestrial cusps and magnetosheath (B-R+, 2018) or the Chinese-French SVOM mission to observe Gamma Ray Bursts. JAXA's Jupiter-orbiting JUXTA X-ray instrument concept, also laid essential groundwork [START_REF] Ezoe | JUXTA: A new probe of X-ray emission from the Jupiter system[END_REF].

For the NASA heliophysics mission-concept COMPASS, the onboard X-ray Imager (XRImass including shielding: 15 kg; power: 15 W) is based on AEPEX/AXIS heritage [START_REF] Marshall | The AEPEX mission: Imaging energetic particle precipitation in the atmosphere through its bremsstrahlung X-ray signatures[END_REF]. The XRI provides a 22° FoV with 1° resolution (50-200 km moon resolution during flybys; L-shell resolution on Jupiter during PJ). While modern radiation-tolerant X-ray detectors can be built to survive Jupiter's particle background, noise is a challenge. Fig. 4c shows that despite this the signal:noise will achieve the science goals. To handle the extraordinary PJ X-ray signal of 10,000 cts/s, the instrument connects 16 X-ray detectors (each handling 6x10 4 cts/s).

Jupiter-orbit X-ray instrumentation and science is now high feasibility and low-risk, opening an exciting new paradigm in heliophysics. An unexplored treasure trove of discoveries awaits Jupiter-orbit X-ray instrumentation, which is essential for addressing an array of fundamental questions on the transfer of energy and particles in high energy systems. Through such observations, heliophysics will provide an irreplaceable stepping-stone to astrophysical systems beyond our solar system.

Fig. 2 :

 2 The removal of Jupiter's radiation belts by moons and the main ring[START_REF] Kollmann | Jupiter's radiation belts as a target for NASA's Heliophysics Division[END_REF]: Energetic Oxygen and Helium intensities throughout the Jovian radiation belts, showing the reduction in energetic particles at the orbits of the moons and ring.

Fig. 4 :

 4 Fig. 4: a) Projection of Jupiter's North pole showing the UV (orange) and X-ray (green dots) aurorae as recorded by the Hubble Space Telescope STIS FUV instrument and Chandra ACIS instrument, on 24 Feb 2003. Each green dot indicates a single X-ray photon.Small green dots show <2 keV photons -predominantly CX lines from precipitating heavy ions. Large green dots show >2 keV photons from electron bremsstrahlung. Bremsstrahlung is co-located with the UV main oval, while the heavy ion aurorae occur poleward of this. The 10° latitude-longitude spaced grid is fixed in Jovian System III coordinates with 180° toward the bottom and 270° to the left(B-R+, 2008). b) XMM-Newton high resolution spectra from Jupiter (blue crosses), combining the RGS 1+2 spectra. Overlaid in red is the best fit model, which includes a CX model, representative of Jupiter's soft X-ray aurorae, and scattered solar X-ray lines to represent the Jovian disk emission (B-R+, 2007) c) Jupiter X-ray aurora Signal to Noise Ratio in and out of the radiation belts with the COMPASS X-ray Imager (XRI).

Exploring Fundamental Particle Acceleration and Loss Processes in Heliophysics through an Orbiting X-ray Instrument in the Jovian System

A White Paper for the 2024-2033 Solar and Space Physics (Heliophysics) Decadal Survey


Ralph Kraft

Maxim Markevitch

Caroline Kilbourne

Joseph S Adams

Hiroki Akamatsu

Mohammadreza Ayromlou

Simon R Bandler

Marco Barbera

Douglas A Bennett

Anil Bhardwaj

Veronica Biffi

Dennis Bodewits

Ákos Bogdán

Massimiliano Bonamente

Stefano Borgani

Graziella Branduardi-Raymont

Joel N Bregman

Joseph N Burchett

Jenna Cann

Jenny Carter

Priyanka Chakraborty

Eugene Churazov

Robert A Crain

Renata Cumbee

Romeel Davé

Michael Dipirro

Klaus Dolag

W Bertrand Doriese

Jeremy Drake

William Dunn

Megan Eckart

Dominique Eckert

Stefano Ettori

William Forman

Massimiliano Galeazzi

Amy Gall

Efrain Gatuzz

Natalie Hell

Edmund Hodges-Kluck

Caitriona Jackman

Amir Jahromi

Fred Jennings

Christine Jones

Philip Kaaret

Patrick J Kavanagh

Richard L Kelley

Ildar Khabibullin

Chang-Goo Kim

Dimitra Koutroumpa

Orsolya Kovács

K D Kuntz

Erwin Lau

Shiu-Hang Lee

Maurice Leutenegger

Sheng-Chieh Lin

Carey Lisse

Ugo Lo Cicero

Lorenzo Lovisari

Dan Mccammon

Seán Mcentee

François Mernier

Eric D Miller

Daisuke Nagai

Michela Negro

Dylan Nelson

Jan-Uwe Ness

Paul Nulsen

Anna Ogorzałek

Benjamin D Oppenheimer

Lidia Oskinova

Daniel Patnaude

Ryan W Pfeifle

Annalisa Pillepich

Paul Plucinsky

David Pooley

Frederick S Porter

Scott Randall

Elena Rasia

John Raymond

Mateusz Ruszkowski

Kazuhiro Sakai

Arnab Sarkar

Manami Sasaki

Kosuke Sato

Gerrit Schellenberger

Joop Schaye

Aurora Simionescu

Stephen J Smith

James F Steiner

Jonathan Stern

Yuanyuan Su

Ming Sun

Grant Tremblay

Nhut Truong

James Tutt

Eugenio Ursino

Sylvain Veilleux

Alexey Vikhlinin

Stephan Vladutescu-Zopp

Mark Vogelsberger

Stephen A Walker

Kimberly Weaver

Dale M Weigt

Jessica Werk

Norbert Werner

Scott J Wolk

Congyao Zhang

William W Zhang

Irina Zhuravleva

John Zuhone

Line Emission Mapper (LEM): Probing the physics of cosmic ecosystems

come

The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s that will answer the outstanding questions of the Universe's structure formation. It will also provide transformative new observing capabilities for every area of astrophysics, and to heliophysics and planetary physics as well. LEM's main goal is a comprehensive look at the physics of galaxy formation, including stellar and black-hole feedback and flows of baryonic matter into and out of galaxies. These processes are best studied in X-rays; as emphasized by the 2020 Decadal Survey, emission-line mapping is the pressing need in this area. LEM will use a large microcalorimeter array/IFU (that builds on Athena XIFU technology developments), covering a 30 × 30 field with 10 angular resolution, to map the soft Xray line emission from objects that constitute galactic ecosystems. These include supernova remnants, star-forming regions, superbubbles, galactic outflows (such as the Fermi/eROSITA bubbles in the Milky Way and their analogs in other galaxies), the Circumgalactic Medium in the Milky Way and other galaxies, and the Intergalactic Medium at the outskirts and beyond the confines of galaxies and clusters. LEM's 1-2 eV spectral resolution in the 0.2-2 keV band will make it possible to disentangle the faintest emission lines in those objects from the bright Milky Way foreground, providing groundbreaking measurements of the physics of these plasmas, from temperatures, densities, chemical composition to gas dynamics. While the mission is optimized to provide critical observations that will push our understanding of galaxy formation, LEM will provide transformative capability for all classes of astrophysical objects, from the Earth's magnetosphere, planets and comets to the interstellar medium and X-ray binaries in nearby galaxies, AGN, and cooling gas in galaxy clusters. In addition to pointed observations, LEM will perform a shallow all-sky survey that will dramatically expand the discovery space.

MISSION CONCEPT

The Line Emission Mapper (LEM) is an X-ray Probe concept aimed at one of the fundamental unsolved problems of modern astrophysics -galaxy formation. Galaxies are governed by the competi-R 200 Fig. 1 -Why soft X-rays? Predicted fraction of Oxygen ions in a CGM halo of a Milky-Way mass galaxy (simulations using EAGLE model 16 ; dotted line is the virial radius; vertical scale is linear). Probing this circumgalactic gas is critical to understanding the physics of galaxies. The temperature of the halo is such that the dominant species over most of the halo is OVII, whose emission (along with that of OVIII) lies in the soft X-ray band -the energy of the Oxygen He α resonant line is 574 eV. The lower ionization species are observable in the UV, but modeling and extrapolation are required to deduce the properties of the CGM. An X-ray instrument will probe the bulk of the CGM directly, complementing the UV data on the cooler gas phases. tion of gravity and the energetic "feedback" from the forming stars and black holes. Theoretical studies predict that key information on these processes is encoded in the tenuous Circumgalactic Medium (CGM) and even more rarefied Intergalactic Medium (IGM), which together contain most of the baryonic matter in the Universe 3 . Most of that matter has so far eluded detection, much less detailed study. The Astro2020 Decadal Survey has identified mapping CGM and IGM in emission as the key missing observing capability and a major "discovery area." LEM is designed to provide this new capability in the soft X-ray band, where the dominant CGM and IGM ion species emit (Fig. 1). While the importance of studying these hidden baryons has been recognized for decades, only now has the technology arrived that makes it possible.

LEM is a single telescope that consists of an Xray mirror with a large collecting area and moderate angular resolution, and a cryogenic microcalorime-1

LINE EMISSION MAPPER

Grazing-incidence X-ray mirror: 1600 cm 2 effective area at E=0. ter array (Fig. 2). Both are optimized for the 0.2-2 keV energy band, where the most abundant CGM/IGM ion species have multiple bright emission lines. Because the CGM and IGM are diffuse sources, mapping them requires a nondispersive imaging spectrometer, or an integral field unit (IFU) as they are known in other wavebands. (Chandra HETG and LETG, XMM-Newton RGS and the ARCUS concept are dispersive spectrometers whose full spectral resolution is available only for point sources.) Furthermore, as will be seen below, our own Milky Way places a bright screen in front of the very faint IGM and CGM around other galaxies. Removing this foreground requires eV-class spectral resolution. These two conditions require a microcalorimeter. Such imaging X-ray calorimeters have not been flown before, with the exception of the short-lived Hitomi, to be replaced in 2023 by XRISM. LEM will provide a leap beyond XRISM, and surpass in relevant capabilities other missions planned for the more distant future. Table 1 compares LEM to future X-ray microcalorimeters -namely, XRISM Resolve, Athena XIFU (to be launched in late 2030s) and HUBS (under study). The effective area and "grasp" as functions of en-ergy for LEM, XRISM and Athena are compared in Fig. 3. We note that Athena is currently undergoing redefinition and as of this writing its final instrument parameters are unknown. The above comparisons are done with the pre-2022 Athena concept.

The LEM concept is being prepared for the NASA 2023 Astrophysics Probes call for proposals. If selected, LEM will be launched in 2032.

X-ray mirror

The grazing-incidence mirror will consist of many pairs (for two reflections) of thin monocrystalline silicon shells, coated with either Ir or Pt to provide X-ray reflectivity. The mirror's 1.5m diameter and 4m focal length are selected to provide a large effective area at the energies of interest (E < 2 keV) and a broad angular field of view, covering a Milky Way like galaxy at z = 0.01 (40 Mpc), to fit within the detector of a feasible linear size. This choice of geometry means that the effective area at higher energies is low. The mirror is being developed at NASA GSFC. The angular resolution requirements imposed by the main science drivers is modest, 10 half-power diameter (HPD) or 15 square pixel enclosing 90% energy. A resolution of 2.8 HPD has Fig. 3 -LEM effective area and grasp -the product of the effective area and the field of view solid angle -compared to XRISM Resolve and Athena XIFU (prior to 2022 reformulation). Grasp is the quantity relevant for mapping faint extended objects with sizes comparable to or greater than the field of view (such as nearby galaxies, clusters, the IGM, or the Milky Way structures) -for such targets, the number of photons collected in a given exposure is directly proportional to grasp. -LEM compared with the requirements for the future X-ray nondispersive spectrometers onboard XRISM (JAXA/NASA), Athena (ESA/NASA) and HUBS (Chinese Space Agency). For XRISM, based on Hitomi, the actual spectral resolution is expected to be 5 eV. The main advantages of LEM are its large grasp and high spectral resolution. * For Athena, pre-reformulation requirements are listed.

LINE EMISSION MAPPER

already been demonstrated in the lab for a small subset of the mirror (Fig. 4). The mirror is designed to have a constant PSF and insignificant vignetting across the 30 field of view.

Detector

The LEM detector is a cryocooled array of Transition-Edge Sensor (TES) microcalorimeters. It will be built at NASA GSFC using the technology developed for Athena XIFU and the Lynx concept by the same team 9,11 . The array will consist of 13,806 absorber pixels (equivalent of 118 × 118 square) with a 290 µm pitch, in hexagonal arrangement. For a 4m focal length, it will cover a solid angle equal to a 29.4 square.

The inner 1062 absorber pixels (a 7 × 7 square array) will each be connected to a single TES and provide an energy resolution of 1 eV (FWHM). The rest of the array will consist of "hydras," where 2 × 2 absorber pixels are connected to a single TES, which will determine which absorber was hit with a photon based on the different time constants for the different absorbers 23 . These pixels will provide a resolution of 2 eV (FWHM) for E < 2 keV. The absorber thickness will be optimized for the low X-ray HPD=2.8" 5 shell pairs, full illumination, 4.5 keV 15" pixel Fig. 4 -Point Spread Function measurement for a subset of the mirror consisting of 5 pairs of reflective shells. The image is elongated because it comes from an incomplete mirror. A 2.8 HPD is achieved for this subset; the requirement is 10 HPD (or, equivalently, for 90% of the photons to fall in the 15 detector pixel) for the whole LEM mirror that will combine approximately 3500 shell pairs. energies and the sensor will take advantage of the narrow energy band of the mirror. Figure 5 shows the measured line response for a prototype LEM array 24 , which already nearly meets our requirements both for the single pixels and the hydras. The position discrimination within the hydra has also been demonstrated at least for E > 300 eV. 24 LEM will use the IR and optical blocking filters developed for Athena XIFU, scaled up in size for a wider viewing cone of the LEM mirror.

The calorimeter array is cooled to the 35 -50 mK operating temperature by the Adiabatic Demagnetization Refrigerators built at NASA GSFC and the cryocooler built by Lockheed Martin.

A modulated X-ray source will operate in orbit and illuminate the array with a 1.5 keV Al Kα near-monochromatic line for continuous calibration of the energy response. The resulting accuracy on the pixel energy gain, 0.25 eV at 1.5 keV (and proportionally lower at lower energies), corresponds to a 50 km/s accuracy (1σ) on the relative velocity measurements using the Doppler shift.

Spacecraft and orbit

The spacecraft will be built by Lockheed Martin Corp. The main mode of operations will be long pointed observations. LEM will also be able to slew to perform efficient scans of large areas of the sky -such as the whole sky ( §3.2). While LEM is not a time-domain mission and not required to have a fast response capability, it will be able to respond to the highest-profile events on a several-day timescale from first notice to pointing -similar to Chandra. The area of the sky accessible at any particular moment will be approximately half the sky; this fraction is currently under study.

The orbit for LEM is currently under study. The Sun-Earth Lagrange points L1 and L2 would provide a stable thermal environment for the microcalorimeter and the high observing efficiency to take full advantage of the 5 yr mission minimum lifetime. A possibility to extend the field of regard to include observations of the Earth's magnetosphere from the L1 orbit is being considered.

SCIENCE DRIVERS

Below is a brief description of the science investigations that will be enabled by LEM. They will be Fig. 5 -Spectral resolution for a prototype LEM detector array 24 , measured for the single-absorber pixels (the central array) and the 4-pixel hydras (the rest of the detector). The measurements were performed at E = 1.5 keV. The blue dotted line is the intrinsic Al line profile (arbitrary normalization) and the black line is the model that includes detector broadening. The performance already nearly meets the LEM requirements.

LINE EMISSION MAPPER

presented in more detail in several forthcoming papers.

Physics of galaxy formation

Galaxies form from the collapse of primordial matter density fluctuations under the effect of gravity, which is counteracted by feedback from stars and the central supermassive black holes that pushes the gas out of the galaxy. The result is a circulation of gas and metals (synthesized by stars and supernovae in galaxies) into and out of galaxies -a galactic ecosystem. Understanding this ecosystem is key to understanding galaxy formation. Quoting Astro2020, "the key missing link in unveiling the physics driving galaxy growth is to measure the properties of the diffuse gas within, surrounding, and between galaxies," and these three locations constitute the main LEM science drivers.

CGM. At the mass scale of the Milky Way, the observable galaxy properties exhibit a dramatic change -from star-forming disks thought to be fed by cold gas streams, to passive ellipticals surrounded by hot hydrostatic gas. Apparently, this reflects the shift in the balance of the various galaxyshaping physical processes. Galaxies around this mass should thus offer a particularly sensitive probe of those processes. Indeed, cosmological simulations of the MW-mass galaxies that include different physics make strikingly different predictions for the X-ray properties of the outer regions of the circumgalactic halos (Fig. 6). At present, these predictions are entirely unconstrained by observations, and detecting and measuring this X-ray signal can tell us MW-mass halo (log M * =10.7-10.9) OVII f line emission counts / arcmin 2 / Ms Fig. 6 -Predicted brightness profiles of the OVII emission for Milky Way mass galaxies from three cosmological simulations (TNG 13 , EAGLE 22,5 , Simba 6 ). OVII is abundant at T = (0.3 -2) × 10 6 K that is characteristic of CGM. Each curve is a median profile for 50 simulated galaxies. The expected LEM counts are shown for a 2 eV wide interval centered on the forbidden line of the OVII triplet at z = 0.01. R 200 1.5R 500 . These numerical models include different physical prescriptions for feedback, and the resulting predicted X-ray brightness at large radii differs by over an order of magnitude. The CGM extent and spatial structure are sensitive diagnostics of the feedbak physics; detecting and mapping this emission will strongly constrain the physical processes shaping the galaxies.

LINE EMISSION MAPPER

LEM 1 Ms

O VII O VIII Fe XVII O VII O VIII

Milky Way foreground

CCD resolution

CGM emission, z=0.01 Fig. 7 -Simulated LEM spectrum of the CGM of a galaxy twice as massive as the Milky Way (from TNG simulations 13 ), at z = 0.01 or 40 Mpc, for a 1 Ms exposure and the 2 eV resolution of the full detector. Blue line shows the spectrum of the foreground, red line includes the CGM signal. The spectrum is extracted from a broad annulus between 0.25 -0.75 R 500 . Point sources detected with the LEM angular resolution are excluded. Because the Milky Way emits in the same spectral lines but is over an order of magnitude brighter, it is impossible to disentangle the two with the present CCD instruments such as Chandra and XMM-Newton (green line). A nondispersive spectrometer with 2 eV resolution allows the study of the faint CGM for galaxies at z 0.01, where they can also be well-resolved spatially.

which physical processes shape the CGM.

LEM will be able to map the CGM (defined here as the gaseous halo within the galaxy virial radius R 200 ) in the emission lines of the dominant ion species, OVII, OVIII and FeXVII. The high spectral resolution will allow us to disentangle the faint signal from the CGM of external galaxies from the overpowering Milky Way foreground using the galaxy's redshift. Because the Milky Way itself has a CGM halo that emits in the same atomic transitions -and we are immersed in its bright central region and see that emission across the sky -the ability to separate the redshifted extragalactic signal by energy is a critical advantage of LEM, as illustrated in Fig. 7. The current X-ray CCD detectors lack spectral resolution for this approach.

For the Milky Way mass halos, LEM will be able to map the CGM emission for individual galaxies (as opposed to stacking the radial profiles of many galaxies) in the brightest emission lines out to R 500 and beyond, and derive the distribution of gas temperature, density, pressure and elemental abundances out to a large fraction of R 500 . The maps are expected to reveal a wealth of spatial structure, possibly including the extragalactic analogs of our own Milky Way's Fermi bubbles (Fig. 8) -if black hole / AGN feedback is as important as theory predicts. Such measurements are only possible because the Milky Way foreground can be separated using the redshift of the signal. The minimum redshift for such separation for the 2 eV detector resolution is ∼ 0.01 (Fig. 7); at this redshift, the R 500 for a Milky Way mass galaxy fills the LEM FOV and the whole galaxy can be mapped in one deep pointing. The LEM angular resolution will allow efficient spatial masking of the background point sources and resolving the interesting linear scales in the halo (e.g., shocks around the Fermi bubbles). A typical CGM halo is optically thin for the emission lines of interest over most of its volume except for the very central regions, where the optical depth for the OVII resonant line becomes significant. LEM will easily resolve the components of the OVII Heα triplet (where the resonant line is affected by scattering while other components are not), which will enable interesting physical diagnostics of the gas.

Using the high-resolution (1 eV) central array, LEM will be able to simultaneously map the veloci- ties in the brighter central regions of the galactic halos. It will detect gas bulk velocities and velocity dispersion predicted for the particularly energetic modes of feedback.

LINE EMISSION MAPPER

IGM. An even more challenging observing target is the IGM, which we define as the diffuse matter outside R 200 of any galactic or cluster halos. The warm-hot phase of it (> 10 5 K) is known as WHIM. It is the ultimate repository of everything expelled from the galaxies over their lifetime, and its metal content thus encodes the history of the feedback processes in the Universe. The IGM regions immediately surrounding massive hot galaxy clusters, where the gas temperature is still above 10 7 are accessible for sensitive low-background CCD instruments, which could use the emission at E > 1 keV. However, most of the IGM has lower temperatures, and its emission will be flooded by the bright Milky Way. LEM will be able to detect the brightest emission lines from the dense regions of the Cosmic Web filaments that connect galaxy clusters, at distances 2R 200 from the clusters and beyond.

We will use optical galaxy redshift surveys to select such filaments at an optimal redshift (z = 0.07 -0.08) to be able to see the most promising IGM emission lines through the Milky Way line forest (Fig. 9) and survey these regions with multiple deep pointings. This is where the large grasp is critically important, as the number of photons collected from a very extended source is directly proportional to it -and the IGM is extremely faint. While we will not be able to detect the continuum emission from the IGM (it is too far below the background), we will detect the faint emission lines of OVII, OVIII and FeXVII (if, that is, the IGM is indeed enriched, as the current theory predicts), and compare the brightness in these lines as a function of the galaxy number density with cosmological simulations, in order to determine when the enrichment of the IGM by galactic ejecta has occurred and what physical processes were responsible.

LEM's large grasp and good angular resolution will allow us to detect the IGM in absorption in the cumulative spectra of the CXB sources in the same fields where we will search for the IGM emission. A comparison of emission and absorption for the same filament will allow us to derive the physical properties of the filament gas. Fig. 9 -Simulated LEM spectrum of the Intergalactic Medium (IGM) in the Cosmic Web. Owing to its high spectral resolution and large grasp, LEM will be able to detect emission from metals in the denser regions of the Cosmic Web filaments, outside the confines of galaxies (and their CGM halos) and galaxy clusters. It is a diagnostic of galaxy formation processes at early epochs. Filaments will be selected from galaxy surveys and the redshift of the IGM will be known, i.e., it is not a blind search for emission lines. The image shows OVIII emission in cosmological simulations (TNG100 12 ). The sample spectrum corresponds to a typical bright region of an IGM filament near a massive cluster (at a distance r ∼ 2R 200 from the cluster), excluding halos of all galaxies and clusters. The eV spectral resolution is essential for separating the faint IGM lines (highlighted by red) from the much brighter Milky Way foreground (cf. the green line that shows the same spectrum at CCD resolution). The line emission from the low-density IGM can be significantly enhanced (in the resonant transitions only), compared to intrinsic thermal emission, by resonant scattering of the CXB photons, if the CXB point sources are excised from the image 8 . This improves the detectability of the IGM emission. This also applies to the outer regions of the CGM, with a significant additional enhancement from the resonant scattering of the photons from the galaxy's own bright central region. This will extend the radius at which the CGM could be detected by LEM. Of course, interpretation of the detected signal will require modeling of the scattered contribution; LEM will be well equipped for such spectral diagnostics because it will resolve the resonant and forbidden transitions for the relevant ions.

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Milky

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Abundances in the ICM. LEM will address the cumulative effect of the galactic feedback from another angle. The ratios of the various elements in the ICM in the outskirts of galaxy clusters, as well as the total metal content of the ICM and the spatial distribution of metals in the ICM, depend on the prevalent star populations and types of supernovae that generated those metals, when it happened, and how the metals mixed into the intergalactic gas 10 . LEM will map the abundances and abundance ratios for O and Fe, but also for a number of less-abundant elements that can be used as sensitive diagnostics for the sources of chemical enrichment. LEM's large grasp and good angular resolution will allow us to efficiently and completely map the abundances of all the interesting elements in nearby groups and clusters in the interesting radial range 0.5R 500 -R 200 .

Velocities of the ICM. LEM will efficiently and completely map the gas velocities and velocity dispersions in nearby clusters and groups, relaxed and dynamic, out to R 200 (using mostly the Fe-L lines). It will determine where the clusters are hydrostatic and where the contribution of turbulence and gas bulk motions into the total energy budget becomes important. This information is key for our understanding of the physics of the large scale structure and cluster formation.

Stellar feedback. LEM will study the gas inside the Milky Way that is part of the galactic ecosystem. It will map the chemical composition Fig. 11 -LEM shallow all-sky survey. The all-sky map 20 in Galactic coordinates overlays soft X-ray emission from SRG/eROSITA (cyan) and γ-ray emission from Fermi (magenta). Top: Example simulated spectra for 10 • × 10 • regions (marked by squares) and 100s depth are shown for an average high-latitude region outside of any bright features ("extragalactic field"), and another for the North Polar Spur; the CXB is included. Purely thermal CIE plasma models are assumed. Red curve shows the NPS spectrum at CCD resolution (e.g., eROSITA). LEM will resolve the forest of lines and gain access to line diagnostics for temperature, nonequilibrium and charge exchange processes. The survey will allow us to map the temperature structure and velocities of the Milky Way inner CGM on few-degree scales -including the expansion of the eROSITA/Fermi bubbles, believed to be produced by the central SMBH or star-forming regions. Numerous other investigations, some of which are labeled in the figure, will become possible.

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and velocities of ejecta in well-resolved supernova remnants Fig. 10). It will also map the physical conditions in the Milky Way and Local Group superbubbles -regions where stellar winds from many massive stars and the supernovae combine and form large cavities in the ISM filled with hot plasma. If superbubbles break, the hot plasma can escape from the galactic disk, forming galactic outflows or fountains and flowing into the CGM. This is the mechanism by which the stellar feedback in galaxies operates. Mapping the X-ray line emission will offer diagnostics of gas out of collisional ionization equilibrium (rapidly heating/cooling) and separating thermal emission from the charge exchange emission that arises from the mixing of cold and hot gas phases. With its large grasp, LEM will be able to efficiently map very extended structures as Orion-Eridanus and other nearby superbubbles.

All-Sky Survey

As described above, the IGM and the CGM of external galaxies are observed through the forest of bright Milky Way emission lines. It will be helpful to find areas in the sky where this foreground is less bright, at least in certain problematic lines such as FeXVII and FeXVIII. (This is similar to Hubble peering into the Lockman Hole to observe high-z galaxies.) The eROSITA and HaloSat sky surveys will be helpful, but lack spectral resolution to detect faint emission lines. LEM has sufficient grasp to perform its own shallow all-sky survey in a relatively modest total exposure -e.g., an all-sky coverage with a useful 10s depth will require 1.6 Ms. We envision performing such shallow pathfinder survey very early in the mission to identify areas in the sky best suited for deep CGM/IGM exposures and for other interesting studies, and add up to 10 times more depth over the mission lifetime.

In addition to the above technical purpose, the first-ever all-sky survey with an imaging calorimeter will open enormous discovery space. With LEM's large effective area, even a shallow depth will be sufficient for groundbreaking studies. The statistical quality and richness of the spectra for typical low-brightness and high-brightness regions of the sky for the eventual full depth are illustrated in Fig. 11. The figure also lists examples of investigations using all-sky or partial-sky data that we can envision at present. Some important aspects of the feedback processes in Milky-Way mass galaxies would be best viewed from inside the Milky Way. LEM will map the velocities of the inner regions of the Milky Way CGM, and in particular, the expansion of the Fermi/eROSITA bubbles, believed to be evidence of feedback from either the SMBH or star-forming regions in the Galactic Center. It will map the temperature structure of the inner CGM across the sky and along the line of sight using lines of the various ion species -something only a calorimeter can do, in the presence of multiple temperature components and solar wind charge exchange on each line of sight. The MW observations will complement the studies of CGM in other galaxies ( §3.1), where LEM will map the outer halos but have limited insight into the interface between the disk and the halo, where the exchange of mass and metals with the outer CGM takes place.

The emission of the lower MW halo (thought to be formed from the exhaust of Galactic chimneys) can be separated from the emission of the Galactic disk ISM by observing a dense absorbing extraplanar cloud, and determining the spectra of the foreground and background to the cloud. LEM's large FOV and high spectral resolution will allow the first definitive separation of the disk and halo spectra for individual plasma diagnostics.

LEM's sensitivity and spectral resolution at low energies will let us unambiguously separate the the heliospheric solar wind charge exchange (SWCX) line emission from thermal emission of the Local Hot Bubble (LHB) and the Galactic halo. This will let us explore the physics of the LHB. The North Polar Spur is another enigmatic structure in the Milky Way sky, possibly projected onto the northern Fermi bubble (Fig. 11); its spectrum is known (at CCD resolution) to be very different from the rest of the sky above the Galactic plane. A survey with LEM will provide the plasma temperature and velocity map for this feature, as well as any possible contribution from the charge exchange mechanism, and may explain its nature.

The Fermi bubbles (as distinct from the eROSITA bubbles that surround them, Fig. 11) contain cosmic ray electrons that generate the synchrotron "microwave haze" observed by WMAP and Planck and the inverse Compton γ-ray emission discovered by Fermi 1 . The synchrotron radiation gen-

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erated by the highest-energy end of this population may be detectable in the soft X-ray band covered by LEM. LEM is a very sensitive instrument for detecting faint continuum emission at E < 1 keV, because it can resolve away the emission lines that dominate the soft X-ray sky. LEM will have the requisite grasp to map the entire Fermi bubbles as part of the all-sky survey and yield constraints on the X-ray synchrotron emission from the southern Fermi bubble. LEM upper limits -or a positive detectionwill provide insights into the energetics of the Fermi bubbles.

Observatory science

In addition to the main science drivers described above (which will be addressed during the 30% of the mission observing time allocated to the science team), LEM will transform all areas of X-ray astrophysics. It is useful to think of LEM as an XMM-Newton with a calorimeter -its imaging capabilities (FOV, angular resolution, effective area) are similar in the E < 2 keV band to EPIC's, while its spectral resolution is 50 times better. Many studies will be serendipitous to the long CGM and IGM exposures, while others will be proposed by general observers (GO) for the remaining 70% of the observing time. Many of them will need only modest exposures. An incomplete sample of possible novel short investigations is given below.

Cooling, AGN feedback, gas mixing in cool cores of galaxy clusters. Where is the coolest gas in the Perseus cluster core and how cool does it get? LEM will use its 1 eV resolution in the central array to derive the temperature distribution for the cooling gas. It will also map the charge exchange emission that may come from the interface between the hot ICM and the molecular gas nebula. If such emission is detected and spatially correlated with the cold nebula (for which at least a LEM-like angular resolution is required), it would provide invaluable insight into the process of gas mixing -a critical piece of the puzzle of the energy balance in the cluster cool cores.

Power spectrum of turbulence in cool galaxy clusters and groups using the velocity structure function. Because of its large grasp, LEM will be able to cover the entire cluster in a short exposure. The full spatial coverage greatly increases the accuracy of the structure function and the power spec-trum measurement. 26 A power spectrum of turbulence will provide the measure of the energy flux from the cluster mergers down the turbulent cascade to the dissipation scale.

Protoclusters at high z. Deep exposures toward IGM and CGM fields will return significant numbers of serendipitous distant galaxy clusters, for which LEM will be able to determine the redshift using its own X-ray spectra. Dedicated surveys can be also devised. LEM will derive chemical abundances for high-z clusters and constrain the cosmological enrichment history.

High-z AGN. Every LEM deep field will have multiple AGN at z > 2.5 with a detectable Fe Kα line. LEM will measure the profiles of these lines to study broad-line region clouds and accretion disk reflection. It will study heavily obscured AGN and expand significantly upon the AGN science derived from current Chandra and XMM-Newton deep fields.

AGN outflows and feedback. With its superior energy resolution, LEM will be able to precisely map fluorescent and photoionization lines and decouple AGN signatures in emission and absorption from ionization by stellar sources to understand the role that AGN outflows and feedback play in galaxy evolution.

Hot ISM and X-ray binaries in nearby galaxies. In face-on spirals, LEM will map the ISM dominated by emission lines and isolate the X-ray binaries (XRB) spatially and using their time variability. In nearby spirals such as M33 (Fig. 12), LEM will be able to study XRB populations and starforming regions that are generally not detected in the Galaxy due to obscuration. We will be able to apply plasma diagnostics to determine whether the gas in and around star-forming regions is in CIE, or whether it has lost a significant amount of energy in expansion, while remaining in a strongly overionized state. This determination will be pivotal for understanding the formation of ISM superbubbles.

In accreting stellar-mass black hole systems, relativistically-broadened O-K and Fe-L lines are fluoresced off the surface of the accretion disk. These features can be used to constrain the accretion disk inner radius and/or black hole spin.

Jellyfish galaxies. LEM will map the process of ram-pressure stripping of galaxies traveling through Fig. 12 -LEM will be able to spatially separate and perform spectroscopy of the ISM and the X-ray binaries in nearby spiral galaxies, such as M33. A simulated 20 ks LEM image (based on the existing XMM-Newton observation) and a sample ISM spectrum are shown. LEM will detect many binaries and probe their variability. It will also produce detailed maps of the line-dominated ISM emission. For such nearby galaxies, the Milky Way foreground cannot be separated using the redshift, but its contamination for most ISM lines is under 10%. In each observation, LEM will provide the same amount of imaging information as XMM-Newton, but 20-30 times more spectral information.

the gas in groups, clusters and the Cosmic Web filaments, and of galaxies undergoing collisions with other galaxies. This is an important mechanism by which the ICM and IGM are enriched by metals. LEM will be particularly well-equipped to detect the stripped gas and study its mixing with the ICM -a process that strongly depends on plasma physics and such properties as plasma viscosity, thermal conduction and the structure of its magnetic fields.

Figure 13 illustrates how LEM will use its energy resolution to separate the cool stripped gas, whose emission is dominated by certain lines, from the hotter ICM, revealing the morphology of the stripped tail, which strongly depends on plasma properties 21 . Furthermore, as stripping can bring neutral gas from the galaxy in direct contact with the hot ICM, one can in principle expect the charge exchange emission from their interface, with its telltale line signatures in the LEM spectra. This is a completely unexplored area that holds great promise for probing the ICM plasma physics. LEM will also study the effect of galaxy collisions and ram pressure stripping on ISM and star formation.

Shocks in outskirts of massive galaxies. As noted above ( §3.1), the emission from the outer regions of the CGM of massive galaxies may be enhanced by resonant scattering of the CXB photons 8 and of the photons from the galaxy's own bright central emission. This "illumination" in the resonant OVII line may help LEM detect and map the outer reaches of the galactic halos, where OVII should be abundant but the intrinsic thermal emission is too faint. It is at those virial regions that we expect to find shock fronts from galaxy/galaxy mergers and matter infall. Recently, a radio phenomenon was discovered that may be caused by those shocks ("odd radio circles," or ORCs, around massive galaxies at several hundred kpc from their centers 15 ). LEM is well-equipped for detecting such shock features via their associated density jumps in thermal plasma, highlighted by the resonantly scattered light. This will uncover an additional piece of the galaxy formation puzzle. Such measurements will have to be done at z > 0.035 in order to move the OVII resonant line from behind the MW OVII triplet.

Light echo on giant molecular clouds in the Galactic Center. The SMBH in the center of our Galaxy is quiescent at present, but may have been more active in past several hundred years. The light

Galaxy stripping

Fig. 13 -LEM will map the process of galaxy stripping and mixing of the stripped gas in detail. A 100 ks LEM exposure of interacting galaxies at z = 0.01 (TNG100 simulations 25 ) -an M tot = 2 × 10 11 M galaxy (at the panel center, with contours showing optical light) after the passage through a group-mass halo (at the lower edge of the panels). The CXB is included, except for its brightest point sources, which are removed. Right: LEM image in a narrow 3 eV interval at the redshifted OVII line (the forbidden component of the triplet, which at this redshift can be resolved from the corresponding Milky Way foreground line as seen in Fig. 7) reveals a "jellyfish" -a trail of cool gas ram-pressure stripped from the infalling galaxy. The hotter group halo does not emit in this line. Such gas tails are a sensitive diagnostic of the plasma physics in galaxy groups and clusters. echo from past explosions will be seen with a time delay in the fluorescent line emission from the GC molecular clouds. 4 While the bright Fe fluorescent line at 6.4 keV is inaccessible for LEM, we will be able to detect such light echo in the Si fluorescence. LEM will require only 4-5 pointings to map the entire region of the giant molecular clouds and map the most recent history of activity of our own SMBH.

Charge exchange emission in the Jovian magnetosphere, planets and comets. LEM will offer revolutionary insights for heliophysics and planetary science. One example is the Jovian system. Jupiter produces the most powerful aurora in the solar system and these are key to understanding which processes govern rapidly-rotating magnetospheres. It is unknown to what extent the X-ray aurora are produced by the precipitation of material that originates in Io's volcanos or whether they are indicative of the direct entry of solar wind particles into the system 7 .

Figure 14 shows how LEM's exquisite energy resolution will finally solve this problem. The figure shows the unique new access LEM provides to the low energy line emissions from planets. This will open an entirely new paradigm, enabling the bright emissions in this spectral range to be utilized to study previously inaccessible objects or to offer a step change in capability to advance previous X-ray studies of Mars, Saturn, comets, the Ice Giants and the heliosphere 2 .

A possibility to extend the LEM calibrated energy band down to E = 0.05 keV is currently under study; while the effective area there would be small, the CX signal at those energies is very luminous. This will also allow diagnostics for species more closely linked to the standard UV lines of CIV, SiIV, NV, and OVI. The sub-eV resolution of LEM's central array would be critical in this very crowded spectral region.

Earth magnetosphere. A possibility to point LEM toward Earth from an L1 orbit is currently under study. If it is possible, LEM will be able to observe the SWCX emission from Earth's magnetopause and magnetosheath, study their dynamics and their response to the varying solar wind pres- b,c) LEM 1 eV resolution spectra for each scenario. Counts per resolution element for a 20 ks exposure is shown; background is not included for clarity (it is negligible because Jupiter is under 1 in size). The most prominent emission lines are marked. The instrument may offer significant sensitivity at E < 0.1 keV with the same high spectral resolution, providing access to never-before-seen charge exchange lines at low energies (an option currently under study; gray area in the plots). LEM will easily distinguish between these models and address key questions on if and when Jupiter's magnetosphere is open to the solar wind -a point of longstanding debate in the heliophysics and planetary science communities. sure -a matter of longstanding interest for the heliophysics community.

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In addition, the LEM observatory will greatly advance the studies of variable black hole winds, flares in exoplanet host stars, protostar accretion rates, interstellar dust in emission and absorption, and numerous other fields. Given the big leap in observing capability that LEM will bring, observers will most certainly come up with experiments that we cannot even foresee at present.

CONCLUSIONS

The Line Emission Mapper Probe will fundamentally transform our understanding of one of the most important questions in modern astrophysics -the formation of galaxies. The unique ability of this observatory to detect the low surface brightness Xray emission from the warm and hot gas in galaxy LINE EMISSION MAPPER halos and in the filamentary structures connecting larger mass structures, and to measure the temperature, entropy, and elemental abundances will allow an understanding of the state of this gas that is, at present, only a matter of theoretical speculation. With its combination of wide field of view X-ray optic and energy-dispersive microcalorimeter array, LEM offers the opportunity for complete dynamical and thermodynamic characterization of this gas out to a large fraction of the virial radius in Milky Way-sized galaxies and beyond the virial radius for galaxy clusters. LEM will detect metals expelled by galaxies over their lifetime and accumulated in the Cosmic Web filaments, and directly measure the imprint of AGN feedback, stellar feedback, and merging over cosmic time in galaxies halos and cluster filaments.

LEM is the X-ray mission for the 2030s, and it should be the first of the exciting new line of astrophysics Probes. The primary science of LEM is of great interest to the entire astrophysical community, not just the X-ray community. Most of the LEM observing time will be dedicated to the guest observer program. The combination of wide field of view and 1-2 eV spectral resolution of the calorimeter offers a unique host of guest observer investigations covering every area of X-ray astrophysics, including studies of AGN, stellar astrophysics, exoplanets, Solar system physics, and time domain and multi-messenger astrophysics. LEM also has significant synergies with virtually every ground-based and space-based observatory planned to be in operation in the 2030s, including SKA, Roman Space Telescope, CMB-S4, and many others. Finally, the technologies required to implement this missionthe lightweight high-resolution X-ray optics and the large microcalorimeter arrays -are rapidly maturing. With small, well-defined, ongoing investments in technology development, LEM is already on the path to meet its key mission milestones. LEM offers the astrophysics community paradigmchanging science in a Probe package.

  5 keV 0.2-2 keV band 15" contains 90% PSF (10" HPD resolution) f o c a l le n g t h 4 m Imaging spectrometer (IFU): TES microcalorimeter array, cryocooled 2 eV resolution (central area 1 eV) 30'×30' field of view 118×118 15" pixels d=1.5m

Fig. 2 -

 2 Fig. 2 -LEM mission design (under development in collaboration with Lockheed Martin Corp.; a snapshot as of November 2022 is shown).

Fig. 8 -

 8 Fig. 8 -Simulated Milky Way-like galaxy at z = 0.01 (TNG50 18,19,14 of the IllustrisTNG simulation suite). Panels are 30 with 15 pixels (LEM field of view and angular resolution). The ellipse shows the size of the optical disk, seen edge-on. Left: Soft X-ray image with a CCD-like (70 eV) spectral resolution centered at the OVIII line, including the Milky Way foreground and CXB (the brightest sources accounting for half of CXB flux have been spatially removed). The bright foreground makes it impossible to detect extended CGM emission. Middle, Right: LEM 1 Ms images of the same galaxy, but in 2 eV wide bins (the LEM spectral resolution) centered on the OVIII and FeXVII CGM emission lines. The MW foreground is almost completely resolved away, taking advantage of the galaxy's known redshift; the use of the narrow bin also removes most of the residual power-law CXB flux. LEM unveils the rich spatial structure of the line-emitting CGM halo -including the analog of the Fermi/eROSITA bubbles produced by the central SMBH. Mapping different emission lines provides a measurement of the plasma temperature.

3 OVIII

 3 Su'face B'ig tness [log p oton s -1 cm -2 a'csec-2 ]

Fig. 10 -

 10 Fig. 10 -Left: Sample spectrum from one pixel in the LEM calorimeter from a 50 ks exposure of the Puppis A supernova remnant. Right: LEM image of Puppis A. The white square shows one LEM FOV and the circle shows the Athena XIFU FOV. LEM will map such large SNRs very efficiently, which is especially valuable for old, large Galactic SNRs. Each LEM pixel will provide an extremely line-rich spectrum, resolving multiple lines for each ion, allowing us to map chemical composition, diagnose nonequilibrium conditions in the plasma and measure velocities of ejecta. A CCD spectrum (Chandra ACIS) is shown for comparison; all spectral features are blended. A dispersive spectrometer (gratings) provides very limited spectral resolution for such extended sources because of spatial blending.

Fig. 14 -

 14 Fig.14 -LEM will revolutionize solar system studies. (a) XMM-Newton EPIC-pn spectrum of Jupiter's Northern X-ray aurora (130 ks). There are two different models that fit the data well, but each offers fundamentally different explanations for the processes that produce Jupiter's aurora. The first model is a solar wind charge exchange model (SWCX), implying entrance of solar wind into the Jovian system. The second model is an Iogenic model, which uses charge exchange on the S and O ions from Io's volcanos. (b,c) LEM 1 eV resolution spectra for each scenario. Counts per resolution element for a 20 ks exposure is shown; background is not included for clarity (it is negligible because Jupiter is under 1 in size). The most prominent emission lines are marked. The instrument may offer significant sensitivity at E < 0.1 keV with the same high spectral resolution, providing access to never-before-seen charge exchange lines at low energies (an option currently under study; gray area in the plots). LEM will easily distinguish between these models and address key questions on if and when Jupiter's magnetosphere is open to the solar wind -a point of longstanding debate in the heliophysics and planetary science communities.

Table 1

 1

	LEM	XRISM Resolve Athena XIFU *	HUBS