P Weltevrede

B B P Perera

L Levin

eds. S Sanidas

S Mereghetti

L Kuiper

A Tiengo

J Hessels

W Hermsen

K Stovall

A Possenti

J Rankin

P Esposito

R Turolla

D Mitra

G Wright

B Stappers

A Horneffer

S Oslowski

M Serylak

J.-M Griessmeier

M Rigoselli

X-rays from the mode-switching PSR B0943+10

Keywords: pulsars: general, X-rays: individual (PSR B0943+10), stars: neutron

New simultaneous X-ray and radio observations of the archetypal mode-switching pulsar PSR B0943+10 have been carried out with XMM-Newton and the LOFAR, LWA and Arecibo radio telescopes in November 2014. They allowed us to better constrain the X-ray spectral and variability properties of this pulsar and to detect, for the first time, the X-ray pulsations also during the X-ray-fainter mode. The combined timing and spectral analysis indicates that unpulsed non-thermal emission, likely of magnetospheric origin, and pulsed thermal emission from a small polar cap are present during both radio modes and vary in a correlated way.

Introduction

Until recently, the study of variability in rotation-powered pulsars was limited to the radio band, where changes in the emission properties related to phenomena occurring on very different time scales, from sub-pulses to long-term moding and intermittence, are routinely observed. On the contrary, rotation-powered neutron stars were generally believed to have a steady luminosity at X-and γ-ray energies, to the point that they are typically used as calibration sources. Some X-ray variability, in the form of short bursts associated with X-ray flux enhancements and spectral changes, has been observed in two allegedly rotation-powered pulsars (Gavriil et al. 2008;Archibald et al. 2016), and some 1 2 Sandro Mereghetti, et al. long term X-ray variations have also been observed in one member of the XDIN class (Hohle et al. 2012). However, all these variable phenomena are believed to result from magnetic activity and are not expected to occur in old radio pulsars with "canonical" magnetic fields (see, e.g., Mereghetti, Pons & Melatos 2015).

The situation has changed with the recent discovery of variations in the X-ray flux of PSR B0943+10 (Hermsen et al. 2013), one of the mode-switching pulsars best studied in the radio band. Here, we briefly review the most important results obtained for PSR B0943+10 and compare its properties with those of other mode-switching pulsars recently observed at X-ray energies.

Figure 1. X-ray spectra of the unpulsed (blue) and pulsed (red) emission from PSR B0943+10 in the Q-mode (left) and B-mode (right). In the top panels the unpulsed flux is fitted with a power law and the pulsed flux with a blackbody. The corresponding residuals are shown in the middle panels. The bottom panels, which show the residuals obtained by fitting the unpulsed flux with a blackbody and the pulsed flux with a power law, demonstrate that this fit is acceptable for the B-mode but not for the Q-mode.

X-ray results

PSR B0943+10 (P = 1.1 s, Ṗ = 3.5×10 -15 s s -1 ) is a relatively old pulsar (τ = 5 Myr) with a dipole magnetic field of 4×10 12 G. In the radio band, at irregular intervals of < ∼ few hours, it alternates between a radio bright (B-) mode, characterized by regularly drifting subpulses, and a quiescent (Q-) mode in which the radio emission is on average fainter and the subpulses have a chaotic pattern (Suleimanova & Izvekova 1984;Bilous 2017).

The faint X-ray emission from PSR B0943+10 (few 10 -15 erg cm -2 s -1 ) was discovered in 2003 (Zhang et al. 2005), but it could be studied in detail only when longer observations with simultaneous radio coverage were obtained in 2011. This made it possible to analyse separately the X-ray data of the two radio modes, leading to the discovery of X-ray variability anti-correlated with the radio flux (Hermsen et al. 2013). It was found that, during the radio-fainter Q-mode, the X-ray flux is brighter by a factor ∼2.5, shows X-ray pulsations, and has a blackbody plus power-law spectrum. In the B-mode, the spectrum was less constrained, being fit equally well by either a blackbody or a power law. We performed a new observing campaign in November 2014, using XMM-Newton and the LOFAR, LWA and Arecibo radiotelescopes (Mereghetti et al. 2016), obtaining Xray esposures (∼120 ks in Q-and ∼175 ks B-mode) longer by factors of 2.4 and 3.5 than those of the previous data, respectively. This allowed us to better constrain the spectral properties of PSR B0943+10 and to detect its X-ray pulsations also during the B-mode. The new data show that the phase-averaged spectrum is well fit by the sum of a power law with photon index ∼2.4 and a blackbody in both radio modes. The blackbody temperatures in the two modes are kT Q = 0.27±0.04 keV and kT B = 0.24±0.03 keV and the emitting radii are of the order of ∼30 m (for d = 890 pc).

By applying a maximum likelihood spectral analysis which used also the timing information of the detected counts, we derived the spectra of the pulsed and unpulsed emission. As shown in Fig. 1 (left), we confirmed that in the Q-mode the pulsed flux is thermal while the unpulsed flux is non-thermal. The blackbody describing the pulsed emission and the power law describing the unpulsed emission are consistent with the two components used to fit the total spectrum of the Q-mode.

The situation is less well constrained for the B-mode: for both the pulsed and unpulsed emission, it is impossible to distinguish between a power law and a blackbody, since they give similarly good fits (Fig. 1,right). However, it is reasonable to assume that the same two components seen in the Q-mode are present, although with a reduced flux, also during the B-mode. In other words, we attribute the pulsations to the thermal component in both modes. When the pulsar changes from the Q to the B-mode, the thermal flux decreases by a factor ∼3 and the non-thermal one decreases by a factor ∼2.

Discussion

The results obtained in the 2014 campaign indicate that the X-ray properties of PSR B0943+10 in the two radio modes are not as different as it was previously thought: a blackbody plus power law spectrum and X-ray pulsations (which can be attributed to the thermal component) are present in both modes, although with different fluxes. Thus, it is not necessary to invoke global magnetospheric changes capable of entirely suppressing one of the spectral components (Hermsen et al. 2013;Mereghetti et al. 2013) when the pulsar switches to its X-ray fainter mode. The small emitting area inferred from the blackbody fits is broadly consistent with the polar cap size expected in the partially screened gap model, which is smaller than that of a pure dipole owing to multipolar field components (Gil et al. 2007).

The mechanism causing the mode-switching in radio pulsars is still unknown and the study of X-ray variability might help to understand the origin of this phenomenon. Indeed, the interesting results obtained for PSR B0943+10 prompted X-ray observations of other mode-switching pulsars. Unfortunately, these old and not very energetic pulsars are faint in X-rays. Up to now, XMM-Newton data have been obtained for PSR B1822-09 (Hermsen et al. 2017) and PSR B0823+26 (Hermsen et al. 2018). Their properties are compared with those of PSR B0943+10 in Table 1.

PSR B1822-09, younger and more energetic than PSR B0943+10, shows a main radio pulse and a radio interpulse separated by ∼ 0.5 in phase and, contrary to PSR B0943+10, is believed to be a nearly orthogonal rotator. Its Q-and a B-mode radio states last only a few minutes and no evidence for X-ray variations was found. Its X-ray spectrum is fit by the sum of two blackbodies with kT 1 = 0.08 keV, kT 2 = 0.19 keV and emitting radii R 1 = 2 km, R 2 = 100 m. Both the hotter and the cooler blackbody components seem to contribute to the X-ray pulsations, which are present in both radio modes. The lack of X-ray variability could be due to the short duration of the radio modes. This could arise, e.g., if the timescale for the X-ray variation is longer than the mode duration. The timing parameters of PSR B0823+26 are very similar to those of PSR B0943+10. It also shows a bright and a quiet radio mode (Sobey et al. 2015), but, contrary to PSR B0943+10, the X-ray flux is higher during the B-mode.

Altough the global picture emerging from these data is still puzzling, the heterogeneous pattern of X-ray/radio variations seen in these few objects, anticipates that future, more sensitive observations will reveal a rich phenomenology that can potentially provide interesting clues on the mode-switching mechanism, and more in general, on the emission processes of rotation-powered pulsars.

Table 1 .

 1 Comparison of three mode-switching pulsars observed in X-rays

		PSR B0943+10	PSR B0823+26	PSR B1822-09
	P [s]	1.1	0.53	0.77
	Ṗ [s s -1 ]	3.5 × 10 -15	1.7 × 10 -15	5.3 × 10 -14
	τ [Myr]	5	5	0.2
	B [G]	2 × 10 12	9.6 × 10 11	6.5 × 10 12
	Ėrot [erg s -1 ]	10 32	4.5 × 10 32	4.6 × 10 33
	Distance [kpc]	0.89	0.31	0.26
	Likely geometry	∼aligned	∼orthogonal	∼orthogonal
	Mode duration	∼hours	∼hours (also nulling)	few minutes
	Radio interpulse	No	Yes	Yes (in Q-mode)
	Radio pre/postcursor precursor in Q-mode postcursor in B-mode	in B-mode
	Drifting subpulses	in B-mode	Yes	No
	X-ray flux	brighter in Q-mode	brighter in B-mode	constant
	LX [erg s -1 ]	∼ 2 × 10 29	∼ 2 × 10 28	∼ 3 × 10 29
	Total X-ray spectrum BB+PL (or BB+BB) BB+BB (or BB+PL)	BB+BB
	Spectral-timing	BB pulsed	favors BB+BB	both BB pulsed
	analysis	PL unpulsed

References

Mereghetti et al.

2016Hermsen et al. 2018Hermsen et al. 2017


Ke Fang
email: kefang@umd.edu

Jaime Álvarez-Muñiz

Rafael Alves Batista

Mauricio Bustamante

Washington Carvalho

Didier Charrier

Ismaël Cognard

Sijbrand De Jong

Krijn D De Vries

Chad Finley

Quanbu Gou

Junhua Gu

Claire Guépin

Jordan Hanson

Hongbo Hu

Kumiko Kotera

Sandra Le Coz

Yi Mao

Olivier Martineau-Huynh

Clementina Medina

Miguel Mostafa

Fabrice Mottez

Kohta Murase

Valentin Niess

Foteini Oikonomou

Frank Schröder

Cyril Tasse

Charles Timmermans

Nicolas Renault-Tinacci

Matías Tueros

Xiang-Ping Wu

Philippe Zarka

Andreas Zech

Yi Zhang

Qian Zheng

Anne Zilles

The Giant Radio Array for Neutrino Detection (GRAND): Present and Perspectives

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The Giant Radio Array for Neutrino Detection (GRAND) aims at detecting ultra-high energy extraterrestrial neutrinos via the extensive air showers induced by the decay of tau leptons created in the interaction of neutrinos under the Earth's surface. Consisting of an array of ∼ 10 5 radio antennas deployed over ∼ 2 × 10 5 km 2 , GRAND plans to reach, for the first time, an all-flavor sensitivity of ∼ 1.5 × 10 -10 GeV cm -2 s -1 sr -1 above 5 × 10 17 eV and a sub-degree angular resolution, beyond the reach of other planned detectors. We describe here preliminary designs and simulation results, plans for the ongoing, staged approach to the construction of GRAND, and the rich research program made possible by GRAND's design sensitivity and angular resolution.

35th International Cosmic Ray Conference -ICRC2017 10-20 July, 2017 Bexco, Busan, Korea

Introduction

The workings of the nature's highest-energy accelerators are a half-century-old mystery [START_REF] Kotera | The Astrophysics of Ultrahigh Energy Cosmic Rays[END_REF]. It is difficult to use ultra-high energy cosmic rays (UHECR, with E > 10 18 eV) to pinpoint a source as these charged particles can be deflected by up to tens of degrees by extragalactic and Galactic magnetic fields. On the contrary, secondary neutrinos, produced when cosmic rays interact with matter inside a source or with the cosmic microwave background (CMB) during intergalactic propagation, serve as direct probes, as neutrinos are not deflected and barely interact over cosmological distances. Two steps need to be achieved toward finding sources of UHE neutrinos:

1. Detecting an UHE neutrino. Depending on the chemical composition and maximum energy of UHECRs, the flux of cosmogenic neutrino background is expected at a level of ∼ 10 -9 -10 -8 GeV cm -2 s -1 sr -1 per flavor at EeV energies. A small number of diffuse neutrinos found in this step, however, will not reveal an UHE neutrino point source.

2. Identifying individual sources. This step requires the detection of a sizable number of neutrinos to allow point sources to stand out of a diffuse background [START_REF] Ahlers | Pinpointing Extragalactic Neutrino Sources in Light of Recent IceCube Observations[END_REF][START_REF] Fang | Identifying ultrahigh-energy cosmic-ray accelerators with future ultrahigh-energy neutrino detectors[END_REF]. For instance, if sources follow a uniform number density of 10 -7 Mpc -3 , a 5σ detection of point sources requires measuring ∼ 200 neutrino events with a sub-degree angular resolution [START_REF] Fang | Identifying ultrahigh-energy cosmic-ray accelerators with future ultrahigh-energy neutrino detectors[END_REF].

Astrophysical tau neutrinos (ν τ ) can be detected through extensive air showers (EAS) induced by tau (τ) decays in the atmosphere [START_REF] Fargion | Horizontal Tau air showers from mountains in deep vally :Traces of Ultrahigh neutrino tau[END_REF][START_REF] Bertou | Tau neutrinos in the Auger Observatory: a new window to UHECR sources[END_REF]. Specifically, a cosmic ν τ can produce tau leptons under the Earth surface via charge-current interactions. The short-lived τ decays in the atmosphere after traveling tens of kilometers and generates an EAS that emits coherent electromagnetic emissions up to frequencies of hundreds of MHz. Such inclined showers have been measured and confirmed by the Auger Engineering Radio Array to have a radio footprint covering several km 2 [START_REF] Kambeitz | Measurement of horizontal air showers with the auger engineering radio array[END_REF]. With practical advantages such as low unit cost, easiness of deployment and maintenance, radio antennas present appealing characteristics for the detection of very inclined showers [START_REF] Schröder | Radio detection of cosmic-ray air showers and high-energy neutrinos[END_REF].

Consisting of ∼ 10 5 radio antennas covering 200, 000 km 2 in a mountainous area, the Giant Radio Array for Neutrino Detection (GRAND) will detect air showers generated by Earth-skimming UHE tau neutrinos with unprecedented sensitivity. Mountain ranges with low electromagnetic background noise are preferable sites, as mountains provide both additional interaction targets and a slanted surface that is more favorable to detect inclined showers compared to flat ground. The antennas are foreseen to operate in the 60-200 MHz band to avoid the short-wave background at lower frequencies. The highest frequencies enable detection of the Cherenkov ring associated with the air shower [START_REF] Alvarez-Muñiz | Coherent radiation from extensive air showers in the ultrahigh frequency band[END_REF], which can be used as an unambiguous signature to reject background.

Detector performance 2.1 Sensitivity

We present a preliminary evaluation of the GRAND sensitivity based on a simulation of the response of 90,000 antennas deployed on a square layout of 60,000 km 2 in a remote mountainous area (Tianshan mountains, XinJiang province, China). The simulations are composed of three All-flavor (º

+ ¹ º) E 2 º ©(E º )
[GeV cm low

E max CR ◀ Fe rich, low E max CR ▶ G R A N D 2 0 0 k ( p r o j . 3 y r ) G R A N D 1 0 k ( p r o j . 3 y r ) º e : º ¹ : º ¿ = 1:1:1 A R I A N N A ( p r o j . 3 y r )
Figure 1: Three-year differential sensitivity of a 10,000 km 2 simulated setup (red thin curve) and of the projected GRAND array (red thick curve). The grey shaded region brackets fluxes of cosmogenic neutrinos from models with different cosmic ray compositions, spectra, and source evolution histories that provide reasonable fits to the Auger UHECR spectrum [START_REF] Kotera | Cosmogenic neutrinos: parameter space and detectabilty from PeV to ZeV[END_REF]. The grey dashed and dotted lines indicate the predicted cosmogenic neutrino flux in a pessimistic scenario where the bulk of UHECRs are iron nuclei. High-energy neutrinos measured by IceCube [START_REF]The IceCube Neutrino Observatory -Contributions to ICRC 2015 Part II: Atmospheric and Astrophysical Diffuse Neutrino Searches of All Flavors[END_REF], UHE neutrino sensitivities of IceCube [START_REF] Aartsen | Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube[END_REF], Auger [START_REF] Aab | Improved limit to the diffuse flux of ultrahigh energy neutrinos from the pierre auger observatory[END_REF], ARA [START_REF]Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole[END_REF] and ARIANA [START_REF] Barwick | A first search for cosmogenic neutrinos with the ARIANNA Hexagonal Radio Array[END_REF] are also shown for reference.

parts: I) ν τ s interact with the rock, producing τ leptons, II) τs propagate and decay, and III) The τ decay products initiate an EAS that triggers the antennas to be read-out.

In part I, standard rock with a density of 2.65 g cm -3 is assumed down to the sea level, and the Earth core is modeled following the Preliminary Reference Earth Model [START_REF] Dziewonski | Preliminary reference Earth model[END_REF]. We simulate the deep inelastic scattering of neutrinos with Pythia6.4, using the CTEQ5d probability distribution functions (PDF) combined with the cross sections from [START_REF] Gandhi | Neutrino interactions at ultrahigh energies[END_REF].

In part II, we simulate the propagation of the produced τs with GEANT4.9, using parameterizations of PDFs of the τ path length and proper time. We simulate τ decays with the TAUOLA package [START_REF] Jadach | Tauola -a library of monte carlo programs to simulate decays of polarized Ï Ď leptons[END_REF]. For the above two parts, 1D tracking of a primary ν τ is performed down to the converted τ decay.

We have refined our previous calculation of part III [START_REF] Martineau-Huynh | Contributions to the 34th International Cosmic Ray Conference (ICRC 2015)[END_REF] with numerical simulations. We first simulate a set of ν τ showers with ZHaireS [START_REF] Alvarez-Muñiz | Monte Carlo simulations of radio pulses in atmospheric showers using ZHAireS[END_REF] at different energies, and determine a conical volume inside which the electric field is above the expected detection threshold of the GRAND antennas. Based on a parametrization of the shape (including angle and height) of the detection cones as a function of energy, we compute the expected shape and position of cones for each neutrino-initiated EAS in our simulation (so far in the 30-80 MHz band). We select the antennas located inside the cones, taking into account signal shadowing by mountains. When the selection consists of 8 or more neighboring antennas in direct view of the τ decay point, we count the primary ν τ as detected.

Our analysis finds that about 50% of detectable neutrino interactions take place inside mountains rather than in the Earth crust. Moreover, some parts of the array provide 2-3 times higher detection rates compared to the average rate of the entire simulated area. These "hotspots" are typically located in a low electromagnetic noise background regions and have desirable geometric features (for e.g., large mountains, slopes facing another mountain range at distances of 30-80 km). An envisioned GRAND setup is composed of smaller sub-arrays of a few 10,000 km 2 each, deployed solely on such favorable sites.

Figure 1 presents the differential 90%-CL sensitivity of a GRAND simulation for a 10,000 km 2 hotspot and the projected full GRAND array limit after a 3-year observation. An all-flavor 90% CL integral limit of 2 × 10 -10 GeVcm -2 s -1 sr -1 is derived for an unbroken E -2 power law spectrum in an aggressive scenario (corresponding to a detection threshold of 30 µV m -1 ; 3.9 × 10 -9 GeVcm -2 s -1 sr -1 in a conservative scenario with a detection threshold of 100 µV m -1 ). For comparison, a prediction of the cosmogenic neutrino flux is shown as the grey shaded region. The flux mostly depends on three properties of UHECRs: spectral index, chemical composition, and maximum energy E max . The shaded area brackets a wide range of parameters that reflect different compositions, source evolution and Galactic to extragalactic transition models, all required to fit the UHECR spectrum measured by Auger [START_REF] Aab | Contributions to the 34th International Cosmic Ray Conference (ICRC 2015)[END_REF]. Specifically, the upper bound corresponds to an UHECR dip model with a pure proton composition and source evolution following the star formation rate (SFR). The lower bound corresponds to a mixed composition with 30% iron nuclei, SFR evolution and E max = 10 19 Z eV. The grey dotted and dashed lines show the most pessimistic scenarios, where UHECRs are mostly composed of iron nuclei which barely produce neutrinos in a photo-disintegration process. We note that the best-fit models, found by a combined fit to the Auger spectrum, X max and the standard derivation of X max measurements [START_REF] Aab | Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory[END_REF], correspond to a cosmogenic neutrino flux level of ∼ 10 -9 GeV cm -2 s -1 sr -1 . For comparison, we show UHE neutrino upper limits of current experiments IceCube [START_REF] Aartsen | Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube[END_REF] and Auger [START_REF] Aab | Improved limit to the diffuse flux of ultrahigh energy neutrinos from the pierre auger observatory[END_REF] (the sensitivity curve of ANITA-II [START_REF] Gorham | Observational constraints on the ultrahigh energy cosmic neutrino flux from the second flight of the ANITA experiment[END_REF] is not shown as it would be too high for the plot). GRAND10k will be as sensitive as the proposed ARA [START_REF]Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole[END_REF] and ARIANA [START_REF] Barwick | A first search for cosmogenic neutrinos with the ARIANNA Hexagonal Radio Array[END_REF] experiments at 1 EeV. The full GRAND array will nominally detect even the lowest expected fluxes of cosmogenic neutrinos.

Following [START_REF] Ardouin | First detection of extensive air showers by the TREND self-triggering radio experiment[END_REF], the angular resolution of the radio array is estimated by the resolution of the time difference between pulses in different antennas. Using a 3 ns precision on the antenna trigger timing, GRAND can reach an angular resolution as accurate as 0.1 • .

The instantaneous field of view covers a band with zenith angles between 86 • and 93 • in the simulation (the zenith angle-dependent effective area is shown in Figure 1 of [START_REF] Martineau-Huynh | Contributions to the 34th International Cosmic Ray Conference (ICRC 2015)[END_REF]). The integrated exposure over 3 years of GRAND operation will cover a large fraction of the sky as shown in Figure 2. With stacking searches, GRAND will be sensitive to transient sources and flaring events with a duration longer than ∼ 1 day. Several interesting UHECR features, such as the Telescope Array hotspot and the Auger hotspot, are located close to GRAND's best sensitivity region.

Background rejection

Radio background from high-energy particles other than cosmic neutrinos should be manageable. The atmospheric neutrino background is mostly negligible above 10 16 eV. EAS generated by muon decay are expected at a rate of a few per century as muons have a significantly longer life time than taus. Rejecting trajectories far from the horizon should effectively suppress the EAS generated by cosmic rays.

Background event rates associated to terrestrial sources such as human activities and thunderstorms are difficult to evaluate. The background measurement of the Tianshan Radio Experiment for Neutrino Detection (TREND) [START_REF] Ardouin | First detection of extensive air showers by the TREND self-triggering radio experiment[END_REF] provides a conservative estimate. TREND consisted of 50 self-triggered antennas deployed in a populated valley of the Tianshan mountains, with an antenna design and sensitivity similar to what is foreseen for GRAND. The observed rate of events triggering six selected TREND antennas separated by ∼800 m over a sample period of 120 live days was found to be around 1 per day, among which two-thirds arrived as bursts of events mostly due to air plane activities. An extrapolation from this measurement leads to an expected background rate of 1 Hz for GRAND for a trigger algorithm based on coincident pulses on neighboring antennas and a rejection of event bursts.

Specific amplitude patterns on the ground (such as beamed emission along the shower axis and signal enhancement along the Cherenkov ring [START_REF] Alvarez-Muñiz | Coherent radiation from extensive air showers in the ultrahigh frequency band[END_REF]), and polarization of signals [START_REF] Aab | Probing the radio emission from air showers with polarization measurements[END_REF] are strong indicators of neutrino-initiated EAS. We are investigating the possibilities of using these signatures as efficient background discrimination tools through both simulations and experiments.

Science case

In addition to a guaranteed detection of cosmogenic neutrinos, GRAND will allow a differentiation of the source types by measuring neutrinos produced inside sources. In Figure 3, we show fluxes of neutrinos from four typical classes of astrophysical sources that can explain UHECR measurements, including galaxy clusters with central sources [START_REF] Murase | Cosmic Rays above the Second Knee from Clusters of Galaxies and Associated High-Energy Neutrino Emission[END_REF][START_REF] Fang | Linking High-Energy Cosmic Particles by Black Hole Jets Embedded in Large-Scale Structures[END_REF], fast-spinning newborn pulsars [START_REF] Fang | Testing the newborn pulsar origin of ultrahigh energy cosmic rays with EeV neutrinos[END_REF], active galactic nuclei [START_REF] Murase | Active Galactic Nuclei as High-Energy Neutrino Sources[END_REF], and afterglows of gamma-ray bursts [START_REF] Murase | High energy neutrino early afterglows from gamma-ray bursts revisited[END_REF]. The GRAND sensitivity is sufficient to observe neutrinos from sources in all cases.

A 5σ identification of individual point sources out of a diffuse background requires ∼ 100 -1000 events with a sub-degree angular resolution for sources that have a local density of 10 -9 -10 -7 Mpc -3 [START_REF] Fang | Identifying ultrahigh-energy cosmic-ray accelerators with future ultrahigh-energy neutrino detectors[END_REF]. Assuming a neutrino flux of 10 -8 GeV cm -2 s -1 sr -1 , the GRAND neutrino sen- [START_REF] Murase | Cosmic Rays above the Second Knee from Clusters of Galaxies and Associated High-Energy Neutrino Emission[END_REF][START_REF] Fang | Linking High-Energy Cosmic Particles by Black Hole Jets Embedded in Large-Scale Structures[END_REF], fast-spinning newborn pulsars [START_REF] Fang | Testing the newborn pulsar origin of ultrahigh energy cosmic rays with EeV neutrinos[END_REF], active galactic nuclei [START_REF] Murase | Active Galactic Nuclei as High-Energy Neutrino Sources[END_REF], and afterglows of gamma-ray bursts [START_REF] Murase | High energy neutrino early afterglows from gamma-ray bursts revisited[END_REF]. High-energy neutrino observed by IceCube [START_REF]The IceCube Neutrino Observatory -Contributions to ICRC 2015 Part II: Atmospheric and Astrophysical Diffuse Neutrino Searches of All Flavors[END_REF], UHE neutrino sensitivities of IceCube [START_REF] Aartsen | Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube[END_REF] and Auger [START_REF] Aab | Improved limit to the diffuse flux of ultrahigh energy neutrinos from the pierre auger observatory[END_REF] are shown for reference. sitivity corresponds to a detection of ∼ 100 events after three years of observation. The unprecedented sensitivity of GRAND is crucial for a direct identification of UHE neutrino point sources.

GRAND will observe UHECRs with an effective area that is at least an order of magnitude larger than Auger. The high statistics will resolve any small-scale anisotropies and features near the end of the cosmic ray spectrum. GRAND will also reach an UHE photon sensitivity exceeding that of current experiments. Future simulation studies will be dedicated to performance of energy and X max reconstructions.

GRAND will contribute in a unique way to the measurement of fast radio bursts (FRB) and giant radio pulses by collecting unprecedented statistics at low frequencies [START_REF] Zarka | Observations of Fast Radio Bursts and perspectives at low frequencies[END_REF]. Simulations are being performed to quantify the detection power with incoherent sum of FRB spectra computed at each antenna. The possibility of GRAND's contribution to additional science topics, such as the study of the epoch of reionization, is also being investigated.

Development plan

The GRAND project aims at building a next-generation neutrino telescope composed of a radio antenna array deployed over 200,000 km 2 . Preliminary simulations indicate that 5% of the array deployed in favorable sites will improve the sensitivity over that of current-generation telescopes by an order of magnitude. The full array will offer a sensitivity that ensures a detection of cosmogenic neutrinos in the most pessimistic scenario, and an identification of individual point sources in an optimistic scenario. GRAND will also be a powerful instrument for UHECR observations with high statistics. Simulation and experimental work is ongoing on technological development and background rejection strategies.

The GRAND development plan consists of several steps. Presently, deployment of GRANDproto [START_REF] Gou | The GRANDproto35 experiment (ICRC 2017 proceedings)[END_REF], which is a hybrid detector composed of 35 3-arm antennas and 24 scintillators, is underway at the site of the TREND experiment, in the Tianshan mountains. The GRANDproto project will demonstrate that EAS can be detected with an autonomous radio array with both high efficiency and high purity. Following this step, a dedicated setup with a size of 300 km 2 will be deployed. This array will establish the autonomous radio detection of very inclined EAS with excellent background rejection, as a validation for the future GRAND layout.

Figure 2 :

 2 Figure 2: Three-year integrated exposure (in units of cm 2 s) of GRAND10k at 3 EeV in Galactic coordinates.

Figure 3 :

 3 Figure3: Three-year differential sensitivity of the GRAND10k and GRAND full array comparing to fluxes of neutrinos from interactions inside astrophysical sources. Four source classes that can explain UHECR measurements are shown: galaxy clusters with central sources[START_REF] Murase | Cosmic Rays above the Second Knee from Clusters of Galaxies and Associated High-Energy Neutrino Emission[END_REF][START_REF] Fang | Linking High-Energy Cosmic Particles by Black Hole Jets Embedded in Large-Scale Structures[END_REF], fast-spinning newborn pulsars[START_REF] Fang | Testing the newborn pulsar origin of ultrahigh energy cosmic rays with EeV neutrinos[END_REF], active galactic nuclei[START_REF] Murase | Active Galactic Nuclei as High-Energy Neutrino Sources[END_REF], and afterglows of gamma-ray bursts[START_REF] Murase | High energy neutrino early afterglows from gamma-ray bursts revisited[END_REF]. High-energy neutrino observed by IceCube[START_REF]The IceCube Neutrino Observatory -Contributions to ICRC 2015 Part II: Atmospheric and Astrophysical Diffuse Neutrino Searches of All Flavors[END_REF], UHE neutrino sensitivities of IceCube[START_REF] Aartsen | Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube[END_REF] and Auger[START_REF] Aab | Improved limit to the diffuse flux of ultrahigh energy neutrinos from the pierre auger observatory[END_REF] are shown for reference.

Acknowledgments

The GRAND and GRANDproto project are supported by the France China Particle Physics Laboratory, the Institut Lagrange de Paris, the APACHE grant (ANR-16-CE31-0001) of the French Agence Nationale de la Recherche, the Natural Science Foundation of China (Nos.11135010, 11105156, 11375209 and 11405180), the Chinese Ministry of Science and Technology, the Youth Innovation Promotion Association of Chinese Academy of Sciences, and the São Paulo Research Foundation FAPESP (grant 2015/15735-1).


M Bailes

E Barr

N D R Bhat

J Brink

S Buchner

M Burgay

F Camilo

D J Champion

J Hessels

G H Janssen

A Jameson

S Johnston

A Karastergiou

R Karuppusamy

V Kaspi

M J Keith

M Kramer

M A Mclaughlin

K Moodley

S Oslowski

A Possenti

S M Ransom

F A Rasio

J Sievers

M Serylak

B W Stappers

I H Stairs

G Theureau Stu

W Van Straten

P Weltevrede

N Wex

MeerTime -the MeerKAT Key Science Program on Pulsar Timing

The MeerKAT telescope represents an outstanding opportunity for radio pulsar timing science with its unique combination of a large collecting area and aperture efficiency (effective area ∼7500 m 2 ), system temperature (T < 20K), high slew speeds (1-2 deg/s), large bandwidths (770 MHz at 20cm wavelengths), southern hemisphere location (latitude ∼ -30 • ) and ability to form up to four sub-arrays. The MeerTime project is a five-year program on the MeerKAT array by an international consortium that will regularly time over 1000 radio pulsars to perform tests of relativistic gravity, search for the gravitational wave signature induced by supermassive black hole binaries in the timing residuals of millisecond pulsars, explore the interiors of neutron stars through a pulsar glitch monitoring programme, explore the origin and evolution of binary pulsars, monitor the swarms of pulsars that inhabit globular clusters and monitor radio magnetars. MeerTime will complement the TRAPUM project and time pulsars TRAPUM discovers in surveys of the galactic plane, globular clusters and the galactic centre. In addition to these primary programmes, over 1000 pulsars will have their arrival times monitored and the data made immediately public. The MeerTime pulsar backend comprises two server-class machines each of which possess four Graphics Processing Units. Up to four pulsars can be coherently dedispersed simultaneously up to dispersion measures of over 1000 pc cm -3 . All data will be provided in psrfits format. The MeerTime backend will be capable of producing coherently dedispersed filterbank data for timing multiple pulsars in the cores of globular clusters that is useful for pulsar searches of tied array beams. The first real-time pulsar profiles have been obtained as part of the MeerKAT commissioning process, and useful scientific data will start to come online through 2017. All MeerTime data will ultimately be made available for public use, and any published results will include the arrival times and profiles used in the results.

MeerKAT Science: On the Pathway to the SKA 25-27 May, 2016 Stellenbosch, South Africa

Introduction

Fifty years after their discovery [START_REF] Hewish | Observation of a Rapidly Pulsating Radio Source[END_REF], radio pulsars remain a fertile area of astronomical research. In the standard model [START_REF] Lorimer | Handbook of Pulsar Astronomy[END_REF], radio pulsars are highly-magnetised neutron stars with magnetic field strengths of 10 8 -10 14 G, some 10 km in radius and with masses of between ∼ 1.2-2 M ⊙ . Magnetic dipole radiation removes energy from the pulsar, and a light-house beam of radio emission sweeps through space and results in a train of regular pulses that can be routinely detected by radio telescopes of sufficient aperture. The energy loss results in a slow spin-down of the pulsar, and by accurately measuring pulse times of arrival at the observatory, a large number of observable parameters become available to pulsar astronomers.

The MeerKAT Key Science Project on Pulsar Timing originated in 2010 when a strong science case was put to an international time allocation committee based upon the then draft sensitivity for the MeerKAT telescope of 220 m 2 /K and bandwidth of 850 MHz. The project received a draft allocation of 7800h and the Key Science Project is now referred to as "MeerTime".

Background

Up until 1974 all known pulsars were solitary objects with spin periods P > 33 ms with limited astrophysical applications. Two major discoveries rocked the pulsar community when improved pulsar survey instrumentation enabled the discovery of faster pulsars. In 1974 the binary PSR B1913+16 was found using the Arecibo telescope [START_REF] Hulse | Discovery of a pulsar in a binary system[END_REF]. It was a 59 ms pulsar orbiting another neutron star every 7.75 hours. Its timing ultimately led to the Nobel prize in physics for verifying the existence of gravitational waves. Then in 1981 the celebrated 1.55 millisecond pulsar, PSR B1937+21 was discovered [START_REF] Backer | A millisecond pulsar[END_REF] . Today the pulsar catalogue [START_REF] Manchester | The Australia Telescope National Facility Pulsar Catalogue[END_REF] lists over 250 binary pulsars (almost 10% of the population) and 350 millisecond pulsars. These pulsars are ideal relativistic laboratories and probes of stellar evolution. Their inherent stability give them applications in the search for gravitational waves. The majority of known pulsars reside in the southern hemisphere and 95% of all pulsars are accessible to MeerKAT with its Southern location and 75 degree elevation limit.

MeerTime Science Themes

MeerTime has three high-priority science themes:

• Relativistic and Binary Pulsars: Pulsars that possess compact binary companions in tight relativistic orbits allow tests of General Relativity and its alternatives that are impossible to perform elsewhere. Binary pulsars are also fossil records of stellar evolution and accretion physics. In addition General Relativity can be used to determine the masses of millisecond pulsars that inform us about the equation of state of nuclear matter.

• Millisecond Pulsar Timing and Gravitational Wave Detection: Millisecond pulsars can be used to search for the signatures of gravitational waves generated by supermassive black hole binaries and/or cosmic strings in the early Universe.

• Globular Cluster Pulsar Timing: Globular clusters are breeding grounds for millisecond pulsars and via partner exchange in their dense cores produce exotic systems that permit experiments otherwise impossible to perform that inform us about General Relativity and also the equation of state of nuclear matter via neutron star spins and masses.

MeerKAT's design (comprising of 64 independent elements) means that it is possible to form up to four sub-arrays, allowing for some novel approaches to pulsar timing. This permits sub-array modes that enable the simultaneous production of pulse arrival times from four different pulsars in different regions of the sky.

The MeerKAT system design originally forecast a sensitivity of some 220 m 2 /K, but it now appears that the aperture efficiency and system temperature may enable a remarkable sensitivity in excess of 400 m 2 /K, virtually quadrupling its efficiency for pulsar timing. For reference the other Southern hemisphere radio telescopes are all less than 70 m 2 /K, although the new ultra-wideband receiver (700 MHz-4 GHz) for the Parkes 64 m radio telescope still make it a highly-effective pulsar telescope, particularly for dispersion measure determinations.

Further classes of pulsar were part of the original science case:

• Young, Glitching and Highly-magnetised pulsars: These objects allow probes of neutron star interiors where physics is at its most extreme.

• The Thousand Pulsar Array: Most pulsars are not timed regularly because of the time commitment required on otherwise over-subscribed instruments. It can be demonstrated that MeerKAT can time over 1000 pulsars a day, and that for a modest increase in MeerTime's time request a legacy dataset could be created that monitored most of the known pulsar population. This would answer questions like, do "normal" pulsars also glitch? Are the period derivatives stable? What are the proper motions of pulsars? Are pulsar pulse profiles really stable? If we monitor a thousand years of pulsar rotation history every year, what unexpected phenomena will we see?

• RRATs (Rotating RAdio Transients): In 2010 the newly discovered population of "RRATs" was a topic of much interest. These objects appeared to be pulsars that only pulsed sporadically. Their birthrate appeared extremely high, yet little was known about the population. This population was the final science theme of the original proposal.

The construction of the MeerKAT telescope is a great opportunity to advance radio pulsar science. The vast majority of pulsars (> 95%) are visible from the MeerKAT and its combination of a large collecting area, cool receivers, rapid slew speed and wide bandwidth makes it a powerful facility for the great plethora of pulsar science.

In this paper we describe the MeerTime pulsar timing experiment. In section 2 we take the telescope's technical specifications and determine the speed with which it can time the radio pulsar population before discussing the pulsar backend subsystem in section 3. Finally, in section 4, we discuss the main science cases for the telescope.

The MeerKAT as a pulsar telescope

Although individual pulses from a pulsar are irregular in their amplitude, shape and polarisation, their mean profile is often remarkably stable. This fact permits extremely accurate pulse arrival times (ToAs) to be derived by integrating a pulsar's pulses for many 100s-thousands of periods and comparing the time-tagged profile against a standard. The error in a pulse ToA σ depends upon the time derivative of the profile and the signal-to-noise ratio snr. A good rule of thumb is that the error in a ToA σ can be approximated by

σ ∼ w/(2 × snr) (2.1)
where w is the half width of the profile. In reality, this relation is only an approximation and assumes that every pulse is identical. A more correct approach would use the derivative of the pulse profile, and make some allowance for the fact that not every pulse is identical.

The radiometer equation for a radio telescope describes the snr expected from a source of flux density S as

snr = SG (BN p t) T rec + T sky P -w w (2.2)
where G is the gain of the telescope in K Jy -1 , B is the processed bandwidth (in Hz), N p is the number of orthogonal polarisations (maximum of 2), t is the integration time in seconds, T rec and T sky are the receiver and system temperature respectively (in K), P is the pulse period and w is the width of the pulse.

The first determination of the MeerKAT's technical specifications are extremely encouraging. The dishes are an offset gregorian and have an effective diameter of 13.965 m and an aperture efficiency of 0.71-0.81. If we adopt 0.76 as a mean efficiency this gives each dish a gain G 0 of

G 0 = 0.76π (13.965/2) 2 2k = 0.042 K Jy -1 (2.3)
where k is Boltzmann's constant. If added coherently, the 64-dish instrument has a total gain of

G = 64G 0 = 2.7 K Jy -1 .
In array release 3, the telescope will possess a total usable bandwidth of 770 MHz of bandwidth in two orthogonal polarisations. Early tests of the receivers suggests that T rec will be less than 20K, possibly as low as ∼15K.

If MeerKAT observes the 2 mJy millisecond pulsar PSR J1909-3744, which has a pulse width of 40 µs for just 15m, the expected snr ∼2700 (pessimistically assuming T rec = 20K) and the ToA error just ∼8 ns (in reality there is likely to be pulse jitter that limits such accuracies being obtained).

Rather than slewing all over the sky looking for MSPs that are currently experiencing scintillation maxima with cumbersome 64-100m-class telescopes, the MeerKAT will be able to split off a sub-array and quickly determine which MSPs are in a "bright state" to further improve observing efficiency with its impressive slew speeds of 1-2 deg/s.

The MeerKAT pulsar backend system

In the mid-late 1990s pulsar astronomers recognised the increases in timing precision that could be achieved by coherently dedispersing the voltages induced in the receiver of a single dish by the deconvolution of the signal with an appropriate filter. Coherent dedispersion used to represent a major challenge to pulsar astronomy, requiring custom boards to digitise the voltages at the requisite rate (1/B) where B is the bandwidth of the backend and capture them in clusters of computers. It was originally thought that a computing cluster might require a large cluster of workstations to achieve the necessary signal processing to coherently dedisperse the radio frequency signals from MeerKAT, but since 2010 there have been several important developments in the creation of coherent dedispersion pulsar processors or "backends".

• The community has developed open-source software that has been largely adopted providing increased rigour and testing. Most of the pulsar community now uses and contributes to the psrchive suite of tools to process and manipulate folded pulsar profile data [START_REF] Hotan | PSRCHIVE and PSRFITS: An Open Approach to Radio Pulsar Data Storage and Analysis[END_REF] .

• A software library (psrdada) that both captures data from UDP streams and moves it to computer's memory (RAM) is available from sourceforge.com that permits the quick development of capture engines. A library (dspsr) that transforms voltage data to coherentlydedispersed folded profiles is also available as an open-source software library [START_REF] Van Straten | DSPSR: Digital Signal Processing Software for Pulsar Astronomy[END_REF] These open-source libraries greatly facilitate the creation of pulsar processing backends.

• Large-N Fourier transforms used to be extremely expensive to compute, requiring vast arrays of CPUs and multiplexing of the data because of the time taken to compute each one. The technological breakthrough of the graphics processing unit (GPU) in consumer games cards has reduced the cost of the necessary computations by more than an order of magnitude since 2010. Almost all pulsar processors now utilise GPUs as the Fourier transform engine.

• Back in 2010, 10-Gb ethernet represented the peak performance one could aspire to when trying to perform lossless data capture. This meant that with 8-bit sampling, several O(4) computers would be required to capture a single stream from MeerKAT's tied array beams. Now it is possible to have a single machine capture over 54 Gb/s of data without loss using dual 40 Gb Network Interface Cards (NICs).

• The use of interferometers for pulsar timing has been steadily increasing. The Westerbork array has been joined by the LEAP project that ties the major European VLBI telescopes into a single coherent beam, and the Very Large Array, LOFAR and the UTMOST project in Australia are all examples of interferometers observing pulsars.

MeerKAT's pulsar timing hardware comprises two machines that were used to prototype the pulsar processor for the SKA at Swinburne University of Technology. These machines can coherently dedisperse two parallel 850 MHz dual-polarisation streams simultaneously and one is currently at the MeerKAT site.

Progress towards regular pulsar timing is continuing. A major breakthrough occurred in Q2 2016 when the first pulsar profile from the Vela pulsar (PSR J0835-4510) was produced from a single beam with data written to disk. This observation confirmed that MeerKAT single dishes were producing very high quality data with system temperatures near the published specification (<20K). Shortly afterwards, the first tied array beam was created on the bright millisecond pulsar PSR J0437-4715 and processed in software from voltages recorded to disk. Although satellite transmissions are present in the band, over 75% of it is "useable". In October the first real-time pulsar profiles were produced validating that the pulsar processor can capture data at the requisite rate and process them. Currently the polyphase filterbank data cannot be correctly coherently dedispersed because of digitally-induced artefacts in the polyphase filterbank frequency channels.

Coherent dedispersion will be required to achieve the ultimate timing precision promised by the system and should be possible when the beam-former and polyphase filterbanks are moved to the SKARAB boards in mid 2017.

3.1 The Max Planck Institute 1.7-3.5 GHz receiver upgrade High dispersion pulsars suffer from multi-path propagation effects that limit their use in pulsar timing experiments. The effects scale as the wavelength to the power 4.4, and hence some pulsars demand the use of the highest frequencies to perform the best science. Much beyond 3 GHz there are relatively few pulsars that still retain enough flux to make this worthwhile, as many pulsars have steep spectral indices (usually between -1 and -3).

Professor Michael Kramer (MPIfR) has been leading a project to upgrade MeerKAT to operate beyond 1.7 GHz with the deployment of 64×1.7-3.5 GHz receivers. This project will enable both wider bandwidths and reduced scattering on many of our higher dispersion measure pulsars. It was recently demonstrated [START_REF] Shannon | Gravitational waves from binary supermassive black holes missing in pulsar observations[END_REF] that some pulsars (e.g. PSR J1909-3744) produce arrival times of exceptional accuracy at these frequencies that are critical for the direct detection of gravitational waves.

By sub-arraying, it will be possible for us to observe pulsars all the way from 0.9 to 3.5 GHz, albeit with reduced gain because the telescope will have to be "sub-arrayed". This will provide an exceptional lever arm to define the variable dispersion measures (a pulsar timing pollutant). When the UHF receivers are in place this can extend all the way from 0.5-3.5 GHz.

The MeerTime Science Case 4.1 Millisecond Pulsar Timing

In MeerTime's original 2010 science case it was anticipated that MeerKAT could independently detect a gravitational wave background after 5 years if the dimensional amplitude exceeded 2 × 10 -15 based upon limits in vogue at the time [START_REF] Verbiest | Timing stability of millisecond pulsars and prospects for gravitational-wave detection[END_REF]. The current best millisecond pulsar for timing accuracy (PSR J1909-3744) already suggests that an amplitude of this magnitude is ruled out [START_REF] Shannon | Gravitational waves from binary supermassive black holes missing in pulsar observations[END_REF] and that gravitational wave detection from pulsars will require international coordination and cooperation. MeerKAT can dramatically increase the pool of MSPs from which a gravitational wave background or individual binaries can be searched for with its unique combination of sensitivity, geographical location, ability to sub-array and the speed at which it can traverse the sky. Reardon et al. (2016) [START_REF] Reardon | Timing analysis for 20 millisecond pulsars in the Parkes Pulsar Timing Array[END_REF] recently reported on the timing of 20 MSPs from Parkes and based upon his residuals and the relative sensitivity of the two telescopes, the MeerKAT should increase the number of pulsars with sub-us residuals from 5 to 16 objects if sensitivity was the only improvement factor, but MeerKAT's ability to subarray and seek out those MSPs that are experiencing scintillation maxima gives us hope that it can do much better than a simple scaling of sensitivities might suggest.

Several studies [START_REF] Liu | Prospects for high-precision pulsar timing[END_REF][START_REF] Osłowski | High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing[END_REF][START_REF] Osłowski | Improving the precision of pulsar timing through polarization statistics[END_REF][START_REF] Shannon | Limitations in timing precision due to single-pulse shape variability in millisecond pulsars[END_REF] have demonstrated that pulsar timing precision is ultimately limited by the stochastic wideband impulse-modulated self-noise (SWIMS, also known as jitter and single-pulse variability) that is intrinsic to the pulsar emission. Consequently, optimal use of full array sensitivity requires the ability to divide it into sub-arrays; furthermore, because it is imperative to account for this noise in high-precision pulsar timing data analysis, the instrumentation for pulsar timing must be updated to produce additional statistical information. It has been demonstrated [START_REF] Osłowski | High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing[END_REF][START_REF] Osłowski | Improving the precision of pulsar timing through polarization statistics[END_REF] that arrival time estimation bias can be mitigated by measuring the periodic correlations of the Stokes parameters and Shannon et al. ( 2014) [START_REF] Shannon | Limitations in timing precision due to single-pulse shape variability in millisecond pulsars[END_REF] have described how jitter noise can be characterised and incorporated in estimates of arrival time precision. Ongoing research by our team will combine these approaches using generalised least squares estimation to simultaneously reduce bias, accurately estimate uncertainty, and increase the sensitivity of experiments such as pulsar timing arrays.

Gravitational waves are just one of the exciting science cases to be realised by timing an array of millisecond pulsars. Timing residuals also contain a wealth of information about the parameters of the parent binary, useful for studies of stellar evolution, the IGM and even our own planetary ephemerides. Since our original proposal the Fermi satellite has unveiled a tremendous population of millisecond pulsars (now up to 350) waiting for an instrument capable of producing accurate arrival times to capitalise on them. MeerKAT is such an instrument.

Relativistic and Binary Pulsar Timing

Pulsars are remarkable laboratories for the study of gravitation. In both the highly relativistic interior and the vicinity of a pulsar (and its binary companion, in case of double neutron-star systems or potential pulsar-black hole system) space-time may significantly deviate from the predictions of General Relativity (GR) [START_REF] Damour | Tensor-scalar gravity and binary-pulsar experiments[END_REF]. Pulsar timing therefore provides a unique tool for probing gravity in the strong field regime, enabling high-precision tests of GR or other theories of gravity. Double-neutron-star systems such as the Double Pulsar [START_REF] Kramer | Tests of General Relativity from Timing the Double Pulsar[END_REF] provide unrivalled probes for testing most aspects of GR. Binary pulsars with a white dwarf companion and hence large mass dipole can set interesting constraints on alternative theories that predict, for instance, the existence of gravitational dipole radiation [START_REF] Antoniadis | A Massive Pulsar in a Compact Relativistic Binary[END_REF]. Resolving tight binary orbits to investigate effects such as the Shapiro delay requires short-spaced observations with high sensitivity. Meanwhile, identifying the weak signatures of subtle relativistic effects needs long-term monitoring with good cadence. MeerKAT's excellent sensitivity (surpassing even our high expectations in 2010) and good frequency coverage will make it the premier telescope for studying Southern-sky pulsars. This will not only improve existing GR tests but will allow us to measure new effects to probe new physics. This is best demonstrated with the unique Double Pulsar, where the precision of tests of gravity will go beyond the current best weak-field tests in the solar system. As has been shown (Kehl 2015, Masters thesis, University of Bonn), we expect to measure the moment-of-inertia of the J0737-3039A in the Double Pulsar for the first time, providing a handle on the equation-of-state of super-dense matter.

With the sensitivity provided by MeerTime, we will also determine masses for both pulsars and their companions. These can be used to test theories of binary evolution [START_REF] Tauris | Formation of millisecond pulsars. I. Evolution of low-mass X-ray binaries with Porb> 2 days[END_REF] and to investigate the distribution of neutron-star masses. In particular, the discovery of massive neutron stars [START_REF] Demorest | A two-solar-mass neutron star measured using Shapiro delay[END_REF][START_REF] Antoniadis | A Massive Pulsar in a Compact Relativistic Binary[END_REF] suggests that high-mass population of neutron stars exists, even possibly resulting from birth [START_REF] Tauris | Formation of millisecond pulsars with CO white dwarf companions -I. PSR J1614-2230: evidence for a neutron star born massive[END_REF][START_REF] Antoniadis | An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic field[END_REF]. As mass statistics improve, we will get closer to identifying the maximum mass possible for a neutron star, itself a constraint on the equation of state.

MeerTime will furthermore provide astrometry (distances, proper motions and hence velocities) for millisecond and binary pulsars, allowing us to infer their birth velocities and constrain asymmetric supernova kicks, particularly in double-neutron-star systems [START_REF] Stairs | The formation of the double pulsar PSR J0737-3039A/B[END_REF].

Globular Cluster Pulsar Timing

Globular clusters are treasure troves of exotic millisecond pulsars, for a recent scientific overview see reference [START_REF] Hessels | Pulsars in Globular Clusters with the SKA[END_REF]. The cores of globular clusters have stellar densities 10 3 -10 4 times greater than in the Galactic field; this promotes the formation of binary systems in which a neutron star can be recycled to millisecond rotation rates via the transfer of matter and angular momentum from a Roche-lobe-filling companion. This extreme stellar density can also lead to exchange interactions, which create bizarre pulsar systems, unlike anything so-far seen in the Galactic field. Currently there are 146 pulsars known in 28 globular clusters -including the fastest-spinning pulsar known [START_REF] Hessels | A Radio Pulsar Spinning at 716 Hz[END_REF], exotic eccentric binaries suitable for neutron star mass measurements [START_REF] Ransom | Twenty-One Millisecond Pulsars in Terzan 5 Using the Green Bank Telescope[END_REF] and a unique triple system with a planetary companion [START_REF] Sigurdsson | A Young White Dwarf Companion to Pulsar B1620-26: Evidence for Early Planet Formation[END_REF].. Literally all of these 146 pulsars are visible to MeerKAT and MeerTime plans a sensitive, and comprehensive globular cluster pulsar timing campaign. Combining MeerKAT timing data with up to three decades of archival measurements from GBT, Arecibo, and Parkes, MeerTime will probe the spin, orbital, and proper motions of these pulsars in unprecedented detail and measure previously inaccessible system parameters that will allow us to probe accretion physics, dense matter, gravitational theories, and the evolution and properties of the clusters themselves in exquisite detail. From a practical point of view, timing globular cluster pulsars also provides a great efficency because in some cases (e.g. M28, 47 Tucanae and Terzan 5) dozens of millisecond pulsars can be observed simultaneously. MeerKAT will revolutionize searches of southern globular clusters via TRAPUM and the long-term timing of these and existing pulsars via MeerTime. To achieve sensitivity to 10 µJy pulsars in these clusters, MeerTime plans typically 1-hr timing sessions for these clusters.

The Thousand Pulsar Array

Not all pulsars pulse regularly. Since the connection between pulsar radio emission and timing properties in the so-called intermittent pulsars was first pointed out [START_REF] Kramer | A Periodically Active Pulsar Giving Insight into Magnetospheric Physics[END_REF] more pulsars exhibiting these properties have been discovered, as well as, e.g., multi-wavelength moding pulsars where the pulse profile changes significantly between two states having different radio and X-ray properties [START_REF] Hermsen | Synchronous X-ray and Radio Mode Switches: A Rapid Global Transformation of the Pulsar Magnetosphere[END_REF]. Secular variations in previously thought stable pulse profiles were also seen for a large sample of ordinary pulsars [START_REF] Lyne | Switched Magnetospheric Regulation of Pulsar Spin-Down[END_REF] with a clear connection between timing irregularities/noise and emission properties/profiles. In this context, progress is being made on understanding pulsar interiors and how/if the observed pulse profile variations could be ascribed to long term free precession. Finally, the LOFAR telescope is revealing the complex imprints of the ISM on pulsar data [START_REF] Archibald | Millisecond Pulsar Scintillation Studies with LOFAR: Initial Results[END_REF]. The Thousand Pulsar Timing Array will provide an opportunity to study the breadth of pulsar phenomenology. That will result in new breakthroughs relating to the interiors, to the magnetosphere and to the environment of pulsars, to ISM and Galactic magnetic field studies, and will lead to improved pulsar timing. MeerKAT is an exceptionally sensitive telescope for this purpose. We estimate that, with the current full MeerKAT sensitivity, we can obtain a high signal to noise (>20) profile of a pulsar with a duty cycle of 5% and a flux density of 0.30 mJy (of which more than 1000 are visible from Meerkat) within a minute of observation. From a sensitivity perspective therefore, it is easy to accommodate regular observations of 1000 pulsars within the requested 16 h per observing epoch.

The Timing of Young and Energetic Pulsars

Young and energetic pulsars are often associated with supernova remnants (SNRs), pulsar wind nebulae (PWNe), and/or high-energy X-ray/gamma-ray point sources [START_REF] Roberts | +3651: A Young Radio Pulsar Coincident with an Unidentified EGRET γ-Ray Source[END_REF]. Timing of young pulsars provides their spin-down rate, which then sets the energy budget powering the PWN and other high-energy emission [START_REF] Camilo | Heartbeat of the Mouse: A Young Radio Pulsar Associated with the Axisymmetric Nebula G359.23-0[END_REF]. Long-term timing provides the proper motion, which is a key ingredient for deciphering the morphology of SNRs and PWNe (including bow shocks). In some cases, it is also possible to measure the neutron star's braking index, revealing the multi-pole nature of the magnetic field and perhaps also its evolution in time [START_REF] Livingstone | 21 Years of Timing PSR B1509-58[END_REF]. Many young pulsars also show glitches, which probe the neutron star interior in a unique way [START_REF] Espinoza | A study of 315 glitches in the rotation of 102 pulsars[END_REF]. With MeerKAT's large sensitivity, it may also be possible to probe the nebulae surrounding some young pulsars through precise characterization of their scattering/dispersion/rotation measure with time or other propagation effects [START_REF] Lyne | Pulsar reflections within the Crab Nebula[END_REF].

Magnetar Pulsar Timing

To date, 4 of 23 known magnetars have been detected at radio wavelengths [START_REF] Camilo | Transient pulsed radio emission from a magnetar[END_REF]. They share some characteristics distinct from those of the normal pulsar population, such as flat radio spectra. They are also extremely variable; 2 of the 4 are currently no longer radio emitters [START_REF] Camilo | Radio Disappearance of the Magnetar XTE J1810-197 and Continued X-ray Timing[END_REF] although this could change, and new ones could be discovered. Through frequent timing observations of the active radio magnetars we aim to obtain a continuous record of their torque, which illuminates the continued release of magnetic energy in the neutron star. Our broader aims are to develop a better understanding of the dynamical behaviour of magnetar magnetospheres, and to establish the conditions under which radio emission takes place therein. Very frequent observations are needed because the torque on a magnetar can change by 10% on weekly timescales.

The Rotating Radio Transients

These objects are no longer being observed as part of our timing project. Their poor positions are not well-mapped to the small tied array beam of MeerTime and they don't appear to be anything except an extension of nulling pulsars.

Observing strategy and data products

To detect the stochastic gravitational wave background or individual sources requires the highest precision possible. This calls for regular observing cadence, preferably of order 20 times per year. These observations will make MeerKAT a critical contributor to the broader International Pulsar Timing Array (IPTA) effort. MeerKAT has a unique opportunity to contribute to the direct detection of gravitational waves. It will be the most sensitive telescope in the Southern hemisphere, and as mentioned before its ability to sub-array means it can employ novel techniques to fully exploit MSP scintillation. It will be possible for sub-arrays to be searching for MSPs to time that are at scintillation maxima, whilst the majority of the antennas are conducting routine timing.

The best observing strategy for binary and millisecond pulsars is to observe entire orbits where practical (this prevents unfortunate covariances between binary and other parameters) and to have occasional "campaigns" when the cusps in Shapiro delays are visible. MeerTime's immediate focus is on systems in which gravitational wave emission is implied by monitoring their orbital period derivatives and on those binaries where pulsar masses can be achieved with MeerKAT's increased sensitivity over existing facilities. Currently, orbital decays have been detected for 8 binary pulsar systems while detections for a further 4-6 (some unpublished pulsars) systems can be expected with MeerKAT. In addition, mass measurements of binary neutron stars should be possible for at least 10 more systems [START_REF] Özel | Masses, Radii, and the Equation of State of Neutron Stars[END_REF]. Full orbits on these systems are possible with MeerKAT but not telescopes like Arecibo or FAST.

Most pulsars in globular clusters are very stable timers. As one of the aimed measurables is the proper motion of clusters, many sessions throughout the year are necessary. Since globular cluster MSPs are in average fainter sources than the Galactic ones, long integrations are a necessity.

To enable science it is essential that the pulse profiles produced by the backend are created in a format accessible via public domain packages such as psrchive. The pulsar processor will create FITS-format folded archives and coherent filterbanks at a nominal dispersion measure, also in FITS. The arrival times will be in reference to the observatory clock, which will ultimately be referenced to UTC-NIST.

The dimension of the scientific data products is manageable (60 TB over five years). Software pipelines will be made open access and available on the project website, as will clock correction files.

Once the data are calibrated they can be fit with the standard pulsar timing packages tempo, tempo2 and PINT.

It is rare that a single epoch of pulsar timing results in a publishable outcome. Instead pulsar parameters slowly become scientifically interesting as the time span increases. Pulsar positions rapidly increase their precision once a year of data is obtained, proper motions usually take a few years to manifest themselves in timing residuals and the discovery of the gravitational wave background is likely to take a decade or so. MeerTime's data release policy is as follows: Data from the 1000 pulsar array will be available immediately. The MSP, relativistic binary and globular pulsar data will be released 18 months after they are recorded. We intend to publish all times of arrival on something like a annual basis in scheduled "data releases". All publications will provide the arrival times and raw data from the observations that led to any claimed results.

Acknowledgements M Bailes acknowledges the Australian Research Council grants CE170100004 (OzGrav) and FL150100148. A. P. and M. Bu. acknowledge the support of the Italian Ministry of Foreign Affairs and International Cooperation, Directorate General for the Country Promotion (Bilateral Grant Agreement ZA14GR02 -Mapping the Universe on the Pathway to SKA). This research is supported by the Max-Planck-Society and by the ERC Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes" (Grant 610058). Pulsar research at UBC is supported by an NSERC Discovery Grant and by the Canadian Institute for Advanced Research. This research is supported by NSF IRES Award 7706217.