domain: chim spi

text:
Weng Fu

Julius Motuzas

David Wang

Christelle Yacou

Anne Julbe

James Vaughan
email: james.vaughan@uq.edu.au

João C Diniz Da Costa

Salt Storage and Induced Crystallisation in Porous Asymmetric Inorganic Membranes

Keywords: inorganic membranes, pore storage, pore size control, pervaporation, crystallisation

Processing brines to recover strategic mineral salts using evaporation ponds requires large surface areas and are slow, even in arid climates. Here we show a novel membrane macropore storage mechanism that induces fast salt crystallisation in mesoporous top-layers in inorganic asymmetric membranes, stemming from 789 million nucleation points per metre square of surface area. During membrane pervaporation, dissolved salts are retained mainly in the macropores of the substrate which subsequently provide ideal conditions for crystal nucleation and growth on the membrane surface upon drying. This novel pore storage mechanism is attained owing to the solution flow modulation of the mesoporous titania and gamma-alumina layers that is counterbalanced by the flow of water during pervaporation. Therefore, pore size control is imperative to avoid flooding in the macroporous substrate. This work further shows the fundamental properties of the salt storage mechanism described by a single salt production coefficient, and a global salt production coefficient for metal chloride salts. This technology could potentially be considered for unlocking and process strategic global minerals from brines.

Introduction

The demand for strategic minerals to meet our contemporary societies need for technological advancement is expanding. Meanwhile, conventional orebody grades are in decline, thus mineral brine mines will be an increasingly important source of salts and metals in the future. A clear example is lithium, a relatively rare metal required for the production of highly efficient batteries for portable electronic devices [START_REF] Fang | Progress and challenges of flexible lithium ion batteries[END_REF], plug-in electrical cars [START_REF] Nemeth | Lithium titanate oxide battery cells for highpower automotive applications -Electro-thermal properties, aging behavior and cost considerations[END_REF] and increasingly for large scale power storage [START_REF] Arbabzadeh | The role of energy storage in deep decarbonization of electricity production[END_REF].

The estimated global lithium brine reserves of 21.6 Mt is 5.6 times larger than non-brine lithium reserves [START_REF] Kesler | Global lithium resources: Relative importance of pegmatite, brine and other deposits[END_REF]. Even larger reserves are the oceans of the world which are last frontier in mineral processing, holding vast quantities of dissolved metal ions of commercial interest, such as lithium (2.3x10 5 Mt), nickel [START_REF] Favero | A binary nickel diimine-MCM-41 supported catalyst and its application in ethylene polymerization[END_REF]600 Mt), magnesium (1.3x10 12 Mt), gold (14.5 Mt), potassium (5.1x10 8 Mt), manganese (500 Mt) and uranium (4,300 Mt) among many others [START_REF] Bardi | Extracting Minerals from Seawater: An Energy Analysis[END_REF]. These elements find broad application in wealth generation (gold), and advanced materials such as magnesium for medical applications [START_REF] Grober | Magnesium in prevention and therapy[END_REF], manganese doped semiconductor quantum dots [START_REF] Mahamuni | Photoluminescence properties of manganese-doped zinc selenide quantum dots[END_REF] and nickel as catalyst [START_REF] Favero | A binary nickel diimine-MCM-41 supported catalyst and its application in ethylene polymerization[END_REF]. In addition, potassium plays a critical role in food production [START_REF] Zörb | Potassium in agriculture -Status and perspectives[END_REF] and uranium is a key element to nuclear power production in a decarbonised economy [START_REF] Pacala | Stabilization wedges: solving the climate problem for the next 50 years with current technologies[END_REF]. However, extracting valuable elements from mineral brines is time and energy intensive and expensive, particularly in the case of diluted metal ions in oceans [START_REF] Sholl | Seven chemical separations to change the world[END_REF].

Mineral brines are traditionally processed in evaporation ponds and industrial crystallisers.

Essentially these processes evaporate water from a brine solution, leading to supersaturation and crystallisation. The crystallisation of evaporation ponds is very slow and highly dependable upon the weather conditions whilst industrial crystallisers deliver fast crystallisation rates though are energy intensive. Since the 1980s, polymeric hydrophobic membranes have become attractive to promote crystal nucleation and growth kinetics [START_REF] Drioli | Progress in membrane crystallisation[END_REF], using membrane distillation, also known as pervaporation. This process is known as membrane crystallisation, with proven applications for crystallising LiCl [START_REF] Quist-Jensen | A study of membrane distillation and crystallization for lithium recovery from high-concentrated aqueous solutions[END_REF], NaCl [START_REF] Ji | Membrane distillationcrystallization of seawater reverse osmosis brines[END_REF][START_REF] Chen | Optimization of operating conditions for a continuous membrane distillation crystallization process with zero salty water discharge[END_REF], Na2CO3 [START_REF] Ye | Membrane crystallization of sodium carbonate for carbon dioxide recovery: effect of impurities on the crystal morphology[END_REF] and MgSO4 [START_REF] Mariah | Membrane distillation of concentrated brines -role of water activities in the evaluation of driving force[END_REF]. Owing to the fast pervaporation of solvent via the hydrophobic polymeric membrane, ideal conditions are created at surface of the membrane at the feed side, leading to supersaturation, nucleation and crystallisation. Subsequently, two further downstream processes are required where (i) crystals are separated from the solution by filters and then (ii) dried in furnaces.

Although the state of the art in polymeric membrane crystallisation is well known, inorganic ceramic membranes remained absent [START_REF] Chabanon | Membranes and crystallization processes: state of the art and prospects[END_REF] in this technological development until recently by the pioneering work of Motuzas and co-workers [START_REF] Motuzas | Novel Inorganic Membrane for the Percrystallization of Mineral, Food and Pharmaceutical Compounds[END_REF] who prepared porous hydrophobic carbon membranes to crystallise NaCl from mineral brine solutions. This process is known as membrane percrystallisation as solutes (i.e., crystals) and solvents were separated at a single step by a wet thin film under vacuum on the permeate side of the membrane. Subsequently, it was demonstrated that the process conditions (i.e., flow rates, temperature and vacuum pressure) in membrane percrystallisation altered the crystal NaCl particle size [START_REF] Madsen | Fine control of NaCl crystal size and particle size in percrystallisation by tuning the morphology of carbonised sucrose membranes[END_REF], and the crystal phase of NiSO4 [START_REF] Madsen | Novel membrane percrystallisation process for nickel sulphate production[END_REF]. The use of hydrophilic inorganic membranes has eluded the field of membrane crystallisation to date. Therefore, this work demonstrates for the first time that hydrophilic inorganic membranes can be used efficiently for crystallising mineral brines by a novel pore storage mechanism under pervaporation. It is shown that salt storage in the macropores of a ceramic membrane substrate under pervaporation can be attained only in the presence of a mesoporous inorganic top-layer with controlled pore sizes. Further, this work demonstrates that fundamental mass transfer properties in the pore storage are a function of feed concentrations for a single salt, and as a function of the molecular weight for chloride salts.

Experimental

Sol-gel synthesis and xerogel characterisation

An aqueous aluminium oxide (20%) sol with an average colloidal particle size of 0.05 μm (pseudoboehmite from Alfa Aesar) was used as received for coating gamma alumina interlayers. The top layers were synthesised using a titania sol-gel method. Briefly, titanium (IV) isopropoxide (TIP, TiC12H28O4, 98%, Sigma-Aldrich) was added drop-wise into a solution of double-distilled water and hydrochloric acid under vigorous stirring. Then the mixture was maintained in a water bath at 30 °C for 3 hours. The molar ratio of the final sol was TIP:HCl:H2O = 1:1:22. The sol was then dried in an oven at 60 °C for 3 h.

The preparation of xerogel (dried gel) was initially carried out by heat treatment at 150 °C to ensure the consolidation of the titania matrix. Subsequently, the titania xerogel was calcined at 350 °C to condense the matrix into a meso-structure. Heat treatment and calcination were carried out at a heating/cooling rate of 1 °C min -1 and a dwell time of 1 h. Nitrogen sorption of titania xerogel samples was conducted on a Micromeritics TriStar 3020 analyser after degassing under vacuum on a Micromeritics VacPrep061 at 200 °C for a minimum of 12 h. The specific surface areas were calculated from the adsorption isotherms via the multi-points BET model at relative pressures of p/po=0.05-0.3. The pore size distribution was determined using Barrett-Joyner-Halenda (BJH) model, from the desorption branch of the isotherms of the mesoporous titania xerogel samples.

Membrane preparation and characterisation

Commercial porous alpha alumina (α-Al2O3) tubes (OD = 1 cm, ID = 0.5 cm, L = 5.5 cm, Ceramic Oxide Fabricator Pty Ltd.) were used as membrane substrates. The substrates were pre-calcined at 1000 °C for 8 h with a ramp rate of 5 °C min -1 to improve the mechanical strength and to remove any organic impurities. Each coated layer was deposited separately by a dip-coating method at immersion and withdrawal rate of 5 cm min -1 and a dwell time of 1 min. Both ends of the substrate were closed prior to dip-coating to allow layers to be coated on the outer shell of the substrate only. After each coated layer, the membranes were calcined in air at 500 °C (gamma alumina interlayers), 150 and 350 °C (titania top layers) at a ramping/cooling rate of 1 °C min -1 and a dwell time of 1 h. The latter followed a procedure proposed by Yacou et al. [START_REF] Yacou | Mesoporous TiO2 based membranes for water desalination and brine processing[END_REF] in order to control the mesopore size of titania thin films. The final prepared membranes consisted of two gamma alumina interlayers and two titania top layers. Microstructural investigation of the membrane layers was carried out via a JEOL field emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) operating at 5 kV at a working distance of 10 mm.

Membrane crystallisation

Synthetic feed solutions were prepared by mixing chloride based salts (mols) with the required amount of double-distilled water (Lw) represented by mols Lw -1 . The salt concentration of the feed solution was determined via standard calibration curves (conductivity versus concentration) using a conductivity meter (labCHEM CP). The membrane outer shell was exposed to a pre-determined salty feed solution at atmospheric pressure and room temperature (~20 °C). The inner shell of the membrane (i.e., permeate side) was evacuated to 18 mbar using a vacuum pump, thus allowing water to evaporate and salt to remain in the pores of the membrane. The effective length of the membrane for pervaporation testing was 4.5 cm, as 0.5 cm at both ends of the substrate were covered with silicon tubes. The membranes were tested at time intervals from 10 to 30 min. The water vapour in the permeate stream was condensed and collected in a liquid nitrogen cold trap located downstream. The collected water was analysed using the conductivity meter to determine if any salt ions were entrained in the water vapour during water evaporation.

After testing, the membranes were transferred to a pre-heated oven at 60 °C for 8 h to ensure a complete crystallization of salts on the TiO2 top layer took place on the outer shell of the membrane.

The pervaporation schematic and salt growth on the membrane surface is depicted in the Appendix Fig. A1. The relative humidity of the oven chamber (RH = 67.3%) was controlled using NaNO3 brine solution. Salt crystals on the surface of the membranes were observed with JEOL field emission scanning electron microscope (FE-SEM, JEOL JSM-7001F) operating at 5 kV at a working distance of 10 mm.

The crystallised salts were removed from the outer surface of the membrane and the collected mass of salt crystals was measured in a high-precision laboratory scale. This allowed the production (J) of crystallised salts to be determined as follows,

𝐽 = 𝑚 𝑠𝑎𝑙𝑡 𝑡𝐴 (𝐸𝑞. 1)
where msalt is the mass of crystallised salt collected at a pre-determined membrane pervaporation time interval t (min), and A (m 2 ) is membrane surface area. The membrane pervaporation followed by oven drying were replicated at least five times for all tested conditions. A Panasonic microwave (Genius 1100 MW) with variable power was also used for drying and crystallising chloride base salts.

The membranes were microwave-dried at 900 W for 2 min. The mass and flux of crystallised chloride base salts after microwave-dried were calculated using a similar procedure as described above for the oven-dried membranes.

Results and discussion

Induced Crystallisation and Membrane Characteristics

Here we demonstrate for the first time that hydrophilic inorganic ceramic membranes are very efficient in processing mineral brines by a novel mechanism of induced salt crystallisation. We observed that under pervaporation of an aqueous NaCl solution, salts concentrated in the pores whilst water permeated through the membrane. Upon drying the membrane, capillary forces in the pores induced evaporation of water, leading to super-saturation, nucleation and crystal growth on the surface of the membrane. the interlayer provides a smooth surface to form a defect free thin top layer [START_REF] Lenaars | The preparation and characterisation of alumina membranes with ultrafine pores. Part 2 -The formation of supported membranes[END_REF]. A fundamental aspect of this induced pore crystallisation is the precise control of the sol-gel method to tailor the inorganic hydrophilic membrane pore diameter (dp). This was achieved by an acid sol-gel synthesis of the titania top layers followed by a two-step calcination to 150 and 350 °C and using a low ramping rate of 1 °C min -1 . A number of hydrophilic membranes were tested under pervaporation of aqueous NaCl solutions. The pore size of macroporous alpha alumina (dp > 50 nm) were too large, resulting in pore wetting and full permeation of the solution. In the case of microporous silica membranes (dp < 0.5 nm or 5 Å) [START_REF] Lin | Cobalt oxide silica membranes for desalination[END_REF], only water diffused through the membranes as the permeation of salt ions were hindered. Pore storage was observed for mesoporous titania top-layers (3 < dp < 5 nm) (Fig. 2 inset) and gamma alumina (3.7 < dp < 8.9 nm) [START_REF] Lenaars | The preparation and characterisation of alumina membranes with ultrafine pores. Part 1 -Microstructural investigation on non-supported membranes[END_REF] interlayers.

Membrane Performance

Fig. 3 shows the stable cyclic performance of the titania/alumina membrane over 10 consecutive cycles of pervaporation and NaCl crystallisation. Increasing the NaCl feed concentration resulted in an increase in the production of NaCl crystals, suggesting an increase in the storage rate of NaCl in the pores of the membranes. Notably, the titania/alumina membrane was able to process a wide range of feed solution concentrations ranging from seawater (0.6 mols Lw -1 NaCl) to hypersaline brines (5.4 mols Lw -1 NaCl).

Novel pore storage mechanism

Salt crystallisation on the membrane surface proceeded via pervaporation as schematically depicted in Fig. 5. Upon contact of the dry membrane with an aqueous salt solution, capillary forces take effect immediately [START_REF] Kornev | Spontaneous penetration of liquids into capillaries and porous membranes revisited[END_REF]. These forces induced salt solution penetration into the pores due to the hydrophilic nature of titania and gamma-alumina layers. Based on the results in Fig. 3 for a feed solution of 5.4 mols Lw -1 , the NaCl production average is 144.5 g m -2 . This is equivalent to a NaCl crust of NaCl with a thickness of 67 μm. The latter is 35 times thicker than the average thickness of the mesoporous titania and gamma-alumina layers of 1.95 μm. Therefore, the mass of salt that causes induced crystallisation can be directly attributed to salt storage in the membrane substrate that accounts to 99.96% of the overall membrane thickness. Theoretical calculations in the Appendix Table A1 clearly confirms that this is the case as there is sufficient pore volume in the membrane substrate to accommodate the NaCl mass that induced crystallisation. It is noteworthy the role played by the mesoporous titania and gamma-alumina layers in avoiding membrane flooding under pervaporation. In this case, the mesoporous layers modulated the flow of salt ion solution into the larger macropores of the substrate. This flow modulation benefited the salt storage mechanism by entraining salt ions in the macropores of the substrate whilst avoiding solution flooding in the permeate side.

In this process, menisci are formed in the macropores of the substrate close to the surface of the inner shell of the tubes under pervaporation, thus controlling the water evaporation via the permeate stream.

The water evaporation at the menisci is controlled by vapour-liquid equilibrium according to the Kelvin equation [START_REF] Lawson | Membrane distillation[END_REF]. The water pervaporated through the membrane was condensed in a liquid nitrogen cold trap and analyses indicated salt rejections in excess of 99%, confirming that salt ion solution was mainly retained in the macropores of the substrate of the membrane.

Upon drying the membrane, induced salt crystallisation on the surface of the mesoporous titania top layer took place. This process is directly controlled by capillary forces. As the macropores of the alpha-alumina substrate are interconnected to the mesopores of gamma-alumina interlayers and that to the mesopores of the titania layer, capillary pressure gradients force the salt ion solution stored in the substrate to permeate from the macropores to the mesopores. This is the case for two different saturated porous materials in hydraulic contact with each other and dried, the material with the largest pores starts to drain first [START_REF] Voronina | The influence of osmotic pressure on poulticing treatments for cultural heritage objects[END_REF]. Upon drying further, capillary forces bring the salt ion solution to the surface of the titania top layer where water evaporation increases rapidly. As a result, salt ions on the membrane surface reach supersaturation and lead to salt nucleation and salt growth induction as evidenced by Fig. 1.

Transport phenomena, salt production and storage coefficients

During the pervaporation stage, salt is being concentrated in the membrane pores (cp) as the process is driven by vacuum evaporation of water. A concentration gradient (c) is thus established between the solution in the membrane pores and the solution in the feed. Fig. 6 shows the continuous built up of NaCl in the membrane pores as a function of time until a maximum salt storage capacity is reached.

At this maximum capacity, the pores are fully saturated with salt solution and equilibrium is achieved between the salt concentration in the feed solution and that in the macropores. In addition, the salt production capacity also increased as a function of the NaCl feed concentration. Hence, the transport phenomenon in this membrane pore storage mechanism is associated with the salt concentration in the feed solution. The driving force for salt storage is at the highest at the beginning of pervaporation (t=0) and minimum at the maximum storage or saturation time (t=tsat). In other words, the membrane pore storage mechanism is characterised by a batch process of salt loading. Interestingly, the production of NaCl as a function of the feed concentration fitted well to a linear relationship (R 2 >0.99) as demonstrated in Fig. 7a. We define the slope of this linear relationship as a salt production coefficient (kt), which provides a fundamental parameter of this novel membrane pore storage process. Owing to the batch process characteristics, the salt production coefficients were plotted against time as shown in Fig. 7b, using data from Fig. A2 in the Appendix where all R 2 >0.99.

After 20 min, NaCl accumulation in the pores reached saturation level (ksat), suggesting an approach to equilibrium when the driving force is no longer significant. These results clearly indicate that the mass storage mechanism in the membrane pores of the hydrophilic inorganic membrane follows Eq.

1 for a given condition, where P is the membrane surface area specific salt production (mols m -2 ), t is the pervaporation time, k is the salt production coefficient (mols Lw mols -1 m -2 ) and cf is the feed salt concentration (mols Lw -1 ).

𝑃 𝑡 = 𝑘 𝑡 𝑐 𝑓 (𝐸𝑞. 2) Essentially salt solutions were stored in the membrane during pervaporation, so the pore storage coefficients are volumetric as a function of the membrane surface area and membrane thickness.

Hence, the mass storage coefficient (St) is the mass of salt mainly stored in the macropores of the substrate (mols m -2 m -1 ), per area (m 2 ) per membrane thickness l (m), which can be derived from Eq.

2 as follows:

𝑆 𝑡 = 𝑃 𝑡 𝑙 (𝐸𝑞. 4)
Likewise, the global salt storage coefficient GSt,x for a membrane thickness l (m) can be derived from Eq. 3. As the slopes of Zt,x and GSt,x remain the same irrespective of the thickness of the membrane, than both global salt production and global salt storage coefficients for chloride hydrates are the same as follows:

𝐺𝑆 𝑡,𝑥 = 𝑍 𝑡,𝑥 (𝐸𝑞. 5)

In principle, the capability of the membrane to store salt is a function of the membrane specific structure. Hence, salt production values increase by increasing the thickness of the substrate, or by increasing the porosity of the substrate provided that membrane flooding is avoided. However, if increased in salt production are proportional to the feed concentration and/or to the chloride salt tested, then the slopes may remain the same. This means that the coefficients kt and Zt may not be affected by the membrane's specific structure if salt production proportionality is maintained.

Conclusions

The induced crystallisation of mineral brines using hydrophilic inorganic membranes occurred via a novel mechanism of membrane pore salt storage during pervaporation. This phenomenon occurred due to the modulation of salt ion solution via the mesopores of titania and gamma-alumina layers, thus avoiding pore flooding in the substrate macropores. Fundamental properties of the membrane pore mass storage were established by determining salt production coefficient for single salts, and a global salt storage coefficient as a function of the molecular weight of chloride salts. These coefficients form the basis for the design of industrial plants for processing brines.

a membrane with 1 m 2 surface area with a solution in equilibrium with the feed temperature. The mass of salt (M) stored in the pores of the membrane is determined by M= pvol. Table A1 shows the NaCl mass stored mainly in the macropores of the substrates which are 99.96% of the total porosity.

The NaCl production results from Fig. 3 are also listed. These results show that theoretical mass of NaCl stored in the substrate is always higher than the mass produced per 1 m 2 of membrane surface area.

Fig. 1 displaysFig. 1 .Fig. 2 )

 112 Fig. 1. SEM image of NaCl crystal pyramid-like shapes on a membrane dried at 60 °C.

Fig. 2 .

 2 Fig. 2. SEM image of an asymmetric titania (top-layer) and gamma alumina (interlayer) where inset shows a pore size distribution based on Barrett, Joyner and Halenda (BJH) model.

Fig. 3 Fig. 4 .

 34 Fig. 3 NaCl production rate for 10 successive cycles of pervaporation of 30 min for various NaCl feed concentrations (0.6, 3.0 and 5.4 mols Lw -1 ). The membranes were oven-dried at 60 °C for 8 h and RH = 67.3%.

Fig. 5 .

 5 Fig. 5. Idealised schematic representing the salt storage in the membrane where is the salt feed solution,is the titania and gamma alumina mesoporous layers, is the water meniscus, is water vapour and is the alpha alumina macroporous substrate.

Fig. 6 .

 6 Fig. 6. NaCl production as a function of pervaporation time for a porous membrane tube thickness of 2.5 mm for various NaCl feed concentration (0.6, 3.0 and 5.4 mols Lw -1 ). The membranes were oven dried at 60 °C for 8 h and RH = 67.3%.

Fig. 7 .Fig. 8 .

 78 Fig. 7. (a) NaCl production versus feed concentration at 30 min pervaporation time, and (b) NaCl production coefficient (kt) versus time.

Table A1 .

 A1 Theoretical NaCl mass stored in the membrane pores for a membrane with 1 m 2 of surface area and pore volume of 675 mL.

	Sol conc.	Sol conc. Av. density	Membrane Mass sol	Membrane NaCl mass	Fig. 3 results NaCl mass
	(mols Lw -1 )	Wt%	(g L -1 )	(gsol)	(gs)	(gs)
	0.6	3.4	1022.3	690.4	23.5	16.2
	3	14.9	1107.8	748.2	111.5	88.9
	5.4	24	1180.4	797.2	191.3	174.0

Acknowledgement

The authors acknowledge the financial support from the Australian Research Council (DP1901002502) and (DP190101734) grants. J. C. Diniz da Costa gratefully thanks the support given by the French "Centre National de la Recherche Scientifique" (CNRS-INC section 15) grant as invited Professor at the Institut Européen des Membranes de Montpellier in France, and the Visiting

Fellowship d by the Associate Laboratory for Green Chemistry -LAQV, financed by the Portuguese National Government funds from FCT/MCTES (UIDB/50006/2020).

Competing interests

The authors declare no competing interests. The alpha-alumina substrate porosity of 36% was determined from helium pycnometry. The substrate's pore volume (pvol) is 675 mL for a membrane tube with a surface area of 1 m 2 , 1 cm outer shell diameter and a wall thickness of 2.5 mm. Table 1 lists the average density () for the NaCl feed concentrations used in this work. The NaCl densities were determined based soaking conditions of a

Appendix


Fei Dong
email: feidong@mosk.tytlabs.co.jp

Akihiko Suda

Toshitaka Tanabe

Yasutaka Nagai

Hideo Sobukawa

Hirofumi Shinjoh

Masahiro Sugiura

Claude Descorme

Daniel Duprez

Dynamic oxygen mobility and a new insight into the role of Zr atoms in three-way catalysts of Pt/CeO2-ZrO2

Keywords: Oxygen mobility, Isotopic exchange (IE), Oxygen storage capacity, CO oxidation, Pt/CeO 2 -ZrO 2 catalysts

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

Introduction

Automotive exhaust is regarded as a major source to the atmospheric pollution. Along with the more stringent environmental regulations imposed on the automobile industry, more efficient catalysts are necessary to detoxify the harmful exhaust, and increasing efforts are being investigated to optimize the efficiency of three-way catalysts. Oxygen storage materials used in three-way catalysts regulate the oxygen storage property through oxygen storage and release, which is regarded as the most important and remarkable feature of the three-way catalyst. A buffer between the lean-rich swings in exhaust gas composition during vehicle operation is achieved in order to keep a stoichiometric atmosphere, so that HC, CO and NO x are efficiently converted [1,2]. Meanwhile, oxygen storage material can suppress the sintering of supported noble metals and enhance the hydrogen formation under a reductive atmosphere. Therefore, work with oxygen storage material is an important approach to enhance the performance of an automotive catalyst.

Oxygen storage and release materials containing ceria in the automotive exhaust catalysts is a typical example of a workable concept for oxygen spillover in the creation of the storage system [3]. Oxygen storage capacity (OSC) is currently used as the key parameter for evaluating the performance of a certain oxygen storage material, and regarded as the most important indicator for the development of an oxygen storage material. Instead of the static or theoretical oxygen storage capacity, the dynamic oxygen mobility or oxygen diffusivity relates more closely with the efficient oxygen storage capacity and the oxygen storage/release rate, or the amount of the oxygen that could be actually stored and released during the transient period involved in the practical catalytic process. The oxygen mobility and oxygen storage capacity are both very critical parameters for the formulation of a real catalysts or making a new catalyst recipe. As a matter of fact, most published papers are concerned with the static oxygen storage capacity, while the dynamic oxygen mobility, especially the oxygen diffusivities on the surface and in the bulk of the oxides, has been scarcely investigated or discussed, more rarely on an in situ and dynamic base.

For studying the spillover of chemisorbed species, isotopic exchange reaction has been taken as a very useful technique. Hall and Lutinski [4] and Carter et al. [5] have studied the exchange of D 2 with the OH groups of Pt/Al 2 O 3 . A similar study over Rh/Al 2 O 3 was reported by Cavanagh and Yates, who followed the H/D exchange by IR spectroscopy [6]. Quantitative measurements of surface diffusivities were carried out by Conner et al. [7,8] by means of a spatially resolved FTIR apparatus. In 1987, Duprez and coworkers developed a method of isotopic exchange for the study of surface mobility, and systematically studied the supported metal catalysts and some mixed oxides [9][10][11][12][13].In our research, in order to establish a characterization method to quantify the oxygen mobility or spillover rates from the metal to the surface of the support and from the surface to the bulk, we applied 18 O/ 16 O isotopic exchange (IE) reaction technique onto three catalysts of Pt/CeO 2 -ZrO 2 , which have the same Ce/Zr ratio of 1:1 but different structures. One important modification we made was to combine CO oxidation reaction after the IE reaction, thus upgrading the previous IE reaction into a dynamic and in situ measurement of the oxygen spillover rates as well as the oxygen storage capacity and oxygen release rate. Through correlating the results obtained from this combined IE reaction with XRD and other physical parameters, we tried to reveal the relationship between the structure and the actual oxygen mobility, to gain deeper insight and understanding to the oxygen storage materials. The obtained results hopefully give us some hints for the actual catalyst formulation and maximize the efficient oxygen storage capacity of some certain oxygen storage material to the theoretical limit, thus to develop more efficient automotive three-way catalysts.

Experimental

Catalyst preparation

Three different types of CeO 2 -ZrO 2 (CZ-O, CZ-D, and CZ-R) with the same Ce/Zr molar ratio of 1 were synthesized [14] and studied respectively. CZ-O was prepared by precipitation of Zr(OH) 4 on a fine ceria powder (ANA KASEI Co., Ltd., 99.9% purity, specific surface area of 120 m 2 /g) using hydrolysis of ZrO(NO 3 ) 2 with aqueous NH 3 . The solid was dried at 90 • C and calcined at 500 • C for 5 h in air. CZ-R was prepared via the same procedure as the CZ-O, except that the oxide was first reduced at 1200 • C for 4 h by CO and then re-oxidized by at 500 • C for 3 h in air. CZ-D was prepared by the mechano-chemical reaction of CeO 2 and ZrO 2 powder with high energy ball milling. Thereafter, Pt (1.0 wt.%) was supported by conventional impregnation using Pt (NH 3 ) 2 (NO 2 ) 2 as platinum precursor and then calcinations at 500 • C for 3 h in air.

Characterization

The specific surface areas were measured by N 2 adsorption, one-point BET method on Micro Sorb 4232 II (Micro Data Co., Ltd.). Pt dispersion was analyzed by CO chemical adsorption in a flow-type adsorption apparatus using CO pulse gas (OHKURA RIKE R6015-S). Pt particle sizes and the grain sizes of CeO 2 -ZrO 2 oxides were determined by Sherrer equation from XRD spectra (RINT2200, Rigaku Co., Ltd., with Cu K␣ radiation).

Isotopic exchange reaction

The isotopic exchange reaction experiments were carried out in our recycle reactor (ca. 100 cm 3 ) coupled to a mass spectrometer (ULVAC QMS), as shown in Fig. 1. The masses 32, 34, 36 (oxygen isotopmers, 16 O 2 , 18 O 16 O and 18 O 2 ) plus mass 28 (to detect a possible leak) were continuously monitored. The vacuum connection to the mass spectrometer was thermo-regulated so as to maintain a constant pressure of 10 -6 Torr, while the pressure in the reaction loop was 50 Torr. Under these conditions, the vacuum connection creates a negligible decrease of the total micro-reactor pressure (2 Torr for 4.0 h for an initial pressure of 50 Torr). Moreover, no gas-phase limiting diffusion was observed. The catalyst samples (ca. 30 mg) were put into a quartz reactor, and then submitted to an "in situ" pretreatment. After a reduction under hydrogen during 15 min at 500 • C, an oxidation by normal oxygen for 60 min at 500 • C, and an evacuation at 500 • C for 1 h. The sample was cooled down to room temperature. A 50 Torr dose of pure 18 O 2 (99.2% of 18 O, ICON) was then introduced at the reaction temperature. The pressure variation of the oxygen Fig. 1. Scheme of 16 O/ 18 O exchange reaction system. isotopmers, P-36 ( 18 O 2 ), P-34 ( 18 O 16 O), and P-32 ( 16 O 2 ) were continuously recorded at the temperature of reaction. The total pressure (P-36, P-34, and P-32) remained virtually constant.

Subsequent CO oxidation

Following the above isotopic exchange reaction, 22.5 Torr of pure CO was introduced into the reaction loop and then oxidation was conducted at 500

Results and discussions

Structures of the CeO 2 -ZrO 2 supports

For the automotive exhaust catalyst, spillover has been developed into a workable concept instead of simply being used to explain a certain phenomenon. For a multi-component catalytic system, such as our metal supported automotive catalysts, to develop a spillover process, at least two distinct phases are needed, namely a donor of the spillover species and an acceptor for this species. These two phases can be in intimate proximity (co-gelled, co-precipitated or one phase impregnating the other) or even simply physically ad-mixed. The analysis and understanding of the structure and the closeness of different components or phases will be very beneficial to picture the spillover process and gain insight into the nature and the functioning of the involved components or materials of automotive catalyst.

Physical properties concerning our CeO 2 -ZrO 2 solids are listed in Table 1. From XRD patterns in Fig. 2, CZ-O of pyrochlore-type structure [15]. XRD pattern of CZ-R almost corresponds to -Ce 0.5 Zr 0.5 O 2. This result indicates that Zr ions were homogeneously substituted in the CeO 2 lattice. CZ-D was also found to have a cubic phase structure. It shows that CZ solid may easily be prepared by high energy ball milling [16]. In the case of CZ-D, due to peak broadening, it is difficult to estimate how homogeneous Zr ions are introduced. Peak broadening could originate either from the small particle size or from a non-homogeneous solid solution. Ce/Zr atomic ratios were determined by EDX analysis during TEM observations [14], although the Ce/Zr ratio for CZ-R is almost constant at 1.0, but this ratio varies between 0.2 and 1.4 for CZ-D. This result means that the distribution of Zr ions into CeO 2 framework in CZ-D is not as homogeneous as in CZ-R. To summarize, the homogeneity of Zr introduction into the CeO 2 framework decreases as: CZ-R > CZ-D > CZ-O. Nagai et al. [17] have verified the structures the three oxides by Ce K-edge and Zr K-edge X-ray absorption fine structure (XAFS).

Oxygen mobility measured by isotopic exchange reaction

As a matter of fact, CO oxidation method has been conventionally used to characterize the oxygen storage material [18], although it is just a kind of overall estimation of the oxygen storage capacity instead of the oxygen mobility. Considering the basics of the 18 O/ 16 O isotopic exchange reaction, the reaction process and route are the same as those of the oxygen in the exhaust on automotive catalysts. With the labeling function of isotopic oxygen of 18 O, the 18 O/ 16 O isotopic exchange reaction could be employed as a dynamic and in situ probe to characterize the oxygen mobility. Moreover, if one can erase the factors from the supported metal and support particle size, the isotopic exchange reaction could be used to access the nature of oxygen storage materials themselves, and for the surface and bulk oxygen diffusion.

The results obtained on Pt/CZ-D at 450 • C were employed to exemplify the IE reaction. The partial pressure curves of three molecules evolved along with the exchange reaction was plotted in Fig. 3(A).At450 • C, the adsorption and decomposition of oxygen molecules on Pt metals would be the fast step, thus the diffusion of oxygen atoms to the top surface from the bulk of oxide would be the control step. The initial reaction region is referred to as Zone 1 corresponded with the surface diffusion, and the reaction region approaching equilibrium is referred to as Zone 2 ascribed to the bulk diffusion.

The contribution from the surface and from the bulk of the oxide to the actual oxygen storage and release during the transient period has been under argument and discussion for many years. The results from isotopic exchange reaction are believed to provide some quantitative evidence to clarify this issue. The derivation of the diffusion coefficients are a kind of opening of the black box, that is, to erase the influences caused by the supported noble metals, the size and the dispersions of metal particles, and the grain size of the support oxides [11]. IE reaction rates and oxygen diffusion coefficients at 410 • C are listed in Table 2. The surface diffusion rate for CZ-R (R 0 e ) is nearly four times larger than the bulk diffusion rate or the equilibrium IE rate (R ∞ e ), while it is 3.8 times larger for CZ-D, and 26 times larger for CZ-O. These results indicated that the contribution from bulk could also be counted as an important part in the total oxygen storage and release performance. As listed in Table 2, the derived surface and bulk oxygen diffusion coefficients of CZ-R are apparently larger than those of CZ-D and CZ-O. The surface and bulk diffusion coefficients relate closely with the nature and the structure of the oxygen storage material. The quantitative evaluation of the diffusion coefficients is very important and useful, to estimate the oxygen storage and release performance, which are very crucial for making a new catalyst recipe and the catalyst formulation, especially for the transient period.

The initial reaction rate or surface diffusion rate for CZ-D seemed a little bit abnormal when correlated with the structures of three kinds of CZ oxides. The parameters of metal particles and the metal dispersion of CZ-D, and the homo-exchange reaction results could be used to clarify this question. Homo-exchange reactions were conducted with the initial equal-mole ratio of 18 O and 16 O, being regarded to reveal the activity and the performance of the supported metals. The homo-exchange results on three CZ catalysts at different temperatures are shown in Fig. 4. The activation energies could be also derived from Arrhenius equation, as Pt/CZ-D (172.0 kJ/mol) > Pt/CZ-O (67.0 kJ/mol) > Pt/CZ-R (19.0 kJ/mol). If correlated with the metal particle sizes, we would find that the bigger metal particle favors homo-isotopic exchange reaction. The activation energy on Pt/CZ-D was the highest among the three catalysts. Compared with the other two catalysts, the oxygen adsorption and decomposition on Pt/CZ-D was somehow restricted, thus the initial reaction rate or surface diffusion rate for CZ-D is the smallest.

Dynamic oxygen storage/release performance and a new insight to the role of Zr atoms played in oxygen storage/release process

Following the above 18 O/ 16 O isotopic exchange reaction, CO was introduced into the system to conduct CO oxidation, as shown in Fig. 3(B). The oxygen storage capacity and the oxygen release rate were derived as shown in Fig. 5.I ti s well known that OSC and oxygen release rates are conventionally employed as the tools to evaluate the performance of oxygen storage materials. IE reaction is very useful to measure or characterize the oxygen mobility instead of the OSC. Through combining with CO oxidation, our designed IE system also enabled us to access the OSC performance. The obtained OSC and oxygen release rates on the three CZ materials were well matched with the results measured by TGA method [16,19], and are in good correspondence with the oxygen mobility and the structural homogeneity of Zr into the frame work of the oxides, as CZ-R > CZ-D > CZ-O (Table 3).

It is well known that the roles of noble metal and cerium played in the oxygen storage/release process are the pro- moter and the oxygen storage material. What kind of role has been played by Zr? So far, Zr is conventionally regarded to contribute to the enlargement of the lattice space, thus facilitating the migration of oxygen atoms. In addition to the reported space effect, we propose the new insight or concept to the role of Zr, "carrier role". That is, Zr in the frame work helps to "capture" oxygen atoms and "transfer" them to Ce atoms. Referring to the oxygen diffusion coefficients in CeO 2 -ZrO 2 oxides based on the method of the isotopic exchange reaction, this is clearly indicated that surface diffusion coefficients on CeO 2 is much higher than on ZrO 2 , while it is smaller than ZrO 2 in the case of bulk diffusion. The CeO 2 -ZrO 2 solid solution gives both a much higher surface and bulk diffusion coefficient. Moreover, we think, at higher temperatures, oxygen atoms are being shared by the whole oxide, instead of being fixed or restricted in some certain positions of the frame work. This assumption still needs to be verified by applying this isotopic exchange reaction technique to other oxides. The improvement of bulk diffusivity by addition of another component will be a meaningful approach for developing more efficient automotive three-way catalysts.

Conclusions

Through the combination with CO oxidation, 18 O/ 16 O isotopic exchange reaction had been upgraded to a compact technique for quantifying the oxygen migration on the surface and in the bulk of the oxides, the efficient OSC and the oxygen release rate of the oxygen storage materials. The measured oxygen surface and bulk diffusivity, oxygen storage capacity and the oxygen release rates showed a good consistency with the homogeneity of the Zr distribution into the oxide framework, as CZ-R > CZ-D > CZ-O. The oxygen storage capacity at 500 • C was in a good match with those obtained by the conventional TGA method. Oxygen diffusion coefficients measured over CeO 2 , ZrO 2 and CeO 2 -ZrO 2 hinted to us an insight to the mechanism for the oxygen storage, where oxygen species are mobile in the oxide support. In addition to the contribution to the space effect, we proposed that zirconium atoms play a role as carriers for the oxygen transfer, although further investigation is needed. The oxygen diffusion coefficient relates closely to the nature of oxygen storage materials. This new insight into the oxygen storage mechanism is hopefully a valuable approach for the development of more efficient oxygen storage materials. We expect that this improved isotopic exchange can be used as a useful probe to access directly the oxygen spillover rate on the surface and in the bulk of the oxide, the dynamic oxygen storage capacity and oxygen release rate.

  was shown to be a mixture of a cubic CeO 2 phase and a tetragonal ZrO 2 phase. CZ-D and CZ-R were shown to be monophasic Ce 0.5 Zr 0.5 O 2 solid solution with a cubic structure. After high-temperature reduction, CeO 2 -ZrO 2 oxide transforms into a pyrochlore-type Ce 2 Zr 2 O 7+δ phase with δ<0.07[15]. In such a phase, Ce and Zr ions are distributed in an ordered arrangement. -Ce 0.5 Zr 0.5 O 2 was obtained by mild oxidation of the pyrochlore-type structure. In the last structure, Ce ions and Zr ions have similar arrangement to that

Fig. 2 .

 2 Fig. 2. XRD patterns of the three CeO 2 -ZrO 2 oxides.

Fig. 3 .

 3 Fig. 3. (A) Partial pressure evolution along with IE reaction on Pt/CZ-D at 450 • C. (B) Partial pressure evolution along with the subsequent CO oxidation (500 • C) on Pt/CZ-D after IE reaction at 450 • C.

Fig. 4 .

 4 Fig. 4. Homo-exchange reaction on three-model three-way catalysts.

Fig. 5 .

 5 Fig. 5. OSC and ORR measured by subsequent CO oxidation at 510 • C.

  • C. The partial pressure variations of C 16 O (P-28), C 16 O 16 O (P-44) and C 16 O 18 O (P-46) were continuously monitored until the reaction reached an equilibrium. The integration of C 16 O 2 and C 16 O 18 Ow a s taken as OSC, and the oxygen release rates were derived from the resulting slopes of C 16 O 2 and C 16 O 18 O.

Table 1

 1 Summary of the three-model catalysts and their physical parameters

	Catalysts	CeO 2 -ZrO 2 preparation	Surface area (m 2 g -1 )	Metal dispersion (%)	Metal particle size (nm)	Oxide grain size (nm)
	1% Pt/CZ-O	Precipitation	104	23	5.2	10
	1% Pt/CZ-R	HTR	3	2	59	200
	1% Pt/CZ-D	Ball milling	37	59	2	20

HTR: high-temperature reduction at 1200 • Cb yC O .

Table 2

 2 Isotopic exchange reaction rates and oxygen diffusion coefficients at 410 • C

	Catalysts		Pt/CZ-R	Pt/CZ-D	Pt/CZ-O
	Initial IE reaction rate, R 0 e (atoms s -1 g -1 ) Equilibrium IE reaction rate, R ∞ e (atoms s -1 g -1 ) Surface diffusion coefficient, D S a (m 2 s -1 )	1.504 × 10 18 3.55 × 10 17 2.98 × 10 -13	3.899 × 10 17 1.029 × 10 17 1.48 × 10 -18	1.578 × 10 18 5.73217 × 10 16 4.23 × 10 -19
	Bulk diffusion coefficient, D B	b (m 2 s -1 )	4.33 × 10 -19	1.20 × 10 -20	7.77 × 10 -22

a D S : surface diffusion coefficient (m 2 s -1 ), S: surface. b D B : bulk diffusion coefficient (m 2 s -1 ), B: bulk.

Table 3

 3 Measured oxygen diffusion coefficient in CeO 2 , ZrO 2 , and CeO 2 -ZrO 2 oxides

	Oxides	CeO 2 ZrO 12 Ce 0.15 Zr 0.85 O 2 53 41
	Bulk diffusion activation energy,	15	10	14
	E a (kJ mol -1 )

2 Surface diffusion coefficient, D S , at 300 • C( ×10 -19 m 2 s -1 ) 21 33 2 Surface diffusion activation energy, E a (kJ mol -1 ) 41 39 49 Bulk diffusion coefficient, D B ,a t 300 • C( ×10 -23 m 2 s -1 )


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An extreme particle accelerator in the Galactic plane: HESS J1826-130

Keywords: Gamma rays, general -gamma rays, observations -gamma rays, individual objects, HESS J1826-130 -ISM, clouds -ISM, cosmic rays

The unidentified very-high-energy (VHE; E > 0.1 TeV) γ-ray source, HESS J1826-130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS data has revealed a steady γ-ray flux from HESS J1826-130, which appears extended with a half-width of 0.21 • ± 0.02 • stat ± 0.05 • sys . The source spectrum is best fit with either a power-law function with a spectral index Γ = 1.78 ± 0.10 stat ± 0.20 sys and an exponential cut-off at 15.2 +5.5 -3.2 TeV, or a broken power-law with Γ 1 = 1.96 ± 0.06 stat ± 0.20 sys , Γ 2 = 3.59 ± 0.69 stat ± 0.20 sys for energies below and above E br = 11.2 ± 2.7 TeV, respectively. The VHE flux from HESS J1826-130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula (PWN), HESS J1825-137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826-130 VHE emission related to PSR J1826-1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826-130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to 200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants (SNRs), molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.

Introduction

The High Energy Stereoscopic System (HESS) Galactic Plane Survey (HGPS) revealed the presence of 16 new γ-ray sources in the very high energy (VHE; E > 0.1 TeV) domain (H.E.S.S. Collaboration et al. 2018). Among them, HESS J1826-130 is located in a region that is exceptionally rich in VHE γray sources, also encompassing the nearby pulsar wind nebula (PWN) HESS J1825-137 and the γ-ray binary system, LS 5039. HESS J1826-130 belongs to a growing class of VHE γ-ray sources affected by source confusion, caused by the influence of luminous neighboring γ-ray emitters, similar to the case of HESS J1641-463 (Abramowski et al. 2014). HESS J1826-130 is located just ∼ 0.7 • to the north of HESS J1825-137 (H.E.S.S. Collaboration et al. 2019a), a bright PWN whose extended emission prevented an earlier discovery of HESS J1826-130 at VHEs. While HESS J1826-130 was clearly distinguishable from HESS J1825-137 in the HGPS, a deep study accounting for its morphological and spectral properties beyond the results obtained with the standard HGPS analysis tools was still lacking. In this paper, the results obtained from the analysis of 215 hours of VHE γ-ray data recorded with the HESS telescopes is presented, yielding an accurate morphological description and a detailed spectral characterization of the source, including an energydependent emission study of the region. The results reported here on HESS J1826-130 are discussed in a multiwavelength context, discussing in particular its possible association with the spatially coincident supernova remnant, G18.45-0.42, and the Eel Nebula [START_REF] Roberts | WE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after the Discovery[END_REF], as well as with the VHE source 2HWC J1825-134 (Abeysekara et al. 2017), which displays emission up to energies of around 100 TeV.

Furthermore, HESS J1826-130 is surrounded by dense molecular clouds that provide support for a hadronic emission scenario. A recent investigation of the diffuse VHE emission around the Galactic center (GC) region demonstrated the presence of PeV particles within the central 10 pc of the Galaxy (HESS Collaboration et al. 2016). This is the first robust detection of a VHE cosmic-ray (CR) accelerator which operates as a PeVatron. The MAGIC Collaboration recently published (MAGIC Collaboration et al. 2020) the results of observations of the diffuse γ-ray emission in the vicinity of the GC. However, the GC alone would not be able to sustain the CR population close to PeV energies today unless the GC PeVatron was more powerful in the past. This opens up the possibility of other active PeV accelerators in the Galaxy that contribute to the observed Galactic CR flux around the spectral feature known as the knee, that is, a potentially new CR source population in the Galaxy. HESS J1826-130 may belong to such a new Galactic CR source population.

In Sect. 2, we present a summary of the multiwavelength (MWL) information available in the field of view (FoV) around HESS J1826-130. The HESS observations and data analysis are reported in Sect. 3, including morphological and spectral results, as well as a study of the contamination from the nearby PWN HESS J1825-137. Results are discussed in Sect. 4, with concluding remarks in Sect. 5.

Multiwavelength environment

As mentioned before, HESS J1826-130 is located in a region with several VHE γ-ray sources. At VHEs, the TeV sky around the source is dominated by one of the brightest and largest PWNs detected at VHEs, HESS J1825-137. This latter source is thought to be powered by PSR J1826-1334 (also known as PSR B1823-13), a powerful pulsar with a spin-down luminosity of ∼ 3 × 10 36 erg s -1 , located about 13' from the peak position of HESS J1825-137 in the VHE band. At GeV energies, both the spectral and the morphological properties of HESS J1825-137 resemble and smoothly connect with those observed at VHEs, as reported in Grondin et al. (2011) using the Fermi-LAT.

The first evidence for γ-ray emission originating from the north of PSR J1826-1334 was found by the detection of 3EG J1826-1302 (also called GeV J1825-1310) in 1999 with the EGRET telescope on-board the Compton Gamma Ray Observatory (Hartman et al. 1999). Follow-up X-ray observations of the same region led to the detection of a diffuse X-ray source, AX J1826.1-1300, in the 2-10 keV energy band (Roberts et al. 2001a). This X-ray source is characterized by two emission peaks located on top of a diffuse X-ray nebula. Further observations of AXJ1826.1-1300 in 2007 with the Chandra X-ray Observatory resolved the source in more detail [START_REF] Roberts | WE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after the Discovery[END_REF]). The southern peak was found to be likely associated with a stellar cluster, whereas the northern peak was resolved and associated with a PWN. Due to a 4 trail of hard X-ray emission connected to this new nebula, the authors called it the Eel Nebula. [START_REF] Roberts | WE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after the Discovery[END_REF] were the first to introduce a new separate VHE γ-ray source north of HESS J1825-137, which they named HESS J1826-131 and associated with the Eel Nebula. Until that time, the remaining VHE emission to the north of HESS J1825-137 was thought to be connected to the bright source itself.

The Fermi/LAT detected the radio-quiet γ-ray pulsar PSR J1826-1256 (Abdo et al. 2009), with a spin down luminosity Ė = 3.6 × 10 36 erg s -1 and a characteristic age τ c = 14.4 kyr, whose position is consistent with the Eel Nebula, representing a significant step forward in our understanding of the various sources contributing to the γ-ray emission from this region. In 2017, HESS J1826-130 was reported as a new source within the HGPS. The HGPS used a dedicated source-identification algorithm based on a multi-Gaussian component fit to a given region of interest covered by the survey (H.E.S.S. Collaboration et al. 2018). In particular for HESS J1826-130, the spatially extended VHE γ-ray emission region matched the position of 3EG J1826-1302, also overlapping with the diffuse X-ray nebula observed with ASCA (which displays a radius of about 15 ; see Fig. 11 in (Roberts et al. 2001b)) as well as the Eel Nebula (with an extension of about 4 [START_REF] Roberts | WE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after the Discovery[END_REF])). The latest results at VHEs of the region include the significant detection of an emission region, 2HWC J1825-134, included in the 2 nd HAWC catalog (Abeysekara et al. 2017). Due to the large spatial extension of the HAWC emission region and the relatively low angular separation between HESS J1826-130 and HESS J1825-137, the two sources were not distinguishable in this catalog. Further studies, however, using a much larger dataset and employing updated analysis methods, provide a statistical improvement at the level Send offprint requests to: H.E.S.S. collaboration, e-mail: contact.hess@hess-experiment.eu Fig. 1. Map of excess counts with energies E > 5 TeV for region around HESS J1826-130 smoothed with a Gaussian kernel of 0.07 • to match HESS PSF at the same energies. The 68% containment radius of the HESS PSF is shown with the white circle in the lower-left corner. The white contours indicate the significance of the emission at the 5σ, 10σ, and 15σ level. The color scale is in units of excess counts per smoothing Gaussian width. The green circle shows the integration region used for deriving the source spectrum (Fig. 3), while the green cross indicates the value and 1σ uncertainty of the best-fit position of the source. The nearby SNRs: G18.45-0.42, G018.6-00.2, and G018.1-00.1 are represented with yellow circles, while the white triangle indicates the position of the γ-ray pulsar PSR J1826-1256. The blue star shows the peak position of the bright nearby source HESS J1825-137 for energies above 0.25 TeV (we employed the position as reported in H.E.S.S. Collaboration et al. (2019a) since the data set analyzed here accounts for a total exposure across all of HESS J1825-137, which is less than that used in its dedicated study). of ∼ 5σ when accounting for the presence of the two HESS VHE sources.

Within the surroundings of HESS J1826-130, there are also three supernova remnants (SNRs): G18.1-0.1, G18.6-0.2, and G18.45-0.42, which are detected in the radio band (Brogan et al. 2006;Anderson et al. 2017). Detailed studies of the gas dynamics and densities in the direction of the region north of HESS J1825-137 were carried out by Voisin et al. (2016), following the discovery of a molecular cloud north of PSR J1826-1334 by [START_REF] Lemiere | 29th International Cosmic Ray Conference[END_REF]. These studies revealed the presence of dense molecular clouds with particle density values up to n H 2 ~7 × 10 2 cm -3 . The bulk of molecular gas in the vicinity of HESS J1826-130 is located at V LSR = 45-60 km/s, corresponding to a kinematic distance of 4.0 kpc, and V LSR = 60-80 km/s, corresponding to a kinematic distance of 4.6 kpc. The former gas distance estimate is consistent with the dispersion measurement of PSR J1826-1334. Voisin et al. (2016) concluded that due to the general spatial match between the molecular gas and the TeV emission, HESS J1826-130 might be explained by a hadronic emission scenario, with the progenitor SNR of HESS J1825-137 being a suitable local CR source candidate. However, a leptonic scenario connected to the Eel Nebula could not be ruled out. Furthermore, the two SNRs, G18.1-0.1 and G18.6-0.2, were considered unlikely to be directly related to the TeV excess due to their offset positions and their small angular diameters.

HESS observations and data analysis results

The High Energy Stereoscopic System is an array of five imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia, 1800 m above sea level. HESS in phase I comprised four 12 m diameter telescopes that have been fully operational since 2004. A fifth telescope (CT5), with a larger mirror diameter of 28 m and newly designed camera (Bolmont et al. 2014), was added to the center of the array and has been operational since September 2012. The HESS phase I array configuration is sensitive to γ-ray energies between 100 GeV and several tens of TeV, while the energy threshold of the array was lowered to 10's of GeV with the addition of CT5. The VHE HESS data presented in this paper were recorded with the HESS phase I array configuration, which can measure extensive air showers with an angular resolution better than 0.1 • and an average energy resolution of 15% for an energy of 1 TeV (Aharonian et al. 2006c).

The VHE γ-ray observations of the FoV around HESS J1826-130 were carried out between 2004 and 2015.

Article number, page 3 of 9

A&A proofs: manuscript no. HESSJ1826-130_An_extreme_particle_accelerator_in_the_Galactic_plane_arxiv About 140 h of observations were recorded with the HESS phase I array configuration (between 2004 and 2012), while ∼95 h were obtained with the HESS phase II array configuration in a split observation mode without participation of the CT5 telescope. These data sets provide a total acceptance-corrected live-time of 215 h of HESS data after the application of quality selection criteria (Aharonian et al. 2006c).

The data have been analyzed with the HESS Analysis Package (HAP) for shower reconstruction, and a multivariate analysis technique (Ohm et al. 2009) was applied to provide an improved distinction between hadron and γ-ray events. A cross-check analysis was performed using an independent calibration and analysis method (de Naurois & Rolland 2009), which gives compatible results with the main analysis.

Detection and morphological analysis

In order to achieve an improved angular resolution and reduce strong contamination coming from the nearby bright source, HESS J1825-137, the source position and morphology of HESS J1826-130 were obtained with a cut configuration optimized for high energies that requires a minimum of 160 photoelectrons per image. The cosmic-ray background level was estimated using the ring background model (Berge et al. 2007). Using the full 215 h dataset, HESS J1826-130 was detected with a statistical significance of 22.6σ 1 , determined following Equation ( 17) in [START_REF] Li | [END_REF]. Figure 1 shows the acceptance-corrected VHE γ-ray excess map of the region around HESS J1826-130 at energies greater than 5 TeV, smoothed with the HESS point spread function (PSF) with a radius of 0.07 • .

Until 2017, in the first two publications by the HESS Collaboration concerning HESS J1825-137 (Aharonian et al. (2006a); Aharonian et al. (2006b)), HESS J1826-130 remained unnoticed and hidden to the standard source detection techniques due to its low brightness with respect to HESS J1825-137. Confirmed as a separate source in 2017 (H.E.S.S. Collaboration et al. 2018), HESS J1826-130 emerges toward higher energies as illustrated in Fig. 2, with reduced contamination from HESS J1825-137, potentially due to its energy-dependent morphology and hinting at a harder VHE spectrum.

The best-fit position and extension of the source have been determined by convolving 2D Gaussian models with the HESS PSF and fitting this to the excess event distribution (for E > 0.4 TeV). The HESS PSF was derived from simulations and modeled as a weighted sum of three 2D Gaussian functions for the morphology analysis. A log-likelihood ratio test (LLRT) was used to select the best morphology model that represents the HESS data. The centroid of the 2D Gaussian corresponding to the bestfit position of HESS J1826-130 is R.A. ( J2000

Spectral analysis

The green circular region with a radius of 0.22 • centered at the best-fit position of HESS J1826-130 (see Fig. 1) was used as the integration region for measuring the differential VHE γ-ray spectrum of the source. The reflected background model (Berge 1 For comparison, HESS J1826-130 is detected at a statistical significance of about 9.4 σ in the HGPS, (H.E.S.S. Collaboration et al. 2018). Table 1. Analysis statistics of the HESS J1826-130 region using different energy thresholds (E th ) corresponding to the VHE γ-ray excess profiles shown in Fig. 2. The events below the energy threshold shown in the E th column were not taken into account in the analysis. The number of events within the source region (N On ), within the ring region used for estimating the background level (N Off ), the alpha factor (α), the corresponding number of excess events, and the Li&Ma statistical significance [START_REF] Li | [END_REF] 0.69 stat ± 2.02 sys ) × 10 -13 cm -2 s -1 TeV -1 , Γ = 1.78 ± 0.10 stat ± 0.20 sys and a cut-off energy of E c = 15.2 +5.5 -3.2 TeV as shown in Fig. 3. The integral flux level above 1 TeV is Φ(> 1 TeV) = (10.2 ± 0.5 stat ± 2.0 sys ) × 10 -13 cm -2 s -1 and corresponds to 4.9% of the Crab Nebula flux at the same energies 2 . A 2σ lower limit on the cut-off energy of the source is derived at ∼ 10 TeV. A fit to a BPL model gives a break energy at 11.2 ± 2.7 TeV, which is compatible with the cut-off energy obtained from the ECPL model. The comparison between the ECPL and the BPL model does not show any preference from obtained p-values. Both models clearly indicate a spectral steepening of the spectrum above 10 TeV. The integral flux is found to be constant within the HESS dataset at different timescales, from run-wise (∼30 min) to year-wise light curves.

The integration region used to measure the VHE spectrum of HESS J1826-130 is strongly contaminated, especially at lower energies, by the relatively softer spectrum of HESS J1825-137. Therefore, the obtained spectrum is affected by this contamination. Taking into account the relatively softer contribution from the 2 Throughout this paper, 1 Crab unit is defined here as Φ(> 1 TeV) = 2.26 × 10 -11 cm -2 s -1 (Aharonian et al. 2006c).

contaminating source, the intrinsic spectrum of HESS J1826-130 is expected to be even harder with respect to the spectral results discussed above.

Contamination study

The estimation of intrinsic spectra for sources strongly contaminated by neighboring bright and extended sources presents a nontrivial challenge. Although morphology studies in different energy bands can provide a powerful tool for new discoveries, such a method does not help in providing a quantitative characterization of the contamination, as systematic uncertainties in the flux estimation are expected to dominate. In general, contamination effects can be neglected if they are at the 10% level or lower, since this is, for example, the typical HESS systematic error on the source flux. For contamination exceeding 10%, in particular for spectral studies, a dedicated analysis is needed.

Performing contamination studies for HESS J1826-130 requires knowledge about the energy-dependent morphology of HESS J1825-137, which is highly asymmetrical. The contamination was estimated in this case using the morphology model that represents the HESS data best fit. The model includes a 2D Gaussian function accounting for HESS J1826-130 along with other 2D Gaussian functions representing the other sources, HESS J1825-137 and the gamma-ray binary system LS 5039, present in the FoV. Taking the ratio of the total number of excess counts within the integration region (Fig. 1, green circle) used for measuring HESS J1826-130's spectrum between the model components can give a quantitative estimation of the contamination. By using such an approach, the contamination from HESS J1825-137 is estimated to be ∼40% below 1.5 TeV and ∼20% above 1.5 TeV, while it is strongly reduced (<10%) at energies above 2.0 TeV. Given the relatively large angular distance of LS 5039 to the integration region (around 2.0 • ) encompassing HESS J1826-130, we find that the contamination from the binary system is negligible.

The spectral contamination from HESS J1825-137 has been estimated based on the fact that as the separation between contaminated and contaminating source increases, one expects the effect of contamination to decrease. To test the biasing impact of the contamination, the HESS J1826-130 integration region was split into a near and far half-circle, referred to as the south and north semi-circles shown in Fig. 4. Spectra from two control regions at both sides of the source position were also extracted in order to determine whether or not the contribution from HESS J1825-137

Article number, page 5 of 9

A&A proofs: manuscript no. HESSJ1826-130_An_extreme_particle_accelerator_in_the_Galactic_plane_arxiv Table 3. Comparison of ECPL spectral models for the HESS J1826-130 integration region (Total), plus the north and south semi-circle regions. The integral flux (in % Crab Nebula) is given for the same energy range (above 1 TeV).

Region

Normalization Index Cut-off energy Integral flux (>1 TeV) (10 -13 cm -2 s -1

2.3

for the north (south) semi-circle is significant. The spectra of the upper halves of these control regions (solid semi-circles in Fig. 4) show insignificant (< 5.0σ) emission, while the lower halves of the control regions (dashed semi-circles in Fig. 4), which are closer to HESS J1825-137, show significant (> 5.0σ) VHE γ-ray emission.

One expects that the contamination in the north semi-circle region should be less with respect to the south semi-circle region, and the spectrum should reflect the intrinsic spectral properties of HESS J1826-130. The spectral fit results, assuming an ECPL model and obtained from the north and south semi-circle regions, are given in Table 3 along with the total results. As expected, the north semi-circle spectrum is slightly harder than those of the total and south semi-circle regions. Although the contamination from HESS J1825-137 could potentially affect the spectrum of HESS J1826-130, our studies demonstrate that the effects on the spectral parameters are at the level of our analysis systematic uncertainties. The intrinsic spectrum of HESS J1826-130, corrected for this contamination, is found to be well described by an ECPL model where Φ 0 = 9.2 × 10 -13 cm -2 s -1 TeV -1 , Γ = 1.7 and a cut-off energy around E c = 16 TeV.

A further investigation of the intrinsic spectrum of HESS J1826-130 was performed using a 3D cube-analysis approach3 . With such a template-based analysis method, uncontaminated spectra can be derived, since all sources located in the FoV are simultaneously taken into account. The 3D analysis results published in [START_REF] Ziegler | [END_REF] are found to be compatible with the results presented here.

Discussion

The study of VHE γ-ray sources displaying spectra extending above several tens of TeV is required to understand the origin of the highest energy CRs close to the knee in the CR spectrum. These studies are particularly promising in cases in which dense gas regions are coincident with the source of interest. In the case of HESS J1826-130, the spatial coincidence of a dense molecular hydrogen region found along the line of sight (see Voisin et al. (2016)), suggests that the radiation could actually be produced by protons with energies of several hundred TeV colliding with this gas. Figure 5 displays the HESS excess map obtained at energies above 20 TeV together with column density profiles derived from Nobeyama 12 CO(1-0) (Umemoto et al. 2017) and SGPS HI data (McClure-Griffiths et al. 2005), for two different gas velocity ranges, 45-60 km/s (corresponding to a kinematic distance of 4.0 kpc) and 60-80 km/s (corresponding to a kinematic distance of 4.6 kpc). The scaling X-factors, X HI = 1.8 × 10 18 cm -2 (K km s -1 ) -1 and X CO = 2.0 × 10 20 cm -2 (K km s -1 ) -1 , were taken from Dickey & Lockman (1990) Fig. 4. VHE γ-ray excess map of HESS J1826-130 for all events saturated for a better visualization, showing the north semi-circle (green), south semi-circle (blue), and control regions for both north (solid white) and south (dashed white) semi-circle regions. The control regions are distributed in a radially symmetric manner around the peak position of HESS J1825-137, indicated with the blue cross, obtained from the dataset and for energies above 0.4 TeV. The cyan contours indicate the significance of the emission at the 15σ, 20σ and 25σ levels. A. W. et al. (2004), respectively. The Nobeyama 12 CO(1-0) data was smoothed to match the SGPS beam size of 2'. The CO and HI distribution display two local maxima that do not coincide with the E > 20 TeV peak, although they still provide target densities at the level of ∼ 5 × 10 2 cm -3 at the position of HESS J1826-130.

The best-fit parent proton population in this scenario would have a spectral index of ∼1.7 and a cut-off energy around 200 TeV 4 . The relative hardness of the photon spectrum could be due to the highest energy protons diffusing into dense clouds, while lower energy protons might still be confined within the accelerating source, or efficiently excluded from the clouds if the diffusion coefficient inside the clouds is suppressed (Gabici et al. 2009). A hadronic scenario in which runaway protons accelerated by one of the coincident or nearby SNRs emit TeV photons when interacting with the dense ambient gas found along the line of sight could explain the hard spectrum of HESS J1826-130. We note, however, that the filled centers of SNR G18.5-0.04 and SNR G18.1-00.1 are possibly too old to be able to accelerate protons up to hundreds of TeV. The γ-ray luminosity, L γ , of the source is 8 × 10 33 erg s -1 for a distance of 4 kpc, corresponding to the estimated distances of the SNRs G18.45-0.42 (Karpova et al. 2019), G018.1-00.1, or G018.6-00.2 5 . This would translate into an energy output in accelerated protons W pp = L γ × t pp , of 6 × 10 49 (n/1 cm -3 ) -1 erg, where the gas density value is n = 5 × 10 2 cm -3 and t pp is the typical timescale for gamma-rays to be produced through neutral pion decay following the collision of relativistic protons.

The relatively hard spectrum found at VHE gamma rays could also be produced in a leptonic scenario by an electron population with a spectral index close to 2.0 and a cut-off at around 70 TeV. These electrons could be accelerated by the pulsar PSR J1826-1256 and up-scatter cosmic microwave background (CMB) and IR photons to produce the observed TeV emission. PSR J1826-1256 is powering the Eel Nebula (PWN G018.5-0.4, [START_REF] Roberts | WE-Heraeus Seminar on Neutron Stars and Pulsars 40 years after the Discovery[END_REF], which displays an elongated X-ray trail of shocked material associated with its fast proper motion through the surrounding medium. An estimate for the magnetic field inside the Eel Nebula of ∼20-30 µG is provided in Keogh (2010). Recently, Duvidovich et al. (2019) made use of XMM-Newton observations of the region around PSR J1826-1256 and discussed such a possibility. Based on the spectral softening found along the nebula, they considered X-rays produced by electrons with energies E e 150 TeV radiating synchrotron emission in a magnetic field B > 2µG. In a parallel study, Karpova et al. (2019) also reported on the X-ray analysis of this very same XMM-Newton dataset, complemented with additional ∼ 90 ks of Chandra observations of the region. These data support the association of HESS J1826-130 with the Eel Nebula, although somewhat harder spectral indices close to the PWN are derived (Γ X ∼ 1.2), followed by softer emission, with Γ X ∼ 2.5 at larger distances along the PWN tail.

The excess map reported in Fig. 5 at E > 20 TeV traces emission from the vicinity of PSR J1826-1256. TeV photons could be produced by these synchrotron X-ray-emitting electrons through inverse Compton scattering off the CMB and IR photon fields. For the latter, an energy density at a distance of 4 kpc ∼0.23 eV cm -3 was assumed (see e.g., Vernetto & Lipari 2016; we note, however, that the distance to PSR J1826-1256 is not very well constrained; e.g., Wang 2011 favored a distance of about 1.2 kpc, whereas Karpova et al. 2019 found a distance of ∼ 3.5 kpc based on an empirical interstellar absorption-distance correlation). The CMB photon field provides u CMB ∼0.26 eV cm -3 . Electron Lorentz factors of γ e ∼ 1.3 × 10 8 and γ e ∼ 3.6 × 10 7 would be required to reach the VHE γ-ray fluxes reported here for CMB and IR seed photons, respectively, in agreement with the estimates in Duvidovich et al. (2019).

The complex region around HESS J1826-130 has also been observed at TeV energies with the HAWC telescopes. A spatially coincident source, 2HWC J1825-13, is included in the 2 nd HAWC catalog (Abeysekara et al. 2017). Furthermore, the HAWC collaboration recently reported the detection of E > 100 TeV emission from eHWC J1825-134 (HAWC Collaboration et al. 2019). Its position and extension are marked with a white dotted circle in Fig. 5. With a half-width of ∼ 0.36 • , the source encompasses a local peak of the 12 CO distribution. It is also compatible with the location of the E > 20 TeV γ-rays from HESS J1825-137, and adjacent to HESS J1826-130 emission E > 20 TeV. On spectral grounds, both HESS J1825-137 and HESS J1826-130 display statistically preferred cut-offs and/or curvature in their spectral analysis (see Sect. 3.2 and H.E.S.S. Col-5 We note that Voisin et al. (2016) discuss the possibility of a larger distance to G018.6-00.2 of 11.4 kpc, disfavoring an association of HESS J1826-130 with G018.6-00.2. laboration et al. 2019a, respectively). However they may still be able to partially contribute to the 100 TeV flux toward eHWC J1825-134 (HAWC Collaboration et al. 2019). In this regard, the preliminary results reported in Salesa Greus & Casanova (2019) seem to support this scenario.

Conclusions

Finally, HESS J1826-130 is a particle accelerator in the Galactic plane capable of delivering TeV emission above several tens of TeV. The deep observation campaign on the source reported in this paper provides several constraints to its spectral and morphological properties, although its origin is still uncertain. The presence of nearby SNRs found in the region and/or another yet unresolved accelerator, together with the gas density properties of the surroundings, could make hadronic interactions responsible for the observed emission. If protons are delivered with energies up to hundreds of TeV, HESS J1826-130 could point to the existence of a distinct population of CR sources that, along with the GC, significantly contribute to the Galactic CR flux around the knee feature. In a leptonic scenario, HESS J1826-130 has been proposed as the counterpart of the Eel Nebula powered by the pulsar PSR J1826-1256. Our observations reveal VHE emission above several tens of TeV that comes from the vicinity of the pulsar. Regarding the association of HESS J1826-130 with the recently discovered 100 TeV emitter, 2HWC J1825-13, it is worth noting that the spectrum reported in Fig. 3 extends at least up to ∼ 43 TeV. Therefore, it could be possible that a fraction of the emission reported from eHWC J1825-134 could be partially contributed by HESS J1826-130 (see Salesa Greus & Casanova 2019). On the other hand, HESS J1825-137 also displays VHE emission above tens of TeV, albeit displaying a Article number, page 7 of 9 softer spectrum. Observations of the region, for example using the LHAASO observatory (Bai et al. 2019), and in the future with the Cherenkov Telescope Array (The CTA Consortium 2019) with its superior angular and energy resolution and sensitivity at energies above 10 TeV, will provide further insights into the origin of HESS J1826-130.

  Fig.1. Map of excess counts with energies E > 5 TeV for region around HESS J1826-130 smoothed with a Gaussian kernel of 0.07 • to match HESS PSF at the same energies. The 68% containment radius of the HESS PSF is shown with the white circle in the lower-left corner. The white contours indicate the significance of the emission at the 5σ, 10σ, and 15σ level. The color scale is in units of excess counts per smoothing Gaussian width. The green circle shows the integration region used for deriving the source spectrum (Fig.3), while the green cross indicates the value and 1σ uncertainty of the best-fit position of the source. The nearby SNRs: G18.45-0.42, G018.6-00.2, and G018.1-00.1 are represented with yellow circles, while the white triangle indicates the position of the γ-ray pulsar PSR J1826-1256. The blue star shows the peak position of the bright nearby source HESS J1825-137 for energies above 0.25 TeV (we employed the position as reported in H.E.S.S.Collaboration et al. (2019a) since the data set analyzed here accounts for a total exposure across all of HESS J1825-137, which is less than that used in its dedicated study). The blue triangle indicates the position of PSR J1826-1334. The white dashed line indicates the orientation and position of the Galactic plane, while the cyan dashed rectangle indicates the area for the extraction of the profiles shown in Fig. 2. Color version of Fig. 1 available in the electronic version.

Fig. 2 .

 2 Fig. 2. Distribution of VHE γ-ray excess along axis of the cyan dashed rectangle indicated in Fig. 1 that encompasses both HESS J1825-137 (broad peak on the left) and HESS J1826-130 (shallower peak on the right, becoming apparent only at high energies) above 0.4 TeV and at E > 5 TeV, 10 TeV, 15 TeV, and 20 TeV. Vertical lines show the position of PSR J1826-1256 (red), PSR J1826-1334 (blue) and SNR G018.45-0.42 (red).

Fig. 3 .

 3 Fig. 3. VHE γ-ray spectrum of HESS J1826-130 extracted from the green circular source region shown in Fig. 1. The black dots show the flux points with 1σ statistical errors. The 1σ and 3σ error bands of the best-fit ECPL model are shown with the blue and light blue shaded regions, respectively, while the best-fit ECPL model itself is shown with the black line. The spectrum shown is binned such that all flux points have a minimum significance level of 2.0σ. The significance of the last flux bin centered at 42.7 TeV is 2.4σ.

Fig. 5 .

 5 Fig. 5. VHE γ-ray excess map of HESS J1826-130 for events above 20 TeV, together with total column density map contours. The black circle and cross show the location of the pulsars PSR J1826-1256 and PSR J1826-1334. The cyan and blue contours show the total column density contours of [2.5, 9.5] × 10 22 cm 2 , derived from Nobeyama 12 CO(1-0) and SGPS HI data, for gas velocities between 45-60 km/s (d: 4 kpc) and 60-80 km/s (d: 4.6 kpc), respectively. The white dashed circle and diamond are the extension and best-fit position of recently reported E > 100 TeV emitting source, eHWC J1825-134 (HAWC Collaboration et al. 2019).

Table 2 .

 2 are given for each energy threshold separately. Comparison of different spectral models fit to the data in the energy range of[0.4, 56.2] TeV, together with the spectral parameter values. In all cases, statistical errors are quoted, while systematics errors are 20% for flux and 0.2 for spectral indices. The likelihoods and p-values are given for each model for comparison. Loglike χ 2 /n.d. f. ×10 -13 cm -2 s -1 TeV -1 ×10 -13 cm -2 s -1

	E th (TeV)	N On	N Off	α	Excess Significance (σ)
	0.4	5544 27006 0.145	1627.6	22.7
	5	569	1685	0.132	346.2	17.8
	10	237	664	0.126	153.4	12.6
	15	118	363	0.125	72.5	8.2
	20	68	220	0.120	41.6	6.2
	et al. 2007) was used to estimate the background. The source
	spectrum was derived using the forward folding technique (Piron
	et al. 2001). Several spectral models were used to fit the data,
	including a simple power-law (PL) model, a broken power-law
	(BPL) model, and a power-law model with an exponential cut-off
	(ECPL; see Table 2). The spectral fit is performed between 0.42
	TeV and 56.

2 TeV. The best-fit model is found to be the ECPL, with a p-value = 0.41 and a likelihood ratio test (LLRT) against the PL model providing a statistical improvement at the ∼ 3.2σ level. The differential VHE emission is thus best represented by dN/dE = Φ 0 (E/1 TeV) -Γ exp(-E/E c ), with Φ 0 = (10.10 ±

  North semi-circle 4.64 ± 0.49 stat ± 0.93 sys 1.69 ± 0.14 stat ± 0.20 sys 15.8+9.6

		TeV -1 )	(TeV)	(% Crab Nebula)
	Total	10.10 ± 0.69 stat ± 2.02 sys 1.78 ± 0.10 stat ± 0.20 sys	15.2 +5.5 -3.2	4.9
			-4.3	2.4
	South semi-circle 5.06 ± 0.49 stat ± 1.01 sys 1.88 ± 0.14 stat ± 0.20 sys	16.4 +11.9 -4.7

We used ctools(Knödlseder, J. et al. 2016) and gammapy(Deil et al. 2017), which include 3D cube analysis approaches optimized for measuring intrinsic spectra from contaminated sources.

We employed the NAIMA fitting software package for these computations[START_REF] Zabalza | International Cosmic Ray Conference (ICRC2015)[END_REF].

Acknowledgements. The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the Helmholtz Association, the Alexander von Humboldt Foundation, the French Ministry of Higher Education, Research and Innovation, the Centre National de la Recherche Scientifique (CNRS/IN2P3 and CNRS/INSU), the Commissariat à l'énergie atomique et aux énergies alternatives (CEA), the U.K. Science and Technology Facilities Council (STFC), the Knut and Alice Wallenberg Foundation, the National Science Centre, Poland grant no. DEC-2017/27/B/ST9/02272, the South African Department of Science and Technology and National Research Foundation, the University of Namibia, the National Commission on Research, Science & Technology of Namibia (NCRST), the Austrian Federal Ministry of Education, Science and Research and the Austrian Science Fund (FWF), the Australian Research Council (ARC), the Japan Society for the Promotion of Science and by the University of Amsterdam. We appreciate the excellent work of the technical support staff in Berlin, Zeuthen, Heidelberg, Palaiseau, Paris, Saclay, Tübingen and in Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation.