Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Abe, S.; Aguasca-Cabot, A.; Agudo, I.; Alvarez Crespo, N.; Antonelli, L. A.; Aramo, C.; Arbet-Engels, A.; Artero, M.; Asano, K.; Aubert, P.; Baktash, A.; Bamba, A.; Baquero Larriva, A.; Baroncelli, L.; Barres de Almeida, U.; Barrio, J. A.; Batkovic, I.; Baxter, J.; Becerra González, J.; Bernardini, E.; Bernardos, M. I.; Bernete Medrano, J.; Berti, A.; Bhattacharjee, P.; Biederbeck, N.; Bigongiari, C.; Bissaldi, E.; Blanch, O.; Bordas, P.; Buisson, C.; Bulgarelli, A.; Burelli, I.; Buscemi, M.; Cardillo, M.; Caroff, S.; Carosi, A.; Cassol, F.; Cauz, D.; Ceribella, G.; Chai, Y.; Cheng, K.; Chiavassa, A.; Chikawa, M.; Chytka, L.; Cifuentes, A.; Contreras, J. L.; Cortina, J.; Costantini, H.; D'Amico, G.; Dalchenko, M.; De Angelis, A.; de Bony de Lavergne, M.; De Lotto, B.; de Menezes, R.; Deleglise, G.; Delgado, C.; Delgado Mengual, J.; della Volpe, D.; Dellaiera, M.; Di Piano, A.; Di Pierro, F.; Di Tria, R.; Di Venere, L.; Díaz, C.; Dominik, R. M.; Dominis Prester, D.; Donini, A.; Dorner, D.; Doro, M.; Elsässer, D.; Emery, G.; Escudero, J.; Fallah Ramazani, V.; Ferrara, G.; Fiasson, A.; Freixas Coromina, L.; Fröse, S.; Fukami, S.; Fukazawa, Y.; Garcia, E.; Garcia López, R.; Gasparrini, D.; Geyer, D.; Giesbrecht Paiva, J.; Giglietto, N.; Giordano, F.; Giro, E.; Gliwny, P.; Godinovic, N.; Grau, R.; Green, D.; Green, J.; Gunji, S.; Hackfeld, J.; Hadasch, D.; Hahn, A.; Hashiyama, K.; Hassan, T.; Hayashi, K.; Heckmann, L. et al.
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Astronomy and Astrophysics

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Context. Several new ultrahigh-energy (UHE) γ-ray sources have recently been discovered by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. These represent a step forward in the search for the so-called Galactic PeVatrons, the enigmatic sources of the Galactic cosmic rays up to PeV energies. However, it has been shown that multi-TeV γ-ray emission does not necessarily prove the existence of a hadronic accelerator in the source; indeed this emission could also be explained as inverse Compton scattering from electrons in a radiation-dominated environment. A clear distinction between the two major emission mechanisms would only be made possible by taking into account multi-wavelength data and detailed morphology of the source.
Aims: We aim to understand the nature of the unidentified source LHAASO J2108+5157, which is one of the few known UHE sources with no very high-energy (VHE) counterpart.
Methods: We observed LHAASO J2108+5157 in the X-ray band with XMM-Newton in 2021 for a total of 3.8 hours and at TeV energies with the Large-Sized Telescope prototype (LST-1), yielding 49 hours of good-quality data. In addition, we analyzed 12 years of Fermi-LAT data, to better constrain emission of its high-energy (HE) counterpart 4FGL J2108.0+5155. We used naima and jetset software packages to examine the leptonic and hadronic scenario of the multi-wavelength emission of the source.
Results: We found an excess (3.7σ) in the LST-1 data at energies E > 3 TeV. Further analysis of the whole LST-1 energy range, assuming a point-like source, resulted in a hint (2.2σ) of hard emission, which can be described with a single power law with a photon index of Γ = 1.6 ± 0.2 the range of 0.3 − 100 TeV. We did not find any significant extended emission that could be related to a supernova remnant (SNR) or pulsar wind nebula (PWN) in the XMM-Newton data, which puts strong constraints on possible synchrotron emission of relativistic electrons. We revealed a new potential hard source in Fermi-LAT data with a significance of 4σ and a photon index of Γ = 1.9 ± 0.2, which is not spatially correlated with LHAASO J2108+5157, but including it in the source model we were able to improve spectral representation of the HE counterpart 4FGL J2108.0+5155.
Conclusions: The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100−30+70 TeV. The low magnetic field in the source imposed by the X-ray upper limits on synchrotron emission is compatible with a hypothesis of a PWN or a TeV halo. Furthermore, the spectral properties of the HE counterpart are consistent with a Geminga-like pulsar, which would be able to power the VHE-UHE emission. Nevertheless, the lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE γ rays can also be explained as π0 decay-dominated hadronic emission due to interaction of relativistic protons with one of the two known molecular clouds in the direction of the source. Indeed, the hard spectrum in the LST-1 band is compatible with protons escaping a shock around a middle-aged SNR because of their high low-energy cut-off, but the origin of the HE γ-ray emission remains an open question.
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The MAGIC Collaboration is integrated by 20 research institutes and university departments from Armenia, Bulgaria, Finland, Germany, Italy, Poland, Spain, Switzerland and USA. The collaboration comprises two 17m diameter telescopes, located at the Roque de los Muchachos Observatory, designed to measure the Cherenkov radiation associated with
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