Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

Abe, S.; Aguasca-Cabot, A.; Agudo, I.; Alvarez Crespo, N.; Antonelli, L. A. search by orcid; 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. search by orcid; 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. search by orcid; 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. search by orcid; Gunji, S.; Hackfeld, J.; Hadasch, D.; Hahn, A.; Hashiyama, K.; Hassan, T.; Hayashi, K.; Heckmann, L.; Heller, M.; Herrera Llorente, J.; Hirotani, K.; Hoffmann, D.; Horns, D.; Houles, J.; Hrabovsky, M.; Hrupec, D.; Hui, D.; Hütten, M.; Imazawa, R.; Inada, T.; Inome, Y.; Ioka, K.; Iori, M.; Ishio, K.; Iwamura, Y.; Jacquemont, M.; Jimenez Martinez, I.; Jurysek, J.; Kagaya, M.; Karas, V.; Katagiri, H.; Kataoka, J.; Kerszberg, D.; Kobayashi, Y.; Kong, A.; Kubo, H.; Kushida, J.; Lainez, M.; Lamanna, G.; Lamastra, A.; Le Flour, T.; Linhoff, M.; Longo, F.; López-Coto, R.; López-Moya, M.; López-Oramas, A.; Loporchio, S.; Lorini, A.; Luque-Escamilla, P. L.; Majumdar, P.; Makariev, M.; Mandat, D.; Manganaro, M.; Manicò, G.; Mannheim, K.; Mariotti, M.; Marquez, P.; Marsella, G.; Martí, J.; Martinez, O.; Martínez, G.; Martínez, M.; Marusevec, P.; Mas-Aguilar, A.; Maurin, G.; Mazin, D.; Mestre Guillen, E.; Micanovic, S.; Miceli, D.; Miener, T.; Miranda, J. M.; Mirzoyan, R.; Mizuno, T.; Molero Gonzalez, M.; Molina, E.; Montaruli, T.; Monteiro, I.; Moralejo, A.; Morcuende, D.; Morselli, A.; Mrakovcic, K.; Murase, K.; Nagai, A.; Nakamori, T.; Nickel, L.; Nievas, M.; Nishijima, K.; Noda, K.; Nosek, D.; Nozaki, S.; Ohishi, M.; Ohtani, Y.; Okazaki, N.; Okumura, A.; Orito, R.; Otero-Santos, J.; Palatiello, M.; Paneque, D.; Pantaleo, F. R.; Paoletti, R.; Paredes, J. M.; Pavletić, L.; Pech, M.; Pecimotika, M.; Pietropaolo, E.; Pirola, G.; Podobnik, F.; Poireau, V.; Polo, M.; Pons, E.; Prandini, E.; Prast, J.; Priyadarshi, C.; Prouza, M.; Rando, R.; Rhode, W.; Ribó, M.; Rizi, V.; Rodriguez Fernandez, G.; Saito, T.; Sakurai, S.; Sanchez, D. A.; Šarić, T.; Saturni, F. G.; Scherpenberg, J.; Schleicher, B.; Schmuckermaier, F.; Schubert, J. L.; Schussler, F.; Schweizer, T.; Seglar Arroyo, M.; Sitarek, J.; Sliusar, V.; Spolon, A.; Strišković, J.; Strzys, M.; Suda, Y.; Sunada, Y.; Tajima, H.; Takahashi, M.; Takahashi, H.; Takata, J.; Takeishi, R.; Tam, P. H. T.; Tanaka, S. J.; Tateishi, D.; Temnikov, P.; Terada, Y. search by orcid; Terauchi, K.; Terzic, T.; Teshima, M.; Tluczykont, M.; Tokanai, F.; Torres, D. F.; Travnicek, P.; Truzzi, S.; Tutone, A.; Uhlrich, G.; Vacula, M.; Vázquez Acosta, M.; Verguilov, V.; Viale, I.; Vigliano, A.; Vigorito, C. F.; Vitale, V.; Voutsinas, G.; Vovk, I.; Vuillaume, T.; Walter, R. search by orcid; Will, M.; Yamamoto, T.; Yamazaki, R.; Yoshida, T.; Yoshikoshi, T.; Zywucka, N.; Balbo, M.; Eckert, D.; Tramacere, A.

doi:10.1051/0004-6361/202245086

LHAASO J2108+5157 is one of the few known unidentified Ultra-High-Energy (UHE) gamma-ray sources with no Very-High-Energy (VHE) counterpart, recently discovered by the LHAASO collaboration. 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 found an excess (3.7 sigma) in the LST-1 data at energies E > 3 TeV. Further analysis in the whole LST-1 energy range assuming a point-like source, resulted in a hint (2.2 sigma) of hard emission which can be described with a single power law with photon index Gamma = 1.6 +- 0.2 between 0.3 - 100 TeV. We did not find any significant extended emission which 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. The LST-1 and LHAASO observations can be explained as inverse Compton-dominated leptonic emission of relativistic electrons with a cutoff energy of 100+70−30 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. The lack of a pulsar in the neighborhood of the UHE source is a challenge to the PWN/TeV-halo scenario. The UHE gamma 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. 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.