Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157
Antonelli, L. A.; Fukazawa, Y.; Gasparrini, D.; Giglietto, N.; Giordano, F.; Hadasch, D.; Katagiri, H.; Kataoka, J.; Longo, F.; Mizuno, T.; Morselli, A.; Paneque, D.; Rando, R.; Takahashi, H.; Torres, D. F.; Tramacere, A.; Vitale, V.; Okumura, A.; Walter, R.; Pietropaolo, E.; Eckert, D.; Hütten, M.; Takata, J.; Chiavassa, A.; Horns, D.; Tluczykont, M.; Luque-Escamilla, P. L.; Lamanna, G.; Teshima, M.; Paredes, J. M.; Tam, P. H. T.; Bamba, A.; Bissaldi, E.; Di Venere, L.; Agudo, I.; Gunji, S.; Karas, V.; Terada, Y.; Martí, J.; Barres de Almeida, U.; Barrio, J. A.; Becerra González, J.; Bernardini, E.; Berti, A.; Blanch, O.; Carosi, A.; Contreras, J. L.; Cortina, J.; De Angelis, A.; De Lotto, B.; Di Pierro, F.; Dominis Prester, D.; Dorner, D.; Doro, M.; Fallah Ramazani, V.; Hahn, A.; Hassan, T.; Hrupec, D.; Kubo, H.; Kushida, J.; López-Coto, R.; Majumdar, P.; Makariev, M.; Manganaro, M.; Mannheim, K.; Mariotti, M.; Martínez, M.; Mazin, D.; Mirzoyan, R.; Moralejo, A.; Nishijima, K.; Noda, K.; Palatiello, M.; Paoletti, R.; Prandini, E.; Rhode, W.; Ribó, M.; Saito, T.; Schweizer, T.; Sitarek, J.; Strzys, M.; Temnikov, P.; Vovk, I.; Will, M.; Balbo, M.; Fiasson, A.; Bulgarelli, A.; Bordas, P.; Maurin, G.; Sanchez, D. A.; Vuillaume, T.; Nakamori, T.; Tajima, H.; Lamastra, A.; Bigongiari, C.; Ceribella, G.; Chai, Y.; Donini, A.; Ferrara, G.; Fukami, S.; Green, D.; Inada, T.; Ishio, K.; Iwamura, Y.; Kerszberg, D.; López-Oramas, A.; Loporchio, S.; Miceli, D.; Miranda, J. M.; Molina, E.; Morcuende, D.; Nozaki, S.; Sakurai, S.; Suda, Y.; Takahashi, M.; Vigorito, C. F.; Asano, K.; Kong, A.; Saturni, F. G.; Baroncelli, L.; Cardillo, M.; Di Piano, A.; Travnicek, P.; Orito, R.; Yamazaki, R.; Aubert, P.; Hayashi, K.; Giro, E.; Yoshida, T.; Godinovic, N.; Kagaya, M.; de Menezes, R.; Martinez, O.; Montaruli, T.; Caroff, S.; Emery, G.; Poireau, V.; Pons, E.; Tutone, A.; Delgado, C.; Abe, S.; Artero, M.; Baxter, J.; Burelli, I.; Cifuentes, A.; D'Amico, G.; Escudero, J.; Gliwny, P.; Grau, R.; Heckmann, L.; Imazawa, R.; Kobayashi, Y.; López-Moya, M.; Lorini, A.; Mas-Aguilar, A.; Miener, T.; Ohtani, Y.; Otero-Santos, J.; Pirola, G.; Podobnik, F.; Priyadarshi, C.; Schleicher, B.; Schmuckermaier, F.; Schubert, J. L.; Spolon, A.; Strišković, J.; Takeishi, R.; Terauchi, K.; Truzzi, S.; Verguilov, V.; Viale, I.; Yamamoto, T.; Pavletić, L.; Green, J.; Murase, K.; Garcia, E.; Elsässer, D.; Tanaka, S. J.; Alvarez Crespo, N.; Díaz, C.; Linhoff, M.; Sliusar, V.; Ohishi, M.; Tokanai, F.; Yoshikoshi, T.; Aguasca-Cabot, A.; Aramo, C.; Arbet-Engels, A.; Baktash, A.; Baquero Larriva, A.; Batkovic, I.; Bernardos, M. I.; Bernete Medrano, J.; Bhattacharjee, P.; Biederbeck, N.; Buisson, C.; Buscemi, M.; Cassol, F.; Cauz, D.; Cheng, K.; Chikawa, M.; Chytka, L.; Costantini, H.; Dalchenko, M.; de Bony de Lavergne, M.; Deleglise, G.; Delgado Mengual, J.; della Volpe, D.; Dellaiera, M.; Di Tria, R.; Dominik, R. M.; Freixas Coromina, L.; Fröse, S.; Garcia López, R.; Geyer, D.; Giesbrecht Paiva, J.; Hackfeld, J.; Hashiyama, K.; Heller, M.; Herrera Llorente, J.; Hirotani, K.; Hoffmann, D.; Houles, J.; Hrabovsky, M.; Hui, D.; Inome, Y.; Ioka, K.; Iori, M.; Jacquemont, M.; Jimenez Martinez, I.; Jurysek, J.; Lainez, M.; Le Flour, T.; Mandat, D.; Manicò, G.; Marquez, P.; Marsella, G.; Martínez, G.; Marusevec, P.; Mestre Guillen, E.; Micanovic, S.; Molero Gonzalez, M.; Monteiro, I.; Mrakovcic, K.; Nagai, A.; Nickel, L.; Nievas, M.; Nosek, D.; Okazaki, N.; Pantaleo, F. R.; Pech, M.; Pecimotika, M.; Polo, M.; Prast, J.; Prouza, M.; Rizi, V.; Rodriguez Fernandez, G.; Šarić, T.; Scherpenberg, J.; Schussler, F.; Seglar Arroyo, M.; Sunada, Y.; Tateishi, D.; Terzic, T.; Uhlrich, G.; Vacula, M.; Vázquez Acosta, M.; Vigliano, A.; Voutsinas, G.; Zywucka (CTA-LST Project), N.
Japan, Spain, Italy, Germany, France, Brazil, Czech Republic, Norway, Switzerland, Croatia, Poland, India, Bulgaria
Abstract
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.