Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157
Authors: 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. , 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. , 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. , Will M. , Yamamoto T. , Yamazaki R. , Yoshida T. , Yoshikoshi T. , Zywucka N. , Balbo M. , Eckert D. , Tramacere A. 
Published: March 2023

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.