Context. Several new ultrahigh-energy (UHE) gamma-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 gamma-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 (3.7 sigma) 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 (2.2 sigma) of hard emission, which can be described with a single power law with a photon index of Gamma= 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 4 sigma and a photon index of Gamma = 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 cuto ff 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 gamma rays can also be explained as pi(0) 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 gamma-ray emission remains an open question.

Multiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157

A. Chiavassa;R. de Menezes;C. F. Vigorito;A. Tramacere
2023-01-01

Abstract

Context. Several new ultrahigh-energy (UHE) gamma-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 gamma-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 (3.7 sigma) 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 (2.2 sigma) of hard emission, which can be described with a single power law with a photon index of Gamma= 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 4 sigma and a photon index of Gamma = 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 cuto ff 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 gamma rays can also be explained as pi(0) 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 gamma-ray emission remains an open question.
2023
673
A75
_
gamma rays: general; radiation mechanisms: non-thermal; pulsars: general; ISM: individual objects: LHAASO J2108+5157
S. Abe; A. Aguasca-Cabot; I. Agudo; N. Alvarez Crespo; L. A. Antonelli; C. Aramo; A. Arbet-Engels; M. Artero; K. Asano; P. Aubert; A. Baktash; A. Bamba; A. Baquero Larriva; L. Baroncelli; U. Barres de Almeida; J. A. Barrio; I. Batkovic; J. Baxter; J. Becerra Gonz??lez; E. Bernardini; M. I. Bernardos; J. Bernete Medrano; A. Berti; P. Bhattacharjee; N. Biederbeck; C. Bigongiari; E. Bissaldi; O. Blanch; P. Bordas; C. Buisson; A. Bulgarelli; I. Burelli; M. Buscemi; M. Cardillo; S. Caroff; A. Carosi; F. Cassol; D. Cauz; G. Ceribella; Y. Chai; K. Cheng; A. Chiavassa; M. Chikawa; L. Chytka; A. Cifuentes; J. L. Contreras; J. Cortina; H. Costantini; G. D???Amico; M. Dalchenko; A. De Angelis; M. de Bony de Lavergne; B. De Lotto; R. de Menezes; G. Deleglise; C. Delgado; J. Delgado Mengual; D. della Volpe; M. Dellaiera; A. Di Piano; F. Di Pierro; R. Di Tria; L. Di Venere; C. D??az; R. M. Dominik; D. Dominis Prester; A. Donini; D. Dorner; M. Doro; D. Els??sser; G. Emery; J. Escudero; V. Fallah Ramazani; G. Ferrara; A. Fiasson; L. Freixas Coromina; S. Fr??se; S. Fukami; Y. Fukazawa; E. Garcia; R. Garcia L??pez; D. Gasparrini; D. Geyer; J. Giesbrecht Paiva; N. Giglietto; F. Giordano; E. Giro; P. Gliwny; N. Godinovic; R. Grau; D. Green; J. Green; S. Gunji; J. Hackfeld; D. Hadasch; A. Hahn; K. Hashiyama; T. Hassan; K. Hayashi; L. Heckmann; M. Heller; J. Herrera Llorente; K. Hirotani; D. Hoffmann; D. Horns; J. Houles; M. Hrabovsky; D. Hrupec; D. Hui; M. H??tten; R. Imazawa; T. Inada; Y. Inome; K. Ioka; M. Iori; K. Ishio; Y. Iwamura; M. Jacquemont; I. Jimenez Martinez; J. Jurysek; M. Kagaya; V. Karas; H. Katagiri; J. Kataoka; D. Kerszberg; Y. Kobayashi; A. Kong; H. Kubo; J. Kushida; M. Lainez; G. Lamanna; A. Lamastra; T. Le Flour; M. Linhoff; F. Longo; R. L??pez-Coto; M. L??pez-Moya; A. L??pez-Oramas; S. Loporchio; A. Lorini; P. L. Luque-Escamilla; P. Majumdar; M. Makariev; D. Mandat; M. Manganaro; G. Manic??; K. Mannheim; M. Mariotti; P. Marquez; G. Marsella; J. Mart??; O. Martinez; G. Mart??nez; M. Mart??nez; P. Marusevec; A. Mas-Aguilar; G. Maurin; D. Mazin; E. Mestre Guillen; S. Micanovic; D. Miceli; T. Miener; J. M. Miranda; R. Mirzoyan; T. Mizuno; M. Molero Gonzalez; E. Molina; T. Montaruli; I. Monteiro; A. Moralejo; D. Morcuende; A. Morselli; K. Mrakovcic; K. Murase; A. Nagai; T. Nakamori; L. Nickel; M. Nievas; K. Nishijima; K. Noda; D. Nosek; S. Nozaki; M. Ohishi; Y. Ohtani; N. Okazaki; A. Okumura; R. Orito; J. Otero-Santos; M. Palatiello; D. Paneque; F. R. Pantaleo; R. Paoletti; J. M. Paredes; L. Pavleti??; M. Pech; M. Pecimotika; E. Pietropaolo; G. Pirola; F. Podobnik; V. Poireau; M. Polo; E. Pons; E. Prandini; J. Prast; C. Priyadarshi; M. Prouza; R. Rando; W. Rhode; M. Rib??; V. Rizi; G. Rodriguez Fernandez; T. Saito; S. Sakurai; D. A. Sanchez; T. ??ari??; F. G. Saturni; J. Scherpenberg; B. Schleicher; F. Schmuckermaier; J. L. Schubert; F. Schussler; T. Schweizer; M. Seglar Arroyo; J. Sitarek; V. Sliusar; A. Spolon; J. Stri??kovi??; M. Strzys; Y. Suda; Y. Sunada; H. Tajima; M. Takahashi; H. Takahashi; J. Takata; R. Takeishi; P. H. T. Tam; S. J. Tanaka; D. Tateishi; P. Temnikov; Y. Terada; K. Terauchi; T. Terzic; M. Teshima; M. Tluczykont; F. Tokanai; D. F. Torres; P. Travnicek; S. Truzzi; A. Tutone; G. Uhlrich; M. Vacula; M. V??zquez Acosta; V. Verguilov; I. Viale; A. Vigliano; C. F. Vigorito; V. Vitale; G. Voutsinas; I. Vovk; T. Vuillaume; R. Walter; M. Will; T. Yamamoto; R. Yamazaki; T. Yoshida; T. Yoshikoshi; N. Zywucka (CTA-LST Project); M. Balbo; D. Eckert; A. Tramacere
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