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arXiv · 2508.01934

HAWC, VERITAS, Fermi-LAT and XMM-Newton follow-up observations of the unidentified ultra-high-energy gamma-ray source LHAASO J2108+5157

The VERITAS collaboration·C. B. Adams·P. Bangale·W. Benbow·J. H. Buckley·Y. Chen·J. L. Christiansen·A. J. Chromey·M. Escobar Godoy·S. Feldman·Q. Feng·J. Foote·L. Fortson·A. Furniss·W. Hanlon·O. Hervet·C. E. Hinrichs·J. Holder·Z. Hughes·T. B. Humensky·W. Jin·P. Kaaret·M. Kertzman·M. Kherlakian·D. Kieda·T. K. Kleiner·N. Korzoun·S. Kumar·M. J. Lang·M. Lundy·G. Maier·M. J. Millard·P. Moriarty·R. Mukherjee·W. Ning·R. A. Ong·M. Pohl·E. Pueschel·J. Quinn·P. L. Rabinowitz·K. Ragan·P. T. Reynolds·D. Ribeiro·E. Roache·I. Sadeh·L. Saha·G. H. Sembroski·R. Shang·D. Tak·A. K. Talluri·J. V. Tucci·J. Valverde·D. A. Williams·S. L. Wong·J. Woo The HAWC collaboration·:·R. Alfaro·C. Alvarez·J. C. Arteaga-Vel'azquez·D. Avila Rojas·R. Babu·E. Belmont-Moreno·A. Bernal·K. S. Caballero-Mora·A. Carramiñana·S. Casanova·U. Cotti·J. Cotzomi·E. De la Fuente·C. de León·D. Depaoli·P. Desiati·N. Di Lalla·R. Diaz Hernandez·M. A. DuVernois·K. Engel·T. Ergin·C. Espinoza·K. L. Fan·N. Fraija·S. Fraija·J. A. García-González·F. Garfias·A. Gonzalez Muñoz·M. M. González·J. A. Goodman·S. Groetsch·J. P. Harding·S. Hernández-Cadena·I. Herzog·D. Huang·F. Hueyotl-Zahuantitla·P. Hüntemeyer·A. Iriarte·S. Kaufmann·A. Lara·J. Lee·H. León Vargas·A. L. Longinotti·G. Luis-Raya
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Abstract

We report observations of the ultra-high-energy gamma-ray source LHAASO J2108$+$5157, utilizing VERITAS, HAWC, Fermi-LAT, and XMM-Newton. VERITAS has collected $\sim$ 40 hours of data that we used to set ULs to the emission above 200 GeV. The HAWC data, collected over $\sim 2400$ days, reveal emission between 3 and 146 TeV, with a significance of $7.5~\sigma$, favoring an extended source model. The best-fit spectrum measured by HAWC is characterized by a simple power-law with a spectral index of $2.45\pm0.11_{stat}$. Fermi-LAT analysis finds a point source with a very soft spectrum in the LHAASO J2108+5157 region, consistent with the 4FGL-DR3 catalog results. The XMM-Newton analysis yields a null detection of the source in the 2 - 7 keV band. The broadband spectrum can be interpreted as a pulsar and a pulsar wind nebula system, where the GeV gamma-ray emission originates from an unidentified pulsar, and the X-ray and TeV emission is attributed to synchrotron radiation and inverse Compton scattering of electrons accelerated within a pulsar wind nebula. In this leptonic scenario, our X-ray upper limit provides a stringent constraint on the magnetic field, which is $\lesssim 1.5\ \mu$G.

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The VERITAS collaboration, C. B. Adams, P. Bangale, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. J. Chromey, M. Escobar Godoy, S. Feldman, Q. Feng, J. Foote, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, Z. Hughes, T. B. Humensky, W. Jin, P. Kaaret, M. Kertzman, M. Kherlakian, D. Kieda, T. K. Kleiner, N. Korzoun, S. Kumar, M. J. Lang, M. Lundy, G. Maier, M. J. Millard, P. Moriarty, R. Mukherjee, W. Ning, R. A. Ong, M. Pohl, E. Pueschel, J. Quinn, P. L. Rabinowitz, K. Ragan, P. T. Reynolds, D. Ribeiro, E. Roache, I. Sadeh, L. Saha, G. H. Sembroski, R. Shang, D. Tak, A. K. Talluri, J. V. Tucci, J. Valverde, D. A. Williams, S. L. Wong, J. Woo The HAWC collaboration, :, R. Alfaro, C. Alvarez, J. C. Arteaga-Vel'azquez, D. Avila Rojas, R. Babu, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, A. Carramiñana, S. Casanova, U. Cotti, J. Cotzomi, E. De la Fuente, C. de León, D. Depaoli, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, K. Engel, T. Ergin, C. Espinoza, K. L. Fan, N. Fraija, S. Fraija, J. A. García-González, F. Garfias, A. Gonzalez Muñoz, M. M. González, J. A. Goodman, S. Groetsch, J. P. Harding, S. Hernández-Cadena, I. Herzog, D. Huang, F. Hueyotl-Zahuantitla, P. Hüntemeyer, A. Iriarte, S. Kaufmann, A. Lara, J. Lee, H. León Vargas, A. L. Longinotti, G. Luis-Raya. 2025-08-03. HAWC, VERITAS, Fermi-LAT and XMM-Newton follow-up observations of the unidentified ultra-high-energy gamma-ray source LHAASO J2108+5157. https://arxiv.org/abs/2508.01934

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