Testing the molecular cloud paradigm for ultra-high-energy gamma ray emission from the direction of SNR G106.3+2.7
Authors: Alfaro R. , Alvarez C. , Arteaga-Velázquez J. C. , Avila Rojas D. , Ayala Solares H. A. , Babu R. , Belmont-Moreno E. , Bernal A. , Caballero-Mora K. S. , Capistrán T. , Carramiñana A. , Casanova S. , Cotti U. , Cotzomi J. , Coutiño de León S. , De la Fuente E. , de León C. , Depaoli D. , Desiati P. , Di Lalla N. , Diaz Hernandez R. , Dingus B. L. , DuVernois M. A. , Engel K. , Ergin T. , Espinoza C. , Fan K. L. , Fang K. , Fraija N. , Fraija S. , García-González J. A. , Garfias F. , González M. M. , Goodman J. A. , Groetsch S. , Harding J. P. , Hernández-Cadena S. , Herzog I. , Hinton J. , Huang D. , Hueyotl-Zahuantitla F. , Humensky T. B. , Hüntemeyer P. , Kaufmann S. , Kieda D. , Lee W. H. , Lee J. , León Vargas H. , Linnemann J. T. , Longinotti A. L. , Luis-Raya G. , Malone K. , Martinez O. , Martínez-Castro J. , Matthews J. A. , Miranda-Romagnoli P. , Montes J. A. , Moreno E. , Mostafá M. , Najafi M. , Nellen L. , Nisa M. U. , Olivera-Nieto L. , Omodei N. , Pérez Araujo Y. , Pérez-Pérez E. G. , Rho C. D. , Rosa-González D. , Salazar H. , Salazar-Gallegos D. , Sandoval A. , Schneider M. , Serna-Franco J. , Smith A. J. , Son Y. , Springer R. W. , Tibolla O. , Tollefson K. , Torres I. , Torres-Escobedo R. , Turner R. , Ureña-Mena F. , Varela E. , Villaseñor L. , Wang X. , Wang Z. , Watson I. J. , Willox E. , Yu S. , Yun-Cárcamo S. , Zhou H. 
Published: September 2024

Context. Supernova remnants (SNRs) are believed to be capable of accelerating cosmic rays (CRs) to PeV energies. SNR G106.3+2.7 is a prime PeVatron candidate. It is formed by a head region, where the pulsar J2229+6114 and its boomerang-shaped pulsar wind nebula are located, and a tail region containing SN ejecta. The lack of observed gamma ray emission from the two regions of this SNR has made it difficult to assess which region would be responsible for the PeV CRs.

Aims. We aim to characterize the very-high-energy (VHE, 0.1–100 TeV) gamma ray emission from SNR G106.3+2.7 by determining the morphology and spectral energy distribution of the region. This is accomplished using 2565 days of data and improved reconstruction algorithms from the High Altitude Water Cherenkov (HAWC) Observatory. We also explore possible gamma ray production mechanisms for different energy ranges.

Methods. Using a multi-source fitting procedure based on a maximum-likelihood estimation method, we evaluate the complex nature of this region. We determine the morphology, spectrum, and energy range for the source found in the region. Molecular cloud information is also used to create a template and evaluate the HAWC gamma ray spectral properties at ultra-high-energies (UHE, > 56 TeV). This will help probe the hadronic nature of the highest-energy emission from the region.

Results. We resolve one extended source coincident with all other gamma ray observations of the region. The emission reaches above 100 TeV and its preferred log-parabola shape in the spectrum shows a flux peak in the TeV range. The molecular cloud template fit on the higher energy data reveals that the SNR’s energy budget is fully capable of producing a purely hadronic source for UHE gamma rays.

Conclusions. The HAWC observatory resolves one extended source between the head and the tail of SNR G106.3+2.7 in the VHE gamma ray regime. The template fit suggests the highest energy gamma rays could come from a hadronic origin. However, the leptonic scenario, or a combination of the two, cannot be excluded at this time.