Search for Galactic Pevatron candidates in a population of unidentified γ-ray sources

Abstract

Aims. A list of Pevatron candidates is presented to enable deeper observations and dedicated analyses. Methods. Lower limits on the energy cutoff for unidentified γ-ray sources detected in the High Energy Stereoscopic System (HESS) Galactic plane survey were derived. Additional public data from the Very Energetic Radiation Imaging Telescope Array System, HESS, and Milagro experiments were used for MGRO J1908+06 to confirm the limit derived from the HESS Galactic plane survey data and to enable further conclusions on the presence of spectral breaks. Results. Five Pevatron candidates are identified in the HESS Galactic plane survey. The cutoff of the γ-ray spectrum for these sources is larger than 20 TeV at 90% confidence level. The γ-ray sources MGRO J1908+06 and HESS J1641−463, found to be Pevatron candidates in the analysis of the HESS Galactic plane survey catalog, have already been discussed as Pevatron candidates. For MGRO J1908+06, the lower limit on the γ-ray energy cutoff is 30 TeV at 90% confidence level. This is a factor of almost two larger than previous results. Additionally, a break in the γ-ray spectrum at energies between 1 TeV and 10 TeV with an index change ΔΓ >  0.5 can be excluded at 90% confidence level for MGRO J1908+06. The energy cutoff of accelerated particles is larger than 100 TeV at 90% confidence level in a hadronic scenario for all five Pevatron candidates. A hadronic scenario is plausible for at least three of the Pevatron candidates, based on the presence of nearby molecular clouds and supernova remnants.