MGRO J1908+06, an unidentified PeVatron in the Galactic plane
Abstract
The spectrum of galactic cosmic rays (CRs) can be described, in good approximation, with a single power-law up to energies of about 1 PeV ($10^{15}$ eV), meaning that there must be sources in the Galaxy, capable of accelerating protons and nuclei up to these energies (Pevatrons). The association of a $\gamma$-ray source with a PeVatron can be confirmed with a measurement of an exponential cut-off in the $\gamma$-ray spectrum at a few 100 TeV. This feature is expected when CRs are accelerated up to PeV energies and produce $\gamma$-rays interacting with the ambient material. Among the galactic PeVatrons, one of the best candidates is MGRO J1908+06, a remarkable source for its hard spectrum extending up to 100 TeV and beyond, as observed by HAWC and recently confirmed by LHAASO, and with no evidence of a cutoff. It was discovered by Milagro and later confirmed also by HESS and VERITAS that detected three significant emission region in the field. Due to the complexity of the morphological structure of the source and to the limited angular resolution of instruments, the origin of its $\gamma$-ray emission has not yet been unambiguously identified. A few counterparts (the SNR G40.5-0.5, dense molecular clouds illuminated by cosmic rays from the nearby SNR and/or the pulsar PSR J1907+0602) are compatible with the $\gamma$-ray error box ($\sim$0.5$^\circ$), preventing a secure identification of this extreme accelerator and making it difficult to distinguish between an hadronic or leptonic nature of the emission. We have performed a multi-wavelength study of this source, using CO data as well as X-ray (XMM, Chandra) and Fermi LAT data, in order to confirm its PeVatron nature. Our results show that only an hadronic scenario can explain the whole set of multi-wavelength data. We also demonstrated with simulations that future observations in the 10-100 TeV band with the ASTRI Mini-Array can confirm the Pevatron scenario and likely allow us to identify the counterpart thanks to its good angular resolution ($\sim$0.1$^\circ$).
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1387C