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Title: Revealing W51C as a Cosmic-Ray source using Fermi-LAT data

Here, supernova remnants (SNRs) are commonly believed to be the primary sources of Galactic cosmic rays. Despite intensive study of the non-thermal emission of many SNRs the identification of the accelerated particle type relies heavily on assumptions of ambient-medium parameters that are only loosely constrained. Compelling evidence of hadronic acceleration can be provided by detecting a strong roll-off in the secondary γ-ray spectrum below the $${\pi }^{0}$$ production threshold energy of about 135 MeV, the so called "pion bump." Here we use five years of Fermi-Large Area Telescope data to study the spectrum above 60 MeV of the middle-aged SNR W51C. A clear break in the power-law γ-ray spectrum at $${E}_{{\rm{break}}}=290\pm 20\;{\rm{MeV}}$$ is detected with $$9\sigma $$ significance and we show that this break is most likely associated with the energy production threshold of $${\pi }^{0}$$mesons. A high-energy break in the γ-ray spectrum at about 2.7 GeV is found with $$7.5\sigma $$ significance. The spectral index at energies beyond this second break is $${{\rm{\Gamma }}}_{2}={2.52}_{-0.07}^{+0.06}$$ and closely matches the spectral index derived by the MAGIC Collaboration above 75 GeV. Therefore our analysis provides strong evidence to explain the γ-ray spectrum of W51C by a single particle population of protons with a momentum spectrum best described by a broken power law with break momentum $${p}_{{\rm{break}}}\sim 80\;{\rm{G}}{\rm{e}}{\rm{V}}/c.$$ W51C is the third middle-aged SNR that displays compelling evidence for cosmic-ray acceleration and thus strengthens the case of SNRs as the main source of Galactic cosmic rays.
Authors:
 [1] ;  [1]
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States); Erlangen Centre for Astroparticle Physics, Erlangen (Germany)
Publication Date:
OSTI Identifier:
1237020
Report Number(s):
SLAC-PUB-16468
Journal ID: ISSN 1538-4357
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 816; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astrophysics; ASTRO; astroparticle physics; gamma-rays: general; ISM: supernova remnants