REVEALING W51C AS A COSMIC RAY SOURCE USING FERMI-LAT DATA

Abstract

ABSTRACT 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 π 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 break = 290 ± 20 MeV ?> is detected with 9 &sgr; ?> significance and we show that this break is most likely associated with the energy production threshold of π 0 ?> mesons. A high-energy break in the γ-ray spectrum at about 2.7 GeV is found with 7.5 &sgr; ?> significance. The spectral index at energies beyond this second break is Γ 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 break ∼ 80 G e 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.