The Fermi-LAT Collaboration
We present a detailed analysis of the GeV gamma-ray emission toward the
supernova remnant (SNR) G8.7-0.1 with the Large Area Telescope (LAT) onboard
the \emph{Fermi} Gamma-ray Space Telescope. An investigation of the
relationship among G8.7-0.1 and the TeV unidentified source HESS J1804-216
provides us with an important clue on diffusion process of cosmic rays if
particle acceleration operates in the SNR. The GeV gamma-ray emission is
extended with most of the emission in positional coincidence with the SNR
G8.7-0.1 and a lesser part located outside the western boundary of G8.7-0.1.
The region of the gamma-ray emission overlaps spatially-connected molecular
clouds, implying a physical connection for the gamma-ray structure. The total
gamma-ray spectrum measured with LAT from 200 MeV--100 GeV can be described by
a broken power-law function with a break of 2.4 $\pm$ 0.6 (stat) $\pm$ 1.2
(sys) GeV, and photon indices of 2.10 $\pm$ 0.06 (stat) $\pm$ 0.10 (sys) below
the break and 2.70 $\pm$ 0.12 (stat) $\pm$ 0.14 (sys) above the break. Given
the spatial association among the gamma rays, the radio emission of G8.7$-$0.1,
and the molecular clouds, the decay of $\pi^{0}$s produced by particles
accelerated in the SNR and hitting the molecular clouds naturally explains the
GeV gamma-ray spectrum. We also find that the GeV morphology is not well
represented by the TeV emission from HESS J1804$-$216 and that the spectrum in
the GeV band is not consistent with the extrapolation of the TeV gamma-ray
spectrum. The spectral index of the TeV emission is consistent with the
particle spectral index predicted by a theory that assumes energy-dependent
diffusion of particles accelerated in an SNR. We discuss the possibility that
the TeV spectrum originates from the interaction of particles accelerated in
G8.7$-$0.1 with molecular clouds, and we constrain the diffusion coefficient of
the particles.
View original:
http://arxiv.org/abs/1109.3017
No comments:
Post a Comment