Detection of two TeV shell-type remnants at GeV energies with Fermi-LAT: HESS J1731-347 and SN 1006

Abstract: We report the first high-significance GeV gamma-ray detections of supernova remnants HESS J1731-347 and SN 1006, both of which have been previously detected by imaging atmospheric Cherenkov Telescopes above 1 TeV. Using 8 years of Fermi Pass 8 data at energies between 1 GeV and 2 TeV, we detect emission at the position of HESS J1731-347 with a significance of $\sim 5\sigma$ and a spectral index of $\Gamma = 1.66 \pm 0.16_{\rm stat} \pm 0.12_{\rm syst}$. The hardness of the index and the good connection with the TeV spectrum of HESS J1731-347 support an association between the two sources. We also confirm the detection of SN 1006 at $\sim 6\sigma$ with a spectral index of $\Gamma = 1.79 \pm 0.17_{\rm stat} \pm 0.27_{\rm syst}$. The northeast (NE) and southwest (SW) limbs of SN 1006 were also fit separately, resulting in the detection of the NE region ($\Gamma = 1.47 \pm 0.26_{\rm stat}$) and the non-detection of the SW region. The significance of different spectral components for the two limbs is $3.6\sigma$, providing first indications of an asymmetry in the GeV $\gamma$-ray emission.