Analysis of GeV-band $\gamma$-ray emission from supernova remnant RX J1713.7-3946

Context. RX J1713.7–3946 is the brightest shell-type supernova remnant (SNR) of the TeV γ-ray sky. Earlier Fermi-LAT results on low energy γ-ray emission suggested that, despite large uncertainties in the background determination, the spectrum is inconsistent with a hadronic origin.Aims. We update the GeV-band spectra using improved estimates for the diffuse Galactic γ-ray emission and more than double the volume of data. We further investigate the viability of hadronic emission models for RX J1713.7–3946.Methods. We produced a high-resolution map of the diffuse Galactic γ-ray background corrected for the HI self-absorption and used it in the analysis of more than five years worth of Fermi-LAT data. We used hydrodynamic scaling relations and a kinetic transport equation to calculate the acceleration and propagation of cosmic rays in SNR. We then determined spectra of hadronic γ-ray emission from RX J1713.7–3946, separately for the SNR interior and the cosmic-ray precursor region of the forward shock, and computed flux variations that would allow us to test the model with observations.Results. We find that RX J1713.7–3946 is now detected by Fermi-LAT with very high statistical significance, and the source morphology is best described by that seen in the TeV band. The measured spectrum of RX J1713.7–3946 is hard with index γ = 1.53 ± 0.07, and the integral flux above 500 MeV is F = (5.5 ± 1.1) × 10-9 photons cm-2 s-1. We demonstrate that scenarios based on hadronic emission from the cosmic-ray precursor region are acceptable for RX J1713.7–3946, and we predict a secular flux increase at a few hundred GeV at the level of around 15% over ten years, which may be detectable with the upcoming Cherenkov Telescope Array (CTA) observatory