On the Gamma-Ray Emission of W44 and Its Surroundings

We present the analysis of 9.7 yr Fermi-LAT data of the middle-aged supernova remnant W44 and the massive molecular gas complex that surrounds it. We derived a high-quality spectral energy distribution of gamma-radiation of the shell over three decades. The very hard spectrum below 1 GeV supports the earlier claims regarding the hadronic origin of radiation. We also confirm the presence of two extended γ-ray structures located at two opposite edges of the remnant along its major axis. Based on the high-resolution gas maps, we demonstrate that the gamma-ray structures are caused by the enhanced cosmic-ray density rather than the gradient of the gas distribution. We argue that the revealed cosmic-ray "clouds" suggest an anisotropic character of the escape of high-energy particles from the shell along the magnetic field of the remnant.