On the gamma-ray emission of W44 and its surroundings

TL;DR

This study analyzes 9.7 years of Fermi-LAT data on supernova remnant W44, revealing a hadronic gamma-ray spectrum, extended structures caused by cosmic-ray clouds, and insights into particle escape mechanisms.

Contribution

It provides detailed spectral analysis and spatial mapping of gamma-ray emission, highlighting cosmic-ray cloud structures and anisotropic particle escape from W44.

Findings

Hard gamma-ray spectrum supports hadronic origin.

Detection of two extended gamma-ray structures.

Cosmic-ray clouds caused by enhanced cosmic-ray density.

Abstract

We present the analysis of 9.7 years 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 $\gamma$-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.