Fermi-LAT Discovery of GeV Gamma-ray Emission from the Vicinity of SNR W44

TL;DR

This paper reports the detection of GeV gamma-ray emission from the molecular cloud surrounding SNR W44, indicating cosmic ray escape and supporting the idea that supernova remnants are primary sources of Galactic cosmic rays.

Contribution

First detection of gamma-ray emission from molecular clouds around SNR W44 attributed to escaped cosmic rays, providing evidence for supernova remnants as cosmic ray sources.

Findings

Gamma-ray emission from the molecular cloud complex is due to hadronic interactions.

Estimated energy in escaping cosmic rays is (0.3--3)×10^{50} erg.

Supports supernova remnants as main Galactic cosmic ray sources.

Abstract

We report the detection of GeV gamma-ray emission from the molecular cloud complex that surrounds the supernova remnant (SNR) W44 using the Large Area Telescope (LAT) onboard Fermi. While the previously reported gamma-ray emission from SNR W44 is likely to arise from the dense radio-emitting filaments within the remnant, the gamma-ray emission that appears to come from the surrounding molecular cloud complex can be ascribed to the cosmic rays (CRs) that have escaped from W44. The non-detection of synchrotron radio emission associated with the molecular cloud complex suggests the decay of neutral pi mesons produced in hadronic collisions as the gamma-ray emission mechanism. The total kinetic energy channeled into the escaping CRs is estimated to be (0.3--3)x10^{50} erg, in broad agreement with the conjecture that SNRs are the main sources of Galactic CRs.