Fermi-LAT Discovery of Extended Gamma-ray Emission in the Direction of Supernova Remnant W51C

TL;DR

This paper reports the discovery of extended gamma-ray emission from supernova remnant W51C using Fermi LAT, indicating efficient cosmic ray acceleration and shock-cloud interactions, and providing insights into the origin of Galactic cosmic rays.

Contribution

First detection of spatially extended gamma-ray emission from W51C with Fermi LAT, supporting hadronic origin and cosmic ray acceleration in SNRs.

Findings

Gamma-ray emission is spatially extended and consistent with W51C's radio/X-ray extent.

Gamma-ray spectrum shows steepening at high energies.

Luminosity exceeds 10^36 erg/s, making W51C one of the brightest gamma-ray sources in the Galaxy.

Abstract

The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant (~10^4 yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the 0.2-50 GeV band exhibits steepening toward high energies. The luminosity is greater than 1x10^{36} erg/s given the distance constraint of D>5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shock-cloud interactions. The decay of neutral pi-mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to 5x10^{51}(D/6kpc)^2 erg/cm^3. Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays.