GeV Gamma-Rays from Molecular Clouds Illuminated by Particles Diffusing from the Adjacent Supernova Remnant G335.2+0.1 Confined in an Expanding Bubble

TL;DR

This study detects GeV gamma-ray emission from a supernova remnant interacting with a molecular cloud, revealing how cosmic rays from the remnant produce gamma rays through hadronic interactions in an expanding molecular bubble.

Contribution

It provides the first detailed analysis linking gamma-ray emission to SNR-molecular cloud interaction within an expanding bubble, supported by multi-wavelength data and morphological evidence.

Findings

Gamma-ray emission is associated with SNR G335.2+0.1 and a nearby molecular cloud.

The SNR is evolving in an expanding molecular bubble created by stellar wind.

Gamma-ray production is explained by hadronic interactions between escaped protons and the molecular cloud.

Abstract

We report the detection of GeV gamma-ray emission likely associated with supernova remnant (SNR) G335.2+0.1 and the finding of a molecular cloud ($\sim20$--$30^\prime$ in angular size) that is very likely in physical contact with the SNR and responsible for the gamma-ray emission. Using the 16.8 yr Fermi-LAT data, an extended emission, with a significance of 13.5 $\sigma$ and a radius 0.24{\deg} in 0.2--500 GeV in the uniform-disk model, was found to the adjacent east of the SNR. With archival Mopra CO-line data, a large molecular clump at local-standard-of-rest velocity $\sim-48$ to $-43$ km s$^{-1}$ was revealed appearing coincident with the gamma-ray source. The SNR was found located in a cavity encircled by a 'C'-shaped ring-like molecular shell at $-45$ to $-43$ km s$^{-1}$. This morphological agreement, together with the position-velociy diagrams made along lines cutting across the cavity, suggests that the SNR was evolving in the expanding molecular bubble created by the stellar wind of the progenitor with a mass $\gtrsim 15 M_{\mathrm{sun}}$. The giant molecular cloud, visible at around $-46$ km s$^{-1}$, and the associated SNR are thus estimated to lie at a kinematic distance of 3.1 kpc, with the HI absorption taken into account. We suggest that the SNR has entered the radiative phase after the blastwave recently struck the cavity wall. With the evolutionary scenario of the SNR, we demonstrate that the gamma-ray emission reported here can be naturally interpreted by the hadronic interaction between the accelerated protons that escaped from the SNR shock and the eastern large molecular clump.