Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

TL;DR

This study analyzes over 11 years of Fermi-LAT gamma-ray data from Westerlund 2, providing evidence that the gamma-ray emission has a hadronic origin linked to cosmic-ray acceleration in the star-forming region.

Contribution

It offers the first clear identification of the GeV counterpart to a TeV source in Westerlund 2, supporting the hadronic cosmic-ray acceleration hypothesis in star-forming regions.

Findings

FGES J1023.3-5747 is the GeV counterpart of HESS J1023-575.

The gamma-ray emission is likely hadronic in origin.

Star-forming regions may contribute to the Galactic cosmic-ray population.

Abstract

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023-5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2, and the underlying extended source FGES J1023.3-5747. The analysis results in a clear identification of FGES J1023.3-5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023-5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic-ray sea observed in our Galaxy