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

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 analyse 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.

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

“…The reanalysis of the large LAT dataset presented in [4] results in a clear identification of the extended source FGES J1023.3-5747 with the TeV source HESS J1023-575. The matching spectral and morphological agreement, with no signs of cooling features in the size of the source, points to a common origin of the radiation.…”

“…The analysis of the LAT data described above was performed by means of the package (version 0.18.0), based on the F S T [14]. More details on the analysis are described in [4].…”

“…We investigated the spectral and morphological characteristics of the Fermi-LAT source FGES J1023.3-5747, which is spatially in agreement with one of the most massive and well-studied SFRs in our Galaxy: Westerlund 2 [4]. The cluster, which contains an extraordinary ensemble of hot and massive OB stars [5], has been previously associated with an extended TeV source (∼0.2 o ) known as HESS J1023-575 [6,7].…”

“…Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2 [4] for details on the modeling). The red points correspond to the Fermi-LAT data analysis (with only statistical errors), and the purple ones to H.E.S.S.…”

“…Figure 2: Left: Spectral energy distribution of PSR J1023-5746 derived from the on-peak (in red) the Bridge (in blue), and off-peak region (in black). Right: Best-fitting models for FGES J1023.3-5747 spectrum in the pion decay (solid line) and PWN (dashed line) hypotheses (see[4] for details on the modeling). The red points correspond to the Fermi-LAT data analysis (with only statistical errors), and the purple ones to H.E.S.S [7]…”