Gamma-ray emission from the Westerlund 1 region

TL;DR

This study analyzes 4.5 years of Fermi-LAT data to characterize gamma-ray emission from Westerlund 1, revealing extended GeV emission overlapping with TeV signals and exploring potential origins involving pulsar wind nebulae or cosmic ray interactions.

Contribution

It provides the first detailed analysis of GeV gamma-ray emission from Westerlund 1, modeling its morphology and discussing possible physical origins, including electron acceleration and cosmic ray interactions.

Findings

Extended GeV emission with a Gaussian profile (sigma ~0.475 deg)

Partial spatial overlap between GeV and TeV emissions

Proposed scenarios include pulsar wind nebulae and cosmic ray interactions

Abstract

Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and associated with an extended region of TeV emission. Here we report the results of a search for GeV gamma-ray emission in this region. The analysis is based on ~4.5 years of Fermi-LAT data and reveals significantly extended emission which we model as a Gaussian, resulting in a best-fit sigma of sigma_S = (0.475 +/- 0.05) deg and an offset from Wd 1 of ~1 deg. A partial overlap of the GeV emission with the TeV signal as reported by H.E.S.S. is found. We investigate the spectral and morphological characteristics of the gamma-ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a Pulsar Wind Nebula provides a reasonably natural interpretation of the GeV emission, but leaves the TeV emission unexplained. A scenario in which protons are accelerated in or near Wd 1 in supernova explosion(s) and are diffusing away and interacting with molecular material, seems consistent with the observed GeV and TeV emission, but requires a very high energy input in protons, ~10^51 erg, and rather slow diffusion. Observations of Wd 1 with a future gamma-ray detector such as CTA provide a very promising route to fully resolve the origin of the TeV and GeV emission in Wd 1 and provide a deeper understanding of the high-energy (HE) astrophysics of massive stellar clusters.