γ -ray emission from the westerlund 1 region

Abstract

Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and is associated with an extended region of TeV emission. Here, we report the results of a search for GeV γ -ray emission in this region. The analysis is based on ~4.5 yr of Fermi-Large Area Telescope data and reveals significantly extended emission which we model as a Gaussian, resulting in a bestfitting sigma of σS = (0°.475 ± 0°.05) and an offset fromWd 1 of ~1°. A partial overlap of the GeV emission with the TeV signal as reported by the High Energy Stereoscopic System (HESS) is found. We investigate the spectral and morphological characteristics of the γ -ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a pulsarwind 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, ~1051 erg, and rather slow diffusion. Observations of Wd 1 with a future γ -ray detector such as the Cherenkov Telescope Array (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 astrophysics of massive stellar clusters. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.