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Diffuse, Nonthermal X-Ray Emission from the Galactic Star Cluster Westerlund 1

Muno, Michael P.; Law, Casey; Clark, J. Simon; Dougherty, Sean M.; De Grijs, Richard; Portegies Zwart, Simon and Yusef-Zadeh, Farhad (2006). Diffuse, Nonthermal X-Ray Emission from the Galactic Star Cluster Westerlund 1. Astrophysical Journal, 650(1) pp. 203–211.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1086/507175
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Abstract

We present the diffuse X-ray emission identified in Chandra observations of the young, massive Galactic star cluster Westerlund 1. After removing pointlike X-ray sources down to a completeness limit of ≈ ergs s−1, we identify ergs s−1 (2–8 keV) of diffuse emission. The spatial distribution of the emission can be described as a slightly elliptical Lorentzian core with a half-width at half-maximum along the major axis of , similar to the distribution of point sources in the cluster, plus a 5 halo of extended emission. The spectrum of the diffuse emission is dominated by a hard continuum component that can be described as a keV thermal plasma that has a low iron abundance (0.3 solar) or as nonthermal emission that could be stellar light that is inverse Compton scattered by MeV electrons. Only 5% of the flux is produced by a keV plasma. The low luminosity of the thermal emission and the lack of a 6.7 keV iron line suggest that 40,000 unresolved stars with masses between 0.3 and 2 M are present in the cluster, fewer than previously estimated. Moreover, the flux in the diffuse emission is a factor of several lower than would be expected from a supersonically expanding cluster wind, and there is no evidence for thermal remnants produced by supernovae. Less than 10−5 of the mechanical luminosity of the cluster is dissipated as 2–8 keV X-rays, leaving a large amount of energy that either is radiated at other wavelengths, is dissipated beyond the bounds of our image, or escapes into the intergalactic medium.

Item Type: Journal Article
ISSN: 1538-4357
Keywords: stars: winds, outflows — supernova remnants — X-rays: ISM — X-rays: stars
Academic Unit/Department: Science > Physical Sciences
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 12385
Depositing User: Astrid Peterkin
Date Deposited: 21 Nov 2008 07:26
Last Modified: 02 Dec 2010 20:15
URI: http://oro.open.ac.uk/id/eprint/12385
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