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Title:
Bright X-Ray Sources in M31 Globular Clusters
Authors:
Di Stefano, R.; Kong, A. K. H.; Garcia, M. R.; Barmby, P.; Greiner, J.; Murray, S. S.; Primini, F. A.
Affiliation:
AA(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; Also Department of Physics and Astronomy, Tufts University, Medford, MA 02155.), AB(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AC(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AD(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AE(Astrophysical Institute Potsdam, 14482 Potsdam, Germany), AF(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AG(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138)
Publication:
The Astrophysical Journal, Volume 570, Issue 2, pp. 618-636. (ApJ Homepage)
Publication Date:
05/2002
Origin:
UCP
ApJ Keywords:
Galaxies: Individual: Messier Number: M31, Galaxies: Star Clusters- Galaxy: globular clusters: general, X-Rays: Galaxies, X-Rays: Stars
Abstract Copyright:
(c) 2002: The American Astronomical Society
DOI:
10.1086/339283
Bibliographic Code:
2002ApJ...570..618D

Abstract

We have conducted Chandra observations of ~2560 arcmin2 (~131 kpc2) of M31 and find that the most luminous X-ray sources in most of our fields are in globular clusters. Of the 28 globular cluster X-ray sources in our fields, 15 are newly discovered. Approximately one-third of all the sources have LX([0.5-7] keV)>1037 ergs s-1, and approximately one-tenth of all the sources have LX([0.5-7] keV) close to or above 1038 ergs s-1. The most luminous source, in the globular cluster Bo 375, is consistently observed to have LX greater than 2×1038 ergs s-1. (1) We present data on the spectra and/or light curves of the five most luminous M31 globular cluster sources. (2) We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal timescale of the donor star. Our model can produce luminosities of several times 1038 ergs s-1 and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with LX>1037 ergs s-1 constitute a larger fraction of all GC sources than in our Galaxy. (4) We study the possible reasons for this difference between M31 and Galactic globular cluster X-ray sources and identify three promising explanations.
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