The X-ray binary population in M 33. II. X-ray spectra and variability.
GRIMM H.-J., McDOWELL J., ZEZAS A., KIM D.-W. and FABBIANO G.
Abstract (from CDS):
In this paper we investigate the X-ray spectra and X-ray spectral variability of compact X-ray sources for 3 Chandra observations of the Local Group galaxy M33. The observations are centered on the nucleus and the star-forming region NGC 604. In the observations, 261 sources have been detected. For a total of 43 sources the number of net counts is above 100, sufficient for a more detailed spectral fitting. Of these sources, 25 have been observed in more than one observation, allowing the study of spectral variability on timescales of ∼months. A quarter of the sources are found to be variable between observations. However, except for two foreground sources, no source is variable within any observation above the 99% confidence level. Only six sources show significant spectral variability between observations. A comparison of NH values with H I observations shows that X-ray absorption values are consistent with Galactic X-ray binaries and most sources in M33 are intrinsically absorbed. The pattern of variability and the spectral parameters of these sources are consistent with the M33 X-ray source population being dominated by X-ray binaries: Two-thirds of the 43 bright sources have spectral and timing properties consistent with X-ray binaries; we also find two candidates for supersoft sources and two candidates for quasi-soft sources.
Galaxies: Individual: Messier Number: M33 - Galaxies: Local Group - X-Rays: Binaries