The X-ray binary population in M33: II. X-ray spectra and variability

TL;DR

This study analyzes X-ray spectra and variability of sources in M33, revealing that most are X-ray binaries with some super-soft and quasi-soft candidates, and highlights their spectral and temporal behaviors over months.

Contribution

It provides detailed spectral and variability analysis of X-ray sources in M33, identifying the dominant population as X-ray binaries and characterizing their properties.

Findings

Most sources are consistent with X-ray binaries.

A quarter of sources show variability between observations.

Six sources exhibit significant spectral variability.

Abstract

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 ~months timescales. 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 N_H values with HI 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 super-soft sources and two candidates for quasi-soft sources.