Properties of discrete X-ray sources in the starburst spiral galaxy M83

Abstract

We have identified 127 discrete sources in a Chandra observation of M83, with a detection limit of ~ 3 x 10^{36} erg/s in the 0.3-8.0 keV band. We discuss the individual properties of about 20 bright sources with L_x >~ 10^{38} erg/s, and the statistical properties of the whole sample. About 1/3 of the bright sources show X-ray spectra with a bb component at kT <~ 1 keV, plus a p-law component with Gamma ~ 2.5, typical of X-ray binaries (XRBs) in a soft state; another third have p-law spectra with Gamma ~ 1.5, consistent with XRBs in a hard state. Two bright sources show emission lines on a hard p-law continuum, and are probably XRBs surrounded by a photo-ionized nebula or stellar wind. Among the other bright sources, we also identified two SNR candidates, with optically-thin thermal plasma spectra at kT ~ 0.5 keV. The two brightest supersoft sources have T_bb ~ 70 eV and L_x ~ 10^{38} erg/s. Two candidate X-ray pulsars are detected with periods ~ 200 s. Another source corresponds to the core of a background FRII radio galaxy. The discrete sources can be divided into three groups, based on their spatial, color and luminosity distributions. The first group comprises supersoft sources with no detected emission above 1 keV. The second group consists of soft sources with little or no detected emission above 2 keV. They are strongly correlated with H-alpha emission in the spiral arms and starburst nucleus, tracing a young population. Their relative abundance depends on the current level of star-formation. Most of them are likely to be SNRs. The sources in the third group are mostly XRBs (in a soft or hard state), reaching higher X-ray luminosities than sources in the other two groups. Being a mixture of old low-mass and young high-mass systems, the whole group appears to be of intermediate age when correlated with H-alpha.