Properties of discrete X-ray sources in the starburst spiral galaxy M 83

Mullard Space Science Laboratory, University College London, Holmbury St Mary, Surrey RH5 6NT, UK e-mail: Roberto.Soria@mssl.ucl.ac.uk; kw@mssl.ucl.ac.uk

Corresponding author: R. Soria, rs1@mssl.ucl.ac.uk

Received: 3 December 2002
Accepted: 2 July 2003

Abstract

We have identified 127 discrete sources in a Chandra ACIS observation of M 83, with a detection limit of ≈ erg s^-1 in the 0.3–8.0 keV band. We discuss the individual X-ray spectral and time-variability properties of ≈20 bright sources with luminosities 10³⁸ erg s^-1, and the statistical properties of the whole sample. About one third of the bright sources show X-ray spectra with a blackbody component at temperatures 1 keV, plus a powerlaw component with , typical of X-ray binaries in a soft state; another third have powerlaw spectra with , consistent with X-ray binaries in a hard state. Two bright sources show emission lines on a hard powerlaw continuum, and are probably X-ray binaries surrounded by a photo-ionized nebula or stellar wind. Among the other bright sources, we also identified two supernova remnant candidates, with optically-thin thermal plasma spectra at temperatures ~0.5 keV. The two brightest supersoft sources have blackbody temperatures  eV and luminosities ~10³⁸ erg s^-1. Two candidate X-ray pulsars are detected with periods ≈200 s. One X-ray 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 and blackbody spectra at temperatures <100 eV. The second group consists of soft sources with little or no detected emission above 2 keV. They are strongly correlated with Hα emission in the spiral arms and starburst nucleus, tracing a young population. Their relative abundance depends on the current level of star-forming activity in the galaxy. Most of them are likely to be supernova remnants. The sources in the third group are mostly X-ray binaries, 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 the Hα emission. The color-color diagrams allow us to distinguish between sources in a soft and hard state.