3.2. The nature of individual sources

In M83, detailed spectral analysis is possible for sources with an emitted luminosity 10³⁸ erg s^-1. We can easily distinguish (Fig. 6):

• the X-ray nucleus, coincident with the optical/IR nucleus. Its spectrum is consistent with that of a supermassive black hole accreting well below its Eddington rate (L_x 2 × 10³⁸ erg s^-1; M ~ 10⁷ M);
• XRBs in a hard state: their X-ray spectrum is well fitted with a simple power law of photon index 1.3-1.7;
• XRBs in a soft state: their X-ray spectra is softer, dominated by a blackbody or disk-blackbody at 0.5 kT 1, plus a power law of photon index 2.2-3.0 (due to Compton upscattering of the disk photons);
• "emission-line" sources: they show a few prominent metal lines (in particular from Si and Mg). The softer sources are more likely to be young SNRs; the harder ones could be X-ray binaries surrounded by a photoionized nebulae or stellar wind. They latter class would then be analogous to Vela X-1 or Cen X-3 in our Galaxy (eg, Liedahl et al. 2000);
• bright supersoft sources, characterized by a thermal spectrum with blackbody temperature kT 60 eV. They are generally interpreted as accreting white dwarfs with stable nuclear burning on their surface (see also Di Stefano & Kong 2003).

For a more extensive discussion of the spectral and temporal properties of the discrete sources in M83, see Soria & Wu (2003).

Figure 6. Spectral fits to a sample of bright X-ray sources in M83 show different classes of objects. First three rows, from top left: the galactic nucleus; an X-ray binary in a hard state; an X-ray binary in a soft state; a source with soft thermal plasma emission (a young SNR?); a source with a power-law continuum plus line emission (from a photo-ionized stellar wind?); a supersoft source. See Soria & Wu (2003) for further details on the sources and their fitted spectral models. Bottom row: the brightest X-ray source in M83 has an emitted luminosity in excess of 1039 erg s-1. A power-law spectral fit gives a photon index 2.5 ± 0.1. Its lightcurve shows an increase by 40% over the duration of the Chandra observation.

The brightest point source, located 5' south-east of the nucleus, is variable and has an emitted luminosity of 10³⁹ erg s^-1 (Fig. 6, bottom panels). Hence, it can be classified as an ultra-luminous source (ULX).