An ultradeep high-resolution X-ray image of M101: The X-ray source population in a late-type spiral

We have studied the X-ray source population of the face-on spiral galaxy M1 01 (NGC 5457). Within a held of radius 17' (36 kpc at the distance of 7.2 M pc), covered by an ultradeep (229 ks) ROSAT HRI image, 51 X-ray sources are detected with signal-to-noise ratios greater than 3.5. About half of these sources are associated with the galaxy. The luminosity of these galactic s ources individually ranges from similar to 4 x 10(37) to 2 x 10(39) ergs s( -1) in the 0.5-2 keV band. The average luminosity distribution of the sourc es can be characterized by a power-law function: dN/dL(x) = 9.5L(x)(-1.9) s ources per 10(38) ergs s(-1). Combined with archival data from the ROSAT PS PC, the Einstein IPC, and the ASCA GIS, we have examined spatial, spectral, and timing properties of the X-ray sources. In particular, we have explore d the nature of various superluminous X-ray sources with luminosities signi ficantly greater than the Eddington limit (similar to 2 x 10(38) ergs s(-1) ) for a similar to 1.6 M-. object (a neutron star). These X-ray sources, de tected in various ROSAT HRI and PSPC observations, are not transients and a ppear to result from recent massive star formation in outer spiral arms. Th ree superluminous PSPC sources are associated with giant H II complexes and are clearly resolved. Two other superluminous ROSAT HRI sources are likely associated with shell-like supernova (or more likely hypernova) remnants, which are known to be abnormally luminous in optical and/or radio. We furth er identify two superluminous sources, which all show highly absorbed X-ray spectra and time variability during and/or between the observations, as ca ndidates for X-ray binary systems that contain black holes. A comparison of seven nearby spirals shows that their X-ray source luminosity distribution s, normalized by total H I masses, are very similar. But both the number of superluminous X-ray sources and the total X-ray luminosity appear to be co rrelated with the star-forming rate of a galaxy.