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 M101 (NGC 5457). Within a held of radius 17' (36 kpc at the distance of 7.2 Mpc), 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 sources 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 sources can be characterized by a power-law function: dN/dL(x) = 9.5L(x)(-1.9) sources per 10(38) ergs s(-1). Combined with archival data from the ROSAT PSPC, the Einstein IPC, and the ASCA GIS, we have examined spatial, spectral, and timing properties of the X-ray sources. In particular, we have explored the nature of various superluminous X-ray sources with luminosities significantly 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, detected in various ROSAT HRI and PSPC observations, are not transients and appear to result from recent massive star formation in outer spiral arms. Three 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 further identify two superluminous sources, which all show highly absorbed X-ray spectra and time variability during and/or between the observations, as candidates for X-ray binary systems that contain black holes. A comparison of seven nearby spirals shows that their X-ray source luminosity distributions, 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 correlated with the star-forming rate of a galaxy.