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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 field 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. We determine the average luminosity distribution of the sources. 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 for a $\sim 1.6 M_\odot$ object (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. 3 superluminous PSPC sources are associated with giant HII complexes and are clearly resolved. 2 other superluminous RHRI 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 2 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 7 nearby spirals shows that their X-ray source luminosity distributions, normalized by total HI 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.