ChandraX‐Ray Sources in M101

Abstract

A deep (98.2 ks) Chandra Cycle 1 observation has revealed a wealth of discrete X-ray sources as well as diffuse emission in the nearby face-on spiral galaxy M101. From this rich data set we have created a catalog of the 110 sources from the S3 chip detected with a significance of greater than 3 σ. This detection threshold corresponds to a flux of ~10-16 ergs cm-2 s-1 and a luminosity of ~1036 ergs s-1 for a distance to M101 of 7.2 Mpc. The sources display a distinct correlation with the spiral arms of M101 and include a variety of X-ray binaries, supersoft sources, supernova remnants, and other objects of which only ~27 are likely to be background sources. There are only a few sources in the interarm regions, and most of these have X-ray colors consistent with that of background active galactic nuclei. The derived log N- log S relation for the sources in M101 (background subtracted) has a slope of -0.80 ± 0.05 over the range of 1036-1038 ergs s-1. The nucleus is resolved into two nearly identical X-ray sources, each with a 0.5-2.0 keV flux of 4 × 1037 ergs s-1. One of these sources coincides with the optical nucleus, and the other coincides with a cluster of stars 110 pc to the south. The field includes 54 optically identified supernova remnants (SNRs), of which 12 are detected by Chandra. Two of the SNR sources are variable and hence must be compact objects. In total, eight of the X-ray sources show evidence for short-term temporal variation during this observation. Two of these variable sources are now brighter than the ROSAT detection threshold, but they were not detected in the previous ROSAT observations taken in 1992 and 1996. There are also two variable sources previously seen with ROSAT that apparently have faded below the Chandra detection threshold. The brightest source in the field shows extreme long-term and short-term temporal variability. At its peak brightness it has a super-Eddington luminosity greater than 1039 ergs s-1. There are 10 supersoft sources (SSSs) in the field, which can be divided into two distinct subclasses: the brighter class (three objects) has a luminosity of ~1038 ergs s-1 and a blackbody temperature of ~70 eV, whereas the other class (seven objects) is an order of magnitude fainter and has a blackbody temperature of only ~50 eV.