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 dataset we have created a catalog of the 110 sources from the S3 chip detected with a significance of > 3σ. This detection threshold corresponds to a flux of ∼ 10 −16 ergs cm −2 s −1 and a luminosity of ∼ 10 36 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 AGNs. 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 10 36 − 10 38 ergs s −1 . The nucleus is resolved into 2 nearly identical X-ray sources, each with a 0.5 -2.0 keV flux of 4 × 10 37 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 SNR, of which 12 are detected by Chandra. Two of the SNR sources are variable and hence must be compact objects. In total, 8 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 2 variable sources previously seen with ROSAT that apparently have faded below the Chandra detection threshold. The brightest

Classifying X‐Ray Sources in External Galaxies from X‐Ray Colors

“…The main difference between the two distributions, however, is that the nuclear X-ray sources are almost all sources with LMXB colors. In M101, the X-ray sources follow a similar pattern (see also Pence et al 2001), with both types of sources following the spiral structure. There is only a single diffuse source at the center of M101 (there is essentially no bulge in this system), so segregation of central sources is not seen.…”

“…Another source is significantly variable and has a H1 ¼ À0:78 in the long observation. This source has a luminosity of $10 39 ergs s À1 and is the brightest object in M101 (source 98 from Pence et al 2001). It is highly variable and is clearly an accretion source.…”

High-mass X-ray binaries in the Milky Way

“…The properties of individual X-ray binaries in nearby galaxies have been studied for more than a decade, in particular after the launch of the Chandra Xray observatory (see, e.g., Trudolyubov et al, 2001;Pence et al, 2001;Kong, 2003;Swartz et al, 2003). This requires to establish the nature of all X-ray sources, which remains difficult as the spatial resolution of Chandra and of the Hubble Space Telescope are not sufficient to unambiguously identify the counterparts and the nature of most sources.…”

ChandraDiscovery of Luminous Supersoft X‐Ray Sources in M81

“…Pence et al (2001) report the discovery of 10 supersoft sources among 110 objects in M101 using color definitions different than those defined here. Pence et al (2001) combined the spectra of the three most luminous supersoft sources, reportedly having similar spectral shapes, and find a best-fit blackbody temperature of 72 AE 2 eV and a mean (unabsorbed) luminosity of 1:4 Â 10 38 ergs s À1 . These values are similar to the brightest supersoft sources in M81 with the exception of source N1, which has an unabsorbed flux 5-8 times higher.…”

“…The brightest source cannot easily be explained by prevailing theory, while the other two sources for which reliable spectral fits can be made show strong similarities to the hot close binary accreting white dwarf (WD) systems CAL 87 and CAL 83. Some prospects for future observations are discussed (x 6), and a comparison is made to the supersoft source populations observed in M31 with ROSAT (Supper et al 1997;Kahabka 1999) and in M101 with Chandra (Pence et al 2001). …”