A Chandra study of the Circinus galaxy point-source population

We have used the Chandra X-Ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus, which make up approximate to 34% of the total 0.5-10 keV emission. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, although few have optical counterparts within 1 " of their X-ray positions down to limiting magnitudes of m(V)=23-25. At the distance of Circinus (approximate to3.8 Mpc), their intrinsic 0.5-10 keV luminosities range from approximate to2 x 10(37) ergs s(-1) to approximate to4 x 10(39) ergs (-1). One-fourth of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The overall characteristics of the point sources suggest that most are X-ray binaries and/or ultraluminous supernova remnants within Circinus. We are able to analyze the two strongest off-nuclear sources in greater detail and find both to have remarkable properties. The average X-ray luminosities of the two sources are 3.7 x 10(39) ergs s(-1) and 3.4 x 10(39) ergs s(-1). The former displays large and periodic flux variations every 7.5 hr and is well fitted by a multicolor blackbody accretion disk model with T-in = 1.35 keV, properties consistent with an eclipsing greater than or similar to 50 M-circle dot black hole binary. The latter appears to be a young supernova remnant, as it coincides with a nonthermal radio counterpart and an H alpha -detected H II region. This source exhibits both long-term (approximate to4 yr) X-ray variability and a 6.67-6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants.