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 ≈ 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 mV = 23–25. At the distance of Circinus (≈ 3.8 Mpc), their intrinsic 0.5–10 keV luminosities range from ≈ 2×1037 erg s−1 to ≈ 4×1039 erg s−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 ultra-luminous 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× 1039 erg s−1 and 3.4× 1039 erg s−1. The former displays large and periodic flux variations every 7.5 hr and is well fit by a multicolor blackbody accretiondisk model with Tin = 1.35 keV, properties consistent with an eclipsing & 50 M⊙ black-hole binary. The latter appears to be a young supernova remnant, as it coincides with a non-thermal radio counterpart and an Hα-detected H II region. This source exhibits both long-term (≈ 4 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. Subject headings: galaxies: active — galaxies: individual (Circinus) — X-rays: galaxies – X-rays: binaries