IXOcat: A ROSAT HRI Catalog of Intermediate-luminosity X-ray Objects

With the advent of sensitive, high spatial resolution X-ray imaging instruments, it has become possible to study extranuclear X-ray point sources in galaxies other than our own. ROSAT and Chandra X-ray observations have revealed that off-nuclear, compact, Intermediate-luminosity (L_X(2-10 keV) >=10^39.0 erg s^-1) X-ray Objects (IXOs, a.k.a. ULXs) are quite common. We present a catalog of ≈ 100 IXOs derived from all of the public ROSAT HRI images of galaxies (with cz <= 5000 km s^-1) from the Third Reference Catalog of Bright Galaxies (RC3). We have defined the cutoff L_X for IXOs at 10^39.0 erg s^-1 so that it is well above the Eddington luminosity of a 1.4 M_solar black hole (10^38.3 erg s^-1). We find that, on average, one in every five spiral galaxies contains an IXO, and that some galaxies contain an anomalously large number of IXOs. We also present statistics comparing the properties of the IXOs (e.g., number per galaxy, location) with the properties of the host galaxies (e.g., starburst properties such as FIR/B ratio, Hubble type).