The Nature of Ultraluminous Compact X‐Ray Sources in Nearby Spiral Galaxies

Studies were made of ASCA spectra of seven ultra-luminous compact X-raysources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. 1994), M81 X-6(Fabbiano 1988b; Kohmura et al. 1994; Uno 1997), IC 342 Source 1 (Okada et al.1998), Dwingeloo 1 X-1 (Reynolds et al. 1997), NGC 1313 Source B (Fabbiano &Trinchieri 1987; Petre et al. 1994), and two sources in NGC 4565 (Mizuno et al.1999). With the 0.5--10 keV luminosities in the range 10^{39-40} ergs/s, theyare thought to represent a class of enigmatic X-ray sources often found inspiral galaxies. For some of them, the ASCA data are newly processed, or thepublished spectra are reanalyzed. For others, the published results are quoted.The ASCA spectra of all these seven sources have been described successfullywith so called multi-color disk blackbody (MCD) emission arising fromoptically-thick standard accretion disks around black holes. Except the case ofM33 X-8, the spectra do not exhibit hard tails. For the source luminosities notto exceed the Eddington limits, the black holes are inferred to have ratherhigh masses, up to ~100 solar masses. However, the observed innermost disktemperatures of these objects, Tin = 1.1--1.8 keV, are too high to becompatible with the required high black-hole masses, as long as the standardaccretion disks around Schwarzschild black holes are assumed. Similarly highdisk temperatures are also observed from two Galactic transients withsuperluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually highdisk temperature may be explained by the black hole rotation, which makes thedisk get closer to the black hole, and hence hotter.Comment: submitted to ApJ, December 199