Slim-disk model for ultraluminous X-ray sources

The ultraluminous X-ray sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, L-x similar to 10(38)-10(40) ergs s(-1), and rel atively high blackbody temperatures, T(in)similar to1.0-2.0 keV. From the c onstraint that L-x cannot exceed the Eddington luminosity, L-E, we require relatively high black hole masses, M similar to 10-100 M-. however, for suc h large masses the standard disk theory predicts lower blackbody temperatur es, T-in <1.0 keV. To understand a cause of this puzzling fact, we carefull y calculate the accretion flow structure shining at <similar to>L-E, fully taking into account the advective energy transport in the optically thick r egime and the transonic nature of the flow. Our calculations show that at h igh accretion rate ((M) over dot greater than or similar to 30L(E)/c(2)) an apparently compact region with a size of R-in similar or equal to (1-3)r(g ) (with r(g) being the Schwarzschild radius) is shining with a blackbody te mperature of T-in similar or equal to 1.8(M/10 M-.)(-1/4) keV even for the case of a nonrotating black hole. Furthermore, R-in decreases as increases, in (M) over dot increases, contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circ ular orbit. Accordingly, the loci of a constant black hole mass on the H-R diagram (representing the relation between L-x and T-in both on the logarit hm scales) are not straight but bent toward the lower M-direction in the fr ame of the standard disk relation. We also plot the ASCA data of some ULXs on the same H-R diagram, finding th at they all fall on the regions with relatively high masses, M similar to 1 0-30 M-., and high accretion rates, (M) over dot greater than or similar to 10L(E)/c(2). Interestingly, IC 342 source 1, in particular, was observed t o move along the constant M line (not constant R-in line) in our simulation s. This provides firm evidence that at least some ULXs are shining at great er than or similar toL(E) and contain black holes with M similar or equal t o 10-100 M-..