UCL Discovery

Abstract

In the absence of direct kinematic measurements, the mass of an accreting black hole is sometimes inferred from tile X-ray spectral parameters of its accretion disk; specifically, from the temperature and normalization of a disk-blackbody model fit. Suitable corrections have to be introduced when the accretion rate approaches or exceeds the Eddington limit. We summarize phenomenological models that can explain the very high state, with apparently higher disk temperatures and lower inner-disk radii. Conversely, ultraluminous X-ray sources often contain cooler disks with large characteristic radii. We introduce another phenomenological model for this accretion state. We argue that a standard disk dominates the radiative output for radii larger than a characteristic transition radius R-c similar to <(m)over dot> x R-ISCO, where th is the accretion rate in Ecidington units and R-ISCO is the innermost stable orbit. For R-ISCO < R < R-c, most of the accretion power is released via non-thermal processes. We predict the location of such sources in a luminosity-temperature plot. We conclude that BHs with masses similar to 50-100M(circle dot) accreting at th similar to 10-20 may explain the X-ray properties of many ULXs. (c) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.