Testing Accretion Disk Theory in Black Hole X‐Ray Binaries

Abstract

We present results from spectral modeling of three black hole X-ray binaries: LMC X-3, GRO J1655-40, and XTE J1550-564. Using a sample of disk dominated observations, we fit the data with a range of spectral models that includes a simple multitemperature blackbody (DISKBB), a relativistic accretion disk model based on color-corrected blackbodies (KERRBB), and a relativistic model based on non-LTE atmosphere models within an alpha prescription (BHSPEC). BHSPEC provides the best fit for a BeppoSAX observation of LMC X-3, which has the broadest energy coverage of our sample. It also provides the best fit for multiple epochs of Rossi X-ray Timing Explorer (RXTE) data in this source, except at the very highest luminosity (L/L_{Edd} >~ 0.7), where additional physics must be coming into play. BHSPEC is also the best-fit model for multi-epoch RXTE observations of GRO J1655-40 and XTE J1550-564, although the best-fit inclination of the inner disk differs from the binary inclination. All our fits prefer alpha=0.01 to alpha=0.1, in apparent disagreement with the large stresses inferred from the rapid rise times observed in outbursts of these two sources. In all three sources our fits imply moderate black hole spins (a/M ~ 0.1 - 0.8), but this is sensitive to the reliability of independent measurements of these system parameters and to the physical assumptions which underly our spectral models.