Testing Relativistic Accretion Disk Models with GRO J1655-40

Black hole X-ray binaries are ideal environments to test the accretion phenomena in the presence of strong gravitational potentials. KERRBB held an important place in the X-ray spectral continuum method for measuring the black hole spin modeling the emission from the innermost regions of the accretion disk. In this work, we present the results of X-ray spectral analysis using publicly available RXTE data of GRO J1655-40 obtained during the 2005 outburst with the two relativistic accretion disk models, KERRBB and KYNBB. Our analysis showed that both models provide identical results with black hole spin measurements, disk temperature, and disk luminosity when the inner edge of the accretion disk is set at the innermost stable circular orbit (ISCO) for the same accretion rates. We could not obtain reasonable fits for ∼89% of the observations with a fixed black hole spin value at a_* = 0.7 using both models. Allowing the spin parameter to vary improved the fit statistic significantly with reduced χ² values being reduced from ∼10 to 100 to below 2. Both models revealed black hole spin values varying between 0.52 < a_* < 0.94, which can be interpreted as a variable inner edge of the disk throughout different accretion states.