Accretion Disc Evolution in GRO J1655-40 and LMC X-3 with Relativistic and Non-Relativistic Disc Models

Abstract: Black hole X-ray binaries are ideal environments to study the accretion phenomena in strong gravitational potentials. These systems undergo dramatic accretion state transitions and analysis of the X-ray spectra is used to probe the properties of the accretion disc and its evolution. In this work, we present a systematic investigation of $\sim$1800 spectra obtained by RXTE PCA observations of GRO J1655-40 and LMC X-3 to explore the nature of the accretion disc via non-relativistic and relativistic disc models describing the thermal emission in black-hole X-ray binaries. We demonstrate that the non-relativistic modelling throughout an outburst with the phenomenological multi-colour disc model DISKBB yields significantly lower and often unphysical inner disc radii and correspondingly higher ($\sim$50-60\%) disc temperatures compared to its relativistic counterparts KYNBB and KERRBB. We obtained the dimensionless spin parameters of $a_{}=0.774 \pm 0.069 $ and $a_{}=0.752 \pm 0.061 $ for GRO J1655-40 with KERRBB and KYNBB, respectively. We report a spin value of $a_{*}=0.098 \pm 0.063$ for LMC X-3 using the updated black hole mass of 6.98 ${M_{\odot}}$. Both measurements are consistent with the previous studies. Using our results, we highlight the importance of self-consistent modelling of the thermal emission, especially when estimating the spin with the continuum-fitting method which assumes the disc terminates at the innermost stable circular orbit at all times.