Broadband 'spectro-temporal' features of extragalactic black hole binaries LMC X-1 and LMC X-3: An AstroSat perspective

Abstract

We present the first results of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 using all the archival and legacy observations by AstroSat during the period of $2016-2020$. Broadband energy spectra ($0.5-20$ keV) of both sources obtained from the SXT and LAXPC on-board AstroSat are characterized by strong thermal disc blackbody component ($kT_{in}\sim1$keV, $f_{disc}>79\%$) along with a steep power-law ($\Gamma\sim2.4-3.2$). Bolometric luminosity of LMC X-1 varies from $7-10\%$ of Eddington luminosity ($L_{Edd}$) and for LMC X-3 is in the range $7-13\%$ of $L_{Edd}$. We study the long-term variation of the light curve using MAXI data and find the fractional variance to be $\sim25\%$ for LMC X-1 and $\sim53\%$ for LMC X-3. We examine the temporal properties of both sources and obtain fractional rms variability of PDS in the frequency range $0.002-10$ Hz to be $\sim9\%-17\%$ for LMC X-1, and $\sim7\%-11\%$ for LMC X-3. The `spectro-temporal' properties indicate both sources are in thermally dominated soft state. By modelling the spectra with relativistic accretion disc model, we determine the mass of LMC X-1 and LMC X-3 in the range $7.64-10.00$ $M_{\odot}$ and $5.35-6.22$ $M_{\odot}$ respectively. We also constrain the spin of LMC X-1 to be in the range $0.82-0.92$ and that of LMC X-3 in $0.22-0.41$ with 90\% confidence. We discuss the implications of our results in the context of accretion dynamics around the black hole binaries and compare it with the previous findings of both sources.