Broad-Band 'Spectro-Temporal' Features Of Extragalactic Black Hole Binaries Lmc X-1 And Lmc X-3: An Astrosat Perspective

We present the first results of extragalactic black hole (BH) 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. Broad-band energy spectra (0.5-20 keV) of both sources obtained from the Soft X-ray Telescope and Large Area X-ray Proportional Counter on-board AstroSat are characterized by strong thermal disc blackbody component (kT(in) similar to 1keV, f(disc) > 79 per cent) along with a steep power-law (Gamma similar to 2.4-3.2). Bolometric luminosity of LMC X-1 varies from 7 to 10 per cent of Eddington luminosity (L-Edd) and for LMC X-3 is in the range 7-13 per cent of L-Edd. We study the long-term variation of light curve using MAXI data and find the fractional variance to be similar to 25 per cent for LMC X-1 and similar to 53 per cent for LMC X-3. We examine the temporal properties of both sources and obtain fractional rms variability of power density spectrum in the frequency range 0.002-10 Hz to be similar to 9 - 17 per cent for LMC X-1, and similar to 7 - 11 per cent 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 and 5.35-6.22M(circle dot), 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 per cent confidence. We discuss the implications of our results in the context of accretion dynamics around the BH binaries and compare it with the previous findings of both sources.