Multi-mission view of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3: Evolution of broadband spectral features

Extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 are the persistent sources which are usually found in a soft spectral state. In this study, we consider multi-mission (MAXI, NICER, NuSTAR and AstroSat) X-ray observations of both these sources carried out during 2014-2020 to perform a detailed spectral and temporal analysis. Study of long term MAXI lightcurve shows that the flux variability of LMC X-1 is moderate (fractional variability, F_var ∼ 20 %) whereas that of LMC X-3 is high (F_var ∼ 50 %) in 2-10 keV which is associated with the change in spectral state. The energy spectra of LMC X-1 and LMC X-3 are characterized by a multi-color disc blackbody and a Comptonization component, with LMC X-1 having an additional Fe-line emission feature. NICER (0.3 - 10 keV), NuSTAR (3.0 - 40 keV) and AstroSat (0.5 - 20 keV) spectral analysis collectively shows that LMC X-1 has remained in the soft state (disc flux contribution in 0.3-40 keV, f_disc > 80 % , photon index Γ ∼ 2.06 - 4.08) throughout the period of 2014-2020. Mass accretion rate (M ̇) of LMC X-1 during this period, calculated from bolometric luminosity (0.1 - 50 keV) is found to be within 0.07 - 0.24 M_̇Edd (Eddington mass accretion rate). Although LMC X-3 remained in the soft state during most of the observations (f_disc > 95 % , Γ ∼ 2.3), it exhibits a transition into hard state (f_disc ∼ 26 % , Γ ∼ 1.6) and intermediate state (f_disc = 47 - 73 % , Γ ∼ 2.02 - 2.36). M ̇ of LMC X-3 through different spectral states varies in the range 0.01 - 0.42 M_̇Edd . Temporal studies carried out for these observations show that the Power Density Spectra (PDS) in 0.3 - 10 keV follow a red-noise with fractional rms of ∼ 2 % for LMC X-1 and in case of LMC X-3, rms is less during the soft state (∼ 0.08 - 2.35 %), but relatively high in the intermediate (∼ 3.05 - 4.91 %) and hard states (∼ 17.06 %). From spectral modeling of the soft X-ray continuum of NICER and NuSTAR energy spectra with relativistic accretion disc and reflection models, we constrain the spin and accretion rate (M ̇) of the BHs. In case of LMC X-1, spin is estimated to be within 0.85 - 0.94 by continuum-fitting method and 0.93 - 0.94 by Fe-line fitting method and in case of LMC X-3 continuum-fitting yields its value is in the range 0.16 - 0.33 . Finally, we discuss the implications of our findings in the context of accretion disc dynamics around the vicinity of the BHs.