NuSTAR and Swift Observations of the Extragalactic Black Hole X-ray Binaries

TL;DR

This study analyzes simultaneous NuSTAR and Swift observations of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3, revealing their spectral states, black hole spins, and accretion efficiencies with no significant variability or QPOs.

Contribution

First detailed spectral and timing analysis of LMC X-1 and LMC X-3 using combined NuSTAR and Swift data over several years, estimating black hole spins and accretion efficiencies.

Findings

Sources in high soft state with disc temperature ~1 keV

Black hole spins estimated to be 0.92-0.95 for LMC X-1 and 0.19-0.29 for LMC X-3

No significant variability or QPOs detected

Abstract

We present the results obtained from detailed spectral and timing studies of extra-galactic black hole X-ray binaries LMC~X--1 and LMC~X--3, using simultaneous observations with {\it Nuclear Spectroscopic Telescope Array (NuSTAR)} and {\it Neil Gehrels Swift} observatories. The combined spectra in the $0.5-30$~keV energy range, obtained between 2014 and 2019, are investigated for both sources. We do not find any noticeable variability in $0.5-30$~keV light curves, with $0.1-10$~Hz fractional rms estimated to be $<2$\%. No evidence of quasi-periodic oscillations is found in the power density spectra. The sources are found to be in the high soft state during the observations with disc temperature $T_{\rm in}\sim 1$~keV, photon index, $\Gamma > 2.5$ and thermal emission fraction, $f_{\rm disc}>80$\%. An Fe K$\alpha$ emission line is detected in the spectra of LMC~X--1, though no such feature is observed in the spectra of LMC~X--3. From the spectral modelling, the spins of the black holes in LMC~X--1 and LMC~X--3 are estimated to be in the range of $0.92-0.95$ and $0.19-0.29$, respectively. The accretion efficiency is found to be, $\eta \sim 0.13$ and $\eta \sim 0.04$ for LMC~X--1 and LMC~X--3, respectively.