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

TL;DR

This study analyzes the spectral and timing properties of the extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 over several years, revealing their spectral states, accretion rates, and black hole spins using multi-mission data.

Contribution

It provides a comprehensive multi-mission spectral and timing analysis of LMC X-1 and LMC X-3, including state transitions, accretion rates, and spin constraints, which advances understanding of their accretion disc dynamics.

Findings

LMC X-1 remained in the soft state with high disc flux contribution.

LMC X-3 showed transitions from soft to intermediate and hard states.

Black hole spins constrained to 0.85-0.94 for LMC X-1 and 0.16-0.33 for LMC X-3.

Abstract

Extragalactic black hole X-ray binaries LMC X-1 and LMC X-3 are the persistent sources, usually found in soft spectral state. We present the results from spectral and timing study of these sources using MAXI, NICER, NuSTAR and AstroSat observations carried out during 2014-2020. Study of long-term MAXI lightcurve shows that the fractional variability of flux in 2-10keV is moderate ($\sim$20%) in LMCX-1 and high ($\sim$50%) in LMCX-3. The energy spectra of both sources are characterized by a disc blackbody and a Comptonization component with LMCX-1 having an additional Fe-line emission feature. NICER (0.3-10keV), NuSTAR (3.0-40keV) and AstroSat (0.5-20keV) spectral analysis collectively show that LMC X-1 remained in the soft state (disc flux contribution $f_{disc}>80$%, photon index$\Gamma\sim2.06-4.08$) throughout 2014-2020. Mass accretion rate, $\dot{M}$ of LMC X-1 calculated from bolometric luminosity (0.1-50keV) is found to be within $0.07-0.24\dot{M}_{Edd}$ (Eddington mass accretion rate). Although LMC X-3 remained in the soft state ($f_{disc}>95\%,\Gamma\sim2.3$) during most of the time, it exhibits transition into intermediate ($f_{disc}=47-73\%,\Gamma\sim2.02-2.36$) and hard state ($f_{disc}\sim26\%,\Gamma\sim1.6$). $\dot{M}$ of LMC X-3 through different spectral states varies within $0.01-0.42\dot{M}_{Edd}$. Temporal study show that the Power Density Spectra (PDS) in 0.3-10keV follow a red-noise with rms of 2% for LMC X-1 and in case of LMC X-3, it is 0.08-2.35% during the soft state, but relatively high in the intermediate(3.05-4.91%) and hard state($\sim$17.06%). From continuum-fitting method we constrain spin of LMC X-1 and LMC X-3 to be within 0.85-0.94 and 0.16-0.33 respectively and from Fe-line fitting method, spin of LMC X-1 is found to be 0.93-0.94. Finally, we discuss the implication of our findings in the context of accretion disc dynamics around the vicinity of the BHs.