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

Abstract

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, $F_{\mathit{var}}\sim 20\%$) whereas that of LMC X-3 is high ($F_{\mathit{var}}\sim 50\%$) 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, $f_{\mathit{disc}}>80\%$, photon index $\mathrm{\Gamma }\sim 2.06-4.08$) throughout the period of 2014–2020. Mass accretion rate ($\dot{M}$) of LMC X-1 during this period, calculated from bolometric luminosity ($0.1-50$ keV) is found to be within $0.07-0.24$ ${\dot{M}}_{\mathit{Edd}}$ (Eddington mass accretion rate). Although LMC X-3 remained in the soft state during most of the observations ($f_{\mathit{disc}}>95\%,\mathrm{\Gamma }\sim 2.3$), it exhibits a transition into hard state ($f_{\mathit{disc}}\sim 26\%,\mathrm{\Gamma }\sim 1.6$) and intermediate state ($f_{\mathit{disc}}=47-73\%,\mathrm{\Gamma }\sim 2.02-2.36$). $\dot{M}$ of LMC X-3 through different spectral states varies in the range $0.01-0.42$ ${\dot{M}}_{\mathit{Edd}}$. 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 $\sim 2\%$ for LMC X-1 and in case of LMC X-3, rms is less during the soft state ($\sim 0.08-2.35\%$), but relatively high in the intermediate ($\sim 3.05-4.91\%$) and hard states ($\sim 17.06\%$). 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 ($\dot{M}$) 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.

Introduction

The stellar binary systems consisting of a black hole as an accretor and a normal star as its companion are known as Black Hole X-ray Binaries (BH-XRBs). Accretion from the companion star onto the black hole results in the formation of a disc around it which is bright in X-rays. Majority of the BH-XRBs spend most of their time in quiescent state where the X-ray luminosity from the source is very low; beyond detection limit ($<{10}^{35}$ erg cm⁻² sec⁻¹) (McClintock and Remillard, 2006). Such sources are detected in X-rays only when they undergo an outburst where excess flux is emitted in X-rays and hence they are well known as transient X-ray binaries. There are a few XRBs which are always luminous in X-rays and does not attain quiescence and are termed as persistent sources (Chen et al., 1997, Tetarenko et al., 2016, Corral-Santana et al., 2016, Sreehari et al., 2018). While the soft X-ray continuum of the energy spectrum originates from the thermally dominated accretion disc, hard X-rays are produced due to inverse Comptonization of soft photons by the hot corona. Depending on which of these components are dominant in the energy spectra, various spectral states of the BH-XRBs are defined (Tanaka and Lewin, 1995, Chakrabarti and Titarchuk, 1995, McClintock and Remillard, 2006, Sreehari et al., 2019). Thermal disc component dominating over the Comptonized component leads to High-soft state (HSS) or thermal state, during which the source commonly exhibits high disc temperature (${\mathit{kT}}_{\mathit{in}}>1$ keV) and steep spectral index ($\mathrm{\Gamma }>2$). Low Hard State (LHS) is seen when the high energy Comptonized component dominates over the disc component. Energy spectrum is characterised by disc component with low temperature (${\mathit{kT}}_{\mathit{in}}<1$ keV) and dominant Comptonized component with $\mathrm{\Gamma }<2$. Most of the persistent BH sources exhibit either HSS or LHS (Zdziarski et al., 2002), while outbursting sources depict two varieties of intermediate states between LHS and HSS i.e. Hard-Intermediate State (HIMS) and Soft- Intermediate State (SIMS) (Homan et al., 2001, Homan and Belloni, 2005, Belloni, 2005, Nandi et al., 2012, Radhika and Nandi, 2014, Radhika et al., 2016, Sreehari et al., 2019, Baby, 2020).

BH sources also show different temporal properties such as presence of Quasi-periodic Oscillations (QPOs) in the PDS (Psaltis et al., 1999, Méndez et al., 2001, Belloni et al., 2011, Nandi et al., 2012, Ingram and Motta, 2020) during different spectral states. The PDS in HSS is in the form of powerlaw noise component (Belloni et al., 1997) with minimal variability and the QPOs are rarely observed in this state (Homan et al., 2001, Belloni, 2005, Remillard and McClintock, 2006, Nandi et al., 2012, Radhika and Nandi, 2014, Sreehari et al., 2018). Whereas during LHS, PDS contains low frequency QPO along with a flat top noise (Psaltis et al., 1999, Nowak, 2000, van der Klis, 2004, Nandi et al., 2012, Motta, 2016, Ingram and Motta, 2020).

LMC X-1 and LMC X-3 are persistent BH-XRBs located in the Large Magellanic cloud (LMC) at a distance of $\sim 48.1$ kpc (Orosz et al., 2009). Despite of its large distance, they can be observed in a broad X-ray band since minimal Galactic hydrogen column density $n_H$ is present along their line of sight. These sources are well known for the presence of persistent soft spectral state. While LMC X-3 has been found to undergo occasional transition into LHS, LMC X-1 has always been observed in HSS and never seem to have undergone any state transition. Most recently, broadband study of both these sources using AstroSat observations (Bhuvana et al., 2021) confirmed the ‘extreme’ soft nature whose energy spectra are dominated by thermal disc component. Soft-state spectrum of both sources are characterized using thermal disc blackbody component and the hard state spectra of LMC X-3 using high energy Comptonization component along with the disc having a spectral index $\mathrm{\Gamma }\sim 1.7$ (Boyd et al., 2000, Wilms et al., 2001). Temporal studies show that both LMC X-1 and LMC X-3 exhibits moderate variability in soft state with rms $\sim 11.5\%$ and $\sim 17\%$ respectively (Bhuvana et al., 2021) in wide energy band ($3.0-20$ keV). LMC X-3 in its hard state shows a high variability (rms$\sim 40\%$) along with presence of QPO (Boyd et al., 2000).

LMC X-1 is a rapidly spinning BH whose dimensionless spin parameter a is estimated by continuum-fitting method (Zhang et al., 1997) to be ${0.92}_{-0.07}^{+0.05}$ (Gou et al., 2009) and${0.97}_{-0.25}^{+0.02}$ (Steiner et al., 2012) by Fe-line fitting method. LMC X-3 is estimated to have a low spin with $a={0.25}_{-0.29}^{+0.20}$ (Steiner et al., 2010). In our previous work Bhuvana et al. (2021), using recent broadband AstroSat observations, we have constrained the spin of both these sources using continuum-fitting method. Estimated spin value of LMC X-1 and LMC X-3 from this study are in the range $0.82-0.92$ and $0.22-0.41$ respectively. $M_{\mathit{BH}}$ is also constrained using same method whose value is found to be in the range of $7.64-10.00$ ${\mathrm{M}}_{\odot }$ and $5.35-6.22$ ${\mathrm{M}}_{\odot }$ for LMC X-1 and LMC X-3 respectively. These values are consistent with the dynamically estimated values which are $10.9\pm 1.41$ ${\mathrm{M}}_{\odot }$ (Orosz et al., 2009) for LMC X-1 and $6.98\pm 0.56$ ${\mathrm{M}}_{\odot }$ (Soria et al., 2001, Orosz et al., 2014) for LMC X-3. Inclination angle i of the binary system constrained from the dynamical studies of the binary system is $36.38°\pm 1.92°$ (Orosz et al., 2009) and $69.2°\pm 0.72°$ (Orosz et al., 2014) respectively for LMC X-1 and LMC X-3.In this work, we make use of NuSTAR (Harrison et al., 2013), NICER (Gendreau et al., 2016), and AstroSat (Agrawal, 2001) – SXT and LAXPC observations to perform broadband spectral and timing analysis of the extragalactic BH-XRBs LMC X-1 and LMC X-3. MAXI (Matsuoka et al., 2009) data is used to plot the long term lightcurve and Hardness Ratio (HR) of both these sources. A comprehensive analysis is carried out by considering all the observations performed by these instruments during the period of $2014-2020$. We examine the variability of lightcurve, evolution of energy spectra and power spectra during different spectral states. We also study the broadband spectral and temporal properties of these sources by using the simultaneous observations of NICER and NuSTAR.Further, by modelling NICER and NuSTAR observations using relativistic accretion disc and reflection models, we constrain the spin and mass accretion rate of the BH sources. Results from the preliminary analysis of this work have been presented in the ${43}^{\mathit{rd}}$ COSPAR Scientific Assembly (Bhuvana et al., 2021). Organization of this paper is as follows: In Section 2, we present the details of observations and data reduction. Spectral and timing analysis methods are discussed in Section 3 and in Section 4, results from this analysis are presented. Finally in Section 5, we discuss the implication of the results obtained in the context of accretion disc dynamics and conclude the salient findings of this study.