Estimation of the black hole spin in LMC X-1 using AstroSat

TL;DR

This study estimates the high spin of the black hole in LMC X-1 using broadband spectral data from AstroSat, confirming a spin parameter of approximately 0.93 through continuum-fitting methods.

Contribution

First broadband spectral analysis of LMC X-1 with AstroSat data to estimate black hole spin using relativistic disk models.

Findings

Black hole in LMC X-1 has a high spin parameter (~0.93).

The source was observed in a high soft state during the observations.

Results are consistent with previous spin estimates.

Abstract

LMC X-1, a persistent, rapidly rotating, extra-galactic, black hole X-ray binary (BHXB) discovered in 1969, has always been observed in its high soft state. Unlike many other BHXBs, the black hole mass, source distance and binary orbital inclination are well established. In this work, we report the results of simultaneous broadband spectral studies of LMC X-1 carried out using the data from Soft X-ray Telescope and Large Area X-ray Proportional Counter aboard AstroSat as observed on 2016 November 26$^{th}$ and 2017 August 28$^{th}$. The combined spectrum was modelled with a multicolour blackbody emission (diskbb), a Gaussian along with a Comptonization component (simpl) in the energy range 0.7$-$30.0 keV. The spectral analysis revealed that the source was in its high soft state ($\Gamma$=2.67$^{+0.24}_{-0.24}$ and $\Gamma$=2.12$^{+0.19}_{-0.20}$) with a hot disc (kT$_{in}$=0.86$^{+0.01}_{-0.01}$ and kT$_{in}$=0.87$^{+0.02}_{-0.02}$). Thermal disc emission was fit with a relativistic model (kerrbb) and spin of the black hole was estimated to be 0.93$^{+0.01}_{-0.01}$ and 0.93$^{+0.04}_{-0.03}$ (statistical errors) for the two Epochs through X-ray continuum-fitting, which agrees with the previous results.