Broadband mHz QPOs and spectral study of LMC X$-$4 with AstroSat

TL;DR

This study presents broadband timing and spectral analysis of LMC X-4 using AstroSat, revealing coherent pulsations, a quasi-periodic oscillation, and detailed spectral components, enhancing understanding of its accretion processes.

Contribution

First broadband timing and spectral analysis of LMC X-4 with AstroSat, identifying QPOs and detailed spectral features in the 0.5-25 keV range.

Findings

Detected coherent pulsations and a 26 mHz QPO.

Derived spectral parameters including cutoff power law and emission lines.

Estimated luminosity of approximately 2 x 10^{38} erg s^{-1}.

Abstract

We report the results of broadband timing and spectral analysis of data from an AstroSat observation of the High Mass X-ray binary LMC X$-$4. The Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments on-board the AstroSat observed the source in August 2016. A complete X-ray eclipse was detected with the LAXPC. The 3$-$40 keV power density spectrum showed the presence of coherent pulsations along with a $\sim 26$ mHz quasi-periodic oscillation feature. The spectral properties of LMC X$-$4 were derived from a joint analysis of the SXT and LAXPC spectral data. The 0.5$-$25 keV persistent spectrum comprised of an absorbed high energy cutoff power law with photon index of $\Gamma \sim$ 0.8 and cutoff at $\sim$16 keV, a soft thermal component with kT$_{BB} \sim$ 0.14 keV and Gaussian components corresponding to Fe K$_\alpha$, Ne \textsc{ix} and Ne \textsc{x} emission lines. Assuming a source distance of 50 kpc, we determined 0.5--25 keV luminosity to be $\sim 2 \times 10^{38}$ erg s$^{-1}$.