AstroSat view of LMC X-2: Evolution of broadband X-ray spectral properties along a complete Z-track

TL;DR

This study presents the first broadband X-ray spectral analysis of the extragalactic Z-source LMC X-2 along its complete Z-track, revealing spectral evolution, possible outflows, and timing features using AstroSat data.

Contribution

It provides the first detailed spectral evolution of LMC X-2 along its Z-track, highlighting obscured disc presence and potential outflows, based on AstroSat observations.

Findings

Spectral evolution consistent with Comptonized emission

Increased optical depth suggests outflow activity

Weak QPO detected at 2.8 sigma

Abstract

In this paper, we report the first results of the extragalactic Z-source LMC X-2 obtained using the $\sim$ 140 ks observations with {\it Large Area X-ray Proportional Counter (LAXPC)} and {\it Soft X-ray Telescope (SXT)} onboard {\it AstroSat}. The HID created with the {\it LAXPC} data revealed a complete Z-pattern of the source, showing all the three branches. We studied the evolution of the broadband X-ray spectra in the energy range of $0.5-20.0$ keV along the Z-track, a first such study of this source. The X-ray spectra of the different parts of the Z-pattern were well described by an absorbed Comptonized component. An absence of the accretion disc component suggests that the disc is most probably obscured by a Comptonized region. The best fit electron temperature ($kT_e$) was found to be in the range of $1.7-2.1$ keV and optical depth ($\tau$) was found to be in the range of $13.2-17.5$. The optical depth ($\tau$) increased as the source moved from the normal/flaring branch (NB/FB) vertex to the upper part of the FB, suggesting a possible outflow triggered by a strong radiation pressure. The power density spectra (PDS) of HB and NB could be fitted with a pure power-law of index $\alpha$ $\sim$ 1.68 and 0.83 respectively. We also found a weak evidence of QPO (2.8~$\sigma$) in the FB. The intrinsic luminosity of the source varied between $(1.03-1.79)$ $\times$ 10$^{38}$ ergs/s. We discuss our results by comparing with other Z-sources and the previous observations of LMC X-2.