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

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 ($τ$) was found to be in the range of $13.2-17.5$. The optical depth ($τ$) 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 $α$ $\sim$ 1.68 and 0.83 respectively. We also found a weak evidence of QPO (2.8~$σ$) 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.