AstroSat view of LMC X-2: evolution of broad-band X-ray spectral properties along a complete Z-track

Abstract

ABSTRACT In this paper, we report the first results of the extragalactic Z-source Large Magellanic Cloud (LMC) X-2 obtained using the ∼140 ks observations with Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat. The Hardness-Intensity Diagram created with the LAXPC data revealed a complete Z-pattern of the source, showing all the three branches. We studied the evolution of the broad-band 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 (kTe) 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∼1.68 and 0.83, respectively. We also found a weak evidence of quasi-periodic oscillation (2.8σ) in the FB. The intrinsic luminosity of the source varied between (1.03–1.79) × 1038 erg s−1. We discuss our results by comparing with other Z-sources and the previous observations of LMC X-2.