AstroSat view of LMC X-2: evolution of broad-band X-ray spectral properties along a complete Z-track
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2020
- DOI:
- 10.1093/mnras/staa2063
- arXiv:
- arXiv:2007.05938
- Bibcode:
- 2020MNRAS.497.3726A
- Keywords:
-
- accretion;
- accretion discs;
- X-rays: binaries;
- X-rays: individual: LMC X-2;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 9 pages, 8 figures, accepted for publication in MNRAS