AstroSat view of transient low-mass X-ray binary XTE J1701-462: Spectral and temporal evolution along the Z-track

Abstract AstroSat observed transient neutron star low-mass X-ray binary XTE J1701-462 for a total duration of $\sim$ 135 ks during its 2022 outburst. We report the results of a detailed spectral and timing analysis carried out using this data. The source traced a complete ‘Z’ shaped structure in the hardness intensity diagram (HID). The source exhibited an extended horizontal branch and a short-dipping flaring branch in the HID. The spectra of the source were fitted with different approaches. We find that most suitable spectral model comprises emission from a standard multi-color accretion disk (diskbb in XSPEC) and Comptonised radiation from a hot central corona, described by Comptb model of XSPEC. The observed disk component is cool, having a temperature in the range of $\sim 0.28-0.42$ keV and truncated far ( $\sim$ 250 - 1600 km) from the compact object. The Compton corona has an optical depth in the range of $\sim 3.4- 5.1 $ and a temperature in the range of $3.3-4.5$ keV. The disk and corona flux as well as truncation radius vary significantly along the HID. The temperature $kT_{in}$ depends on both luminosity and inner disk radius and hence shows marginal variation as compared to the truncation radius. We discuss possible scenarios to explain the relationship between the spectral evolution and motion of the source along the HID. The timing analysis revealed horizontal branch oscillations (HBOs) in the frequency range $\sim 34-40$ Hz. The frequency and rms strength of HBO vary systematically as the source moves along the horizontal branch (HB). The observed correlation of the HBO properties with the position on the HB is similar to that previously reported in this source using RXTE data during the 2006 outburst of the source. The source also showed normal branch oscillations (NBOs) with frequency $\sim$ 6.7 Hz in the middle and the lower normal branch. The energy-dependent study of the HBO properties suggests that the HBO is stronger in the higher energy band. We also observed very-low frequency noise and band-limited noise (BLN) components in the power density spectra. The break frequency of BLN component was found to be tightly correlated with the HBO frequency. We discuss possible models to explain the origin and nature of the observed features in the PDS.