Exploring the inner-disc region of the atoll source 4U 1705-44 using AstroSat’s SXT and LAXPC observations

Abstract

ABSTRACT For the first time, simultaneous broad-band spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8–70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modelling was performed using the full reflection framework and an inner disc radii of 14 Rg was obtained. A hard power-law tail was noticed in the soft state and hot component fluxes and varying power-law indices point towards a varying corona/sub-Keplerian flow. Based on the spectral fits, the boundary layer radius and magnetospheric radius were constrained to be ∼14–18 and ∼9–19 km, respectively. Cross-correlation function studies were performed between the 0.8–3 keV soft SXT light curve and 10–20 keV hard LAXPC light curve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6–20 km (β = 0.1). Since the inner disc radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disc. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.