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

TL;DR

This study presents the first simultaneous broadband spectral and timing analysis of the atoll source 4U 1705-44 using AstroSat data, revealing insights into the inner disk, corona, and boundary layer structures.

Contribution

It provides a novel combined spectral and timing analysis of 4U 1705-44 with constraints on the inner disk, boundary layer, and coronal geometry during the soft state.

Findings

Inner disk radius ~14 Rg remains stable during observations.

Coronal height constrained to 0.6-20 km based on lag analysis.

Detected correlated and anticorrelated lags suggest a dynamic corona/boundary layer.

Abstract

For the first time, simultaneous broadband 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 modeling was performed using the full reflection framework and an inner disk radii of 14 Rg was obtained. A hard powerlaw tail was noticed in the soft state and hot component fluxes and varying powerlaw 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 $\sim$ 14-18 km and $\sim$ 9-19 km respectively. Cross- Correlation Function studies were performed between the 0.8-3 keV soft SXT lightcurve and 10-20 keV hard LAXPC lightcurve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6-20 km (\b{eta}=0.1). Since the inner disk 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 disk. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.