AstroSat's view of 4U 1735-44: spectral, temporal, and type I X-ray burst studies

This study utilizes the simultaneous broad-band observations of 4U 1735-44 from AstroSat, offering enhanced spectral and temporal resolution, to investigate its spectral properties, temporal behaviour, and burst characteristics. Spectral, type I X-ray burst, and temporal analyses on 4U 1735-44 were performed using AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) observations. The hardness-intensity diagram from LAXPC-20 showed a positive correlation between hardness and intensity, with a pattern resembling the banana branch typical of atoll sources. Spectral analysis carried out in the 0.7-20.0 keV energy range, using the model combination - $\tt {constant}$$\times$$\tt {tbabs}$ ($\tt {nthcomp}$$+$$\tt {diskbb}$$+$$\tt {bbodyrad}$), suggested a cool accretion disc truncated at a large distance from the neutron star in the system. Time-resolved spectral studies of two type I X-ray bursts detected from the source revealed evidence of photospheric radius expansion, allowing for an estimation of the source distance. Temporal analysis showed the presence of low-frequency quasi-periodic oscillation at $\sim$69 Hz (3.3$\sigma$ significance with more than 99 per cent confidence) and prominent noise features below 30 Hz.