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, timing 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 (SXT) and Large Area X-ray Proportional Counter (LAXPC) observations. The hardness-intensity diagram (HID) 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 - constant × tbabs (nthcomp + diskbb + 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 ∼ 69 Hz (3.3σ significance with more than 99 per cent confidence) and prominent noise features below 30 Hz.