Tracing the outburst decay of soft X-ray transients Aql X-1 and 4U 1608-52 with XSPECT

Abstract

XSPECT instrument on-board XPoSat mission is a soft X-ray spectrometer sensitive in the energy band 0.8$-$15 keV. XSPECT has observed several bright neutron star low mass X-ray binaries since launch. Two well known sources, Aql X-1 and 4U 1608-52 which are soft X-ray transients, were observed by XPoSat during the decay phase of their recent outbursts in September 2024 and February 2025 respectively. During XSPECT observations, 4U 1608-52 exhibited a superburst which is a long duration thermonuclear burst, believed to be triggered by carbon burning. We carry out a detailed spectro-temporal analysis of the superburst, tracing its onset, rise, and decay over the next several hours. Using time-resolved spectroscopy, we probe the spectral evolution of the source and find that the persistent emission is suppressed during the superburst and the emission can be described by a gradually cooling blackbody component. The superburst was preceded by a precursor burst which is a normal type-I X-ray burst. We also observe a type-I burst $\sim 5$ days after the superburst, indicating resumption of burst activities which is typically quenched after a superburst. Aql X-1 also exhibited a type-I burst during XSPECT observations. The persistent emission of both the sources can be fitted using a combination of blackbody and disk blackbody emission or, alternatively, using a disk Comptonized by an optically thick plasma. Using the latter model, we find a clear flux dependence of the Comptonization parameters, with both the sources exhibiting harder spectra at higher accretion rates.