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

TL;DR

This study analyzes the decay phase of superbursts in neutron star X-ray binaries Aql X-1 and 4U 1608-52 using the XSPECT instrument, revealing spectral evolution, burst sequences, and accretion-related spectral changes.

Contribution

First detailed spectro-temporal analysis of superburst decay in these sources, including spectral evolution and accretion rate effects, using the XSPECT instrument on XPoSat.

Findings

Superburst caused persistent emission suppression and cooling blackbody spectrum.

Preceded by a normal type-I burst, followed by a burst after 5 days.

Spectral modeling shows flux-dependent Comptonization parameters.

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.