The 2024 outburst of the neutron star LMXB EXO 0748–676: An investigation of bursts and eclipses with astrosat

Abstract

We present a detailed analysis of the Type-I (thermonuclear) X-ray bursts and eclipses observed from the neutron star low-mass X-ray binary EXO 0748–676 with AstroSat during the second known outburst of the source following a 16-year-long quiescence period. We detect three thermonuclear X-ray bursts, with two displaying simultaneous coverage in the soft X-rays. Simultaneous UV observations show evidence of reprocessed burst emissions in the far-ultraviolet band. The time-resolved spectral analysis reveals the photospheric radius expansion (PRE) nature for two bursts. We estimate the distance to the source to be 7.42 ± 0.53 kpc using the peak flux of PRE. Notably, one of the bursts exhibited a secondary peak,  ∼ 30 s after the primary, particularly dominating in the softer X-rays, which reveals a correlation with the evolution of burst hotspot radius with no temperature dependence. The burst properties and corresponding flux values suggest that mixed H/He burning may have fueled the bursts. We also detect evidence of a soft excess during one burst, likely arising from the interaction of the burst photons with the surroundings. We also probe the temporal evolution and the energy dependence of the eclipses, which offer insights into the binary environment. Our study helps gain deeper insight into the physics of burst ignition, flame propagation, the burst-accretion interaction, and the evolution of LMXBs.