Investigation of burst and eclipse behavior of the neutron star LMXB EXO 0748-676 during its 2024 outburst

Neutron Star (NS) low mass X-ray binaries (LMXB) are an excellent playground for probing the equation of state (EoS) of highly dense and gravitating exotic matter in the NS core. Studies of thermonuclear X-ray bursts can offer the most promising constraints to the EoS and provide insights into the interaction of matter in the highly gravitating environment of the LMXBs. Eclipses observed in these systems can unveil information about the binary components, geometry, and any variations in the orbit, like orbital jitter. EXO 0748-676 is a prolific bursting and eclipsing LMXB source. Following its discovery in 1984, it continued in an active state for 24 years, exhibiting a plethora of observation features, making it an important source to constrain the EoS. Since then, it has remained in quiescence for 16 years and only recently went into another outburst in 2024. In this work, we present the temporal and spectral studies conducted using our Target of Opportunity (ToO) AstroSat observation of the latest outburst of EXO 0748-676 conducted in July 2024. We detected three Type-I thermonuclear X-ray bursts and 5 eclipses in the LAXPC light curve in the energy range of 3-80 keV. The LAXPC persistent emission spectrum in the 4.0-20.0 keV energy range is reasonably well described by an absorbed blackbody radiation and power law model, representing the thermal disk/boundary layer and the comptonization by the corona. We modeled the X-ray burst profile using a fast rise and exponential decay model. Time-resolved spectroscopy using an absorbed blackbody radiation model for all the bursts revealed the spreading of the hot spot in the NS surface. Indications of a secondary peak or a hump were observed during the decay of two of the bursts, and we found that for these events, the radius of the hot spot increased suddenly without showing any change in the decay pattern in the hot-spot temperature. We have found evidence of a hard X-ray (≈ 30-80 keV) deficit during the bursts, which suggests Compton cooling of the corona during the burst event. Energy-resolved eclipse profile modeling revealed the ingress, egress, and duration of the eclipses in different energy regimes and their energy-dependent variation, revealing information about the properties of the binary components and their environment. We also observed one simultaneous burst and two simultaneous eclipses in the UVIT data and conducted the X-ray/UV correlation studies. This study helps us gain deeper insight into the physics of burst ignition, the accretion mechanism, and the geometry and environment of LMXB, thereby aiding in constraining the EoS of the exotic matter in NS LMXBs.