Abstract
The wide-field soft X-ray monitoring capability of Einstein Probe (EP) has opened a new window for discovering extragalactic fast X-ray transients (EFXTs), revealing a diverse population of short-duration high-energy transients in soft X-ray band beyond the reach of previous missions. Among them, a growing population of events associated with compact object mergers has emerged, providing a unique opportunity to probe the prompt X-ray emission and the nature of post-merger central engines.
In this talk, I will present a systematic study of these merger-related, also known as Type I, EFXTs detected by EP. Motivated by the theoretical predictions of long-lasting X-ray emission from newborn magnetars following a fraction of binary neutron star mergers, we investigate the observational properties of the Type I events and the prevalence of such emission in compact object mergers. Starting with the study of EP250704a, the first well-characterized Type I EFXT discovered by EP, we compare its properties with the extended emission/internal plateau sample of short gamma-ray bursts (GRBs), and perform a systematic search for short GRB counterparts with EP-WXT. The search candidates reveal similar observational characteristics and suggest that long-lasting soft X-ray emission may be more prevalent (~40%) than previously recognized.
Building upon these results, I will discuss how the observed detection rate can be used to constrain the remnant types of binary neutron star mergers and, further, place constraints on the neutron star equation-of-state. These results also demonstrate that EP is uncovering a previously hidden aspect of merger-driven transients and establishes a new class of electromagnetic counterparts to gravitational wave sources.