Progenitors of Low-redshift Extragalactic Einstein Probe Transients

Abstract

Growing evidence suggests that extragalactic fast X-ray transients discovered by the Einstein Probe (EP-EFXTs) share observational properties with gamma-ray bursts (GRBs). Long GRBs (lGRBs) are traditionally associated with the core collapse of massive stars and are expected to trace the cosmic star formation rate (SFR). Using a nonparametric method to correct for the Eddington–Malmquist bias, we derive unbiased luminosity functions and formation rates from the largest sample of 42 EP-EFXTs with well-determined redshifts. We find that formation rate of EFXTs shows a significant excess over the SFR at low redshifts (z < 1), closely resembling that of lGRBs. Two lines of evidence point to diverse progenitors: (i) multiple EFXTs lie outside the lGRB region in the Amati relation, and (ii) a low-redshift excess is present in the larger sample with measured redshifts. Our derived rate implies that approximately 69% of EFXTs at z < 1 originate from noncollapsar progenitors, while the remaining ∼31% trace the SFR and are consistent with collapsars. Our results demonstrate that EP-EFXTs constitute a hybrid population, providing new insights into the progenitor landscape of fast X-ray transients.