The Formation Rate and Luminosity Function of Fast X-ray transients from Einstein probe

Abstract: Following its launch on January 9, 2024, the Einstein Probe (EP) telescope has detected hundreds of fast X-ray transients (FXTs), yet their physical origins remain elusive. Understanding their luminosity function and formation rate is crucial for elucidating their nature. Based on the latest catalog of EP-detected FXTs, we present, a model-independent, non-parametric approach to derive the luminosity function and formation rate of FXTs. Our analysis reveals significant cosmological luminosity evolution, characterized by a scaling relationship of $(1+z)^{3.58}$. After accounting for this evolution, we establish that the local luminosity function is best represented by a broken power law, with a break luminosity of $(4.17 \pm 0.34) \times 10^{46}$ erg/s. The formation rate exhibits a gentle decline with redshift, following the relation $(1+z)^{-0.21}$, yielding a local rate of approximately $28.0_{-16.4}^{+27.3}$ Gpc$^{-3}$ yr$^{-1}$. This rate is comparable to that of low-luminosity long gamma-ray bursts (LGRBs). Our findings indicate a potential correlation between FXTs and LGRBs, supporting the majority of FXTs may be linked to collapsar progenitors.