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New Evidence for Extragalactic Einstein Probe Transients associated with Long Gamma-ray Bursts

Qin-Mei Li, Qi-Bin Sun, Sheng-Bang Qian, Fu-Xing Li

Abstract

The origin of extragalactic fast X-ray transients (EFXTs) remains a fundamental open question in high-energy astrophysics. The Einstein Probe (EP) mission provides a transformative opportunity to investigate their nature. While mounting observations of EP-discovered EFXTs (EP-EFXTs) suggest a possible connection to long gamma-ray bursts (lGRBs), an in-depth comparative analysis between them remains lacking. Here, we present a comparative analysis of their cosmic formation histories, revealing that EP-EFXTs and lGRBs share a similar evolutionary trend-showing a marked decline at $z<1.0$ and a plateau beyond $1.0<z<5$-which clearly distinguishes them from short GRBs. This result is derived from a rigorously selected sample of EP-EFXTs, using Lynden-Bell's $c^{-}$ method to reconstruct, for the first time, the luminosity function and formation rate of EP-EFXTs without any assumptions. Our findings provide independent evidence that EP-EFXTs and lGRBs may originate from a common progenitor channel.

New Evidence for Extragalactic Einstein Probe Transients associated with Long Gamma-ray Bursts

Abstract

The origin of extragalactic fast X-ray transients (EFXTs) remains a fundamental open question in high-energy astrophysics. The Einstein Probe (EP) mission provides a transformative opportunity to investigate their nature. While mounting observations of EP-discovered EFXTs (EP-EFXTs) suggest a possible connection to long gamma-ray bursts (lGRBs), an in-depth comparative analysis between them remains lacking. Here, we present a comparative analysis of their cosmic formation histories, revealing that EP-EFXTs and lGRBs share a similar evolutionary trend-showing a marked decline at and a plateau beyond -which clearly distinguishes them from short GRBs. This result is derived from a rigorously selected sample of EP-EFXTs, using Lynden-Bell's method to reconstruct, for the first time, the luminosity function and formation rate of EP-EFXTs without any assumptions. Our findings provide independent evidence that EP-EFXTs and lGRBs may originate from a common progenitor channel.

Paper Structure

This paper contains 4 sections, 6 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Data and method
  3. Results
  4. Conclusions and Discussions

Figures (3)

  • Figure 1: Distributions and correlations for the EP-EFXTs sample: Left top: The redshift is correlation with X-ray luminosity; Right top: X-ray luminosity distribution where individual points represent different EP-EFXTs, with the line indicating the sensitivity limit of $1.0 \times 10^{-11}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$; Left bottom: In the Kendall $\tau$ correlation test, the red dotted line represents the null hypothesis ($\tau = 0$), and the measured correlation strength of $k = 3.75$ suggests that the evolutionary dependence between luminosity and redshift has been effectively removed; Right bottom: De-evolved LF following $L = L_{0}(1 + z)^{3.75}$ for our sample of 23 EP-EFXTs, removing the redshift evolution component.
  • Figure 2: Left: The distribution of cumulative LF of EP-EFXTs; Right: Normalized cumulative redshift distribution. .
  • Figure 3: Comparison rate between EP-EFXTs and lGRBs, sGRBs. The lGRB rate and sGRB rate were collected from 2015ApJ...806...44P2015ApJS..218...13Y2019MNRAS.488.5823L and 2018ApJ...852....1Z. The blue line are the FR of 23 EP-EFXTs (pink line exclude the EP250704a). The error bar gives a 1 $\sigma$ error. These fitting lines are normalized at z=1.