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Detecting Population III Gamma-Ray Bursts with Einstein Probe and Space-Based Multi-band Astronomical Variable Objects Monitor

Jun-Jie Wei, Qing-Bo Ma, Xue-Feng Wu

TL;DR

The paper addresses whether Population III GRBs can be detected with upcoming X-ray and gamma-ray missions by combining cosmological N-body/hydrodynamical simulations with a metallicity-based GRB progenitor framework and three Pop III IMF scenarios. A Swift-calibrated population synthesis predicts Pop II/I GRB rates and places upper limits on Pop III GRBs, then propagates these through instrument-specific detection models for EP/WXT and SVOM/ECLAIRs, including luminosity functions and k-corrections. The results show Pop II/I GRBs dominate at z<10 while Pop III GRBs become increasingly significant at higher redshift, potentially constituting up to ~68% of detectable GRBs at z>16 for SVOM and ~28% for EP, depending on the IMF; the calculated upper limits on Pop III rates remain well below a tenth of a per year at z>6. The study demonstrates that detecting Pop III GRBs at very high redshift is feasible and provides concrete, instrument-specific guidelines for identifying Pop III progenitors, offering a path to probing the first stars through time-domain astronomy.

Abstract

High-redshift gamma-ray bursts (GRBs), putative counterparts of massive, low-metallicity Population III (Pop III) stars, are a promising probe of the first stars. We assess the detectability of these Pop III GRBs using a metallicity-based progenitor criterion and cosmological $N$-body/hydrodynamical simulations with three distinct Pop III initial mass functions (IMFs), focusing on the capabilities of the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe (\emph{EP}) and the coded-mask gamma-ray imager (ECLAIRs) aboard the Space-based multi-band astronomical Variable Objects Monitor (\emph{SVOM}). Our population synthesis model, calibrated to \emph{Swift} data, predicts the following Population II/I (Pop II/I) GRB detection rates at $z>6$: $\sim2.4\,\mathrm{events\,yr^{-1}}$ for \emph{EP}/WXT and $\sim0.9\,\mathrm{events\,yr^{-1}}$ for \emph{SVOM}/ECLAIRs. For the IMF with very massive first stars ($\mathrm{100\textrm{--}500\,M_\odot}$), we derive upper limits on the Pop III GRB rate at $z>6$ of $<0.06\,\mathrm{events\,yr^{-1}}$ (\emph{EP}/WXT) and $<0.13\,\mathrm{events\,yr^{-1}}$ (\emph{SVOM}/ECLAIRs), based on the absence of confirmed Pop III progenitors in \emph{Swift} bursts at $z>5.5$. Our results indicate that while Pop III GRBs are subdominant to Pop II/I GRBs at $z<10$, their fractional contribution rises significantly with redshift, reaching $\sim8\%$ ($\sim34\%$) at $z>10$ and $\sim28\%$ ($\sim68\%$) at $z>16$ for \emph{EP}/WXT (\emph{SVOM}/ECLAIRs). This trend is systematically enhanced in the other two IMF models, which adopt a lower stellar mass range of $\mathrm{[0.1,\,100]\,M_\odot}$. We conclude that detecting Pop III GRBs at high redshifts is a realistic prospect, and any GRB detected at $z>16$ is most likely of Pop III origin.

Detecting Population III Gamma-Ray Bursts with Einstein Probe and Space-Based Multi-band Astronomical Variable Objects Monitor

TL;DR

The paper addresses whether Population III GRBs can be detected with upcoming X-ray and gamma-ray missions by combining cosmological N-body/hydrodynamical simulations with a metallicity-based GRB progenitor framework and three Pop III IMF scenarios. A Swift-calibrated population synthesis predicts Pop II/I GRB rates and places upper limits on Pop III GRBs, then propagates these through instrument-specific detection models for EP/WXT and SVOM/ECLAIRs, including luminosity functions and k-corrections. The results show Pop II/I GRBs dominate at z<10 while Pop III GRBs become increasingly significant at higher redshift, potentially constituting up to ~68% of detectable GRBs at z>16 for SVOM and ~28% for EP, depending on the IMF; the calculated upper limits on Pop III rates remain well below a tenth of a per year at z>6. The study demonstrates that detecting Pop III GRBs at very high redshift is feasible and provides concrete, instrument-specific guidelines for identifying Pop III progenitors, offering a path to probing the first stars through time-domain astronomy.

Abstract

High-redshift gamma-ray bursts (GRBs), putative counterparts of massive, low-metallicity Population III (Pop III) stars, are a promising probe of the first stars. We assess the detectability of these Pop III GRBs using a metallicity-based progenitor criterion and cosmological -body/hydrodynamical simulations with three distinct Pop III initial mass functions (IMFs), focusing on the capabilities of the Wide-field X-ray Telescope (WXT) aboard the Einstein Probe (\emph{EP}) and the coded-mask gamma-ray imager (ECLAIRs) aboard the Space-based multi-band astronomical Variable Objects Monitor (\emph{SVOM}). Our population synthesis model, calibrated to \emph{Swift} data, predicts the following Population II/I (Pop II/I) GRB detection rates at : for \emph{EP}/WXT and for \emph{SVOM}/ECLAIRs. For the IMF with very massive first stars (), we derive upper limits on the Pop III GRB rate at of (\emph{EP}/WXT) and (\emph{SVOM}/ECLAIRs), based on the absence of confirmed Pop III progenitors in \emph{Swift} bursts at . Our results indicate that while Pop III GRBs are subdominant to Pop II/I GRBs at , their fractional contribution rises significantly with redshift, reaching () at and () at for \emph{EP}/WXT (\emph{SVOM}/ECLAIRs). This trend is systematically enhanced in the other two IMF models, which adopt a lower stellar mass range of . We conclude that detecting Pop III GRBs at high redshifts is a realistic prospect, and any GRB detected at is most likely of Pop III origin.
Paper Structure (8 sections, 12 equations, 2 figures, 3 tables)

This paper contains 8 sections, 12 equations, 2 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Numerical Simulations
  3. Analysis Method
  4. Event Rate of GRBs
  5. GRBs from Pop II/I Stars
  6. GRBs from Pop III Stars
  7. Pop III GRBs Accessible by EP and SVOM
  8. Summary and Discussions

Figures (2)

  • Figure 1: Cosmic SFR density as a function of redshift for the VMSN (red solid lines), MSN (green dashed lines), and RSN (blue dot-dashed lines) Pop III IMF models. Simulated SFR densities for Pop III and Pop II/I stars are shown as thick and thin lines, respectively. Observational data include: a measurement from IR observations (magenta diamond; 2016MNRAS.461.1100R), spectroscopic lower limits from JWST UV data (orange triangles; 2024ApJ...960...56H2025ApJ...980..138H), and previous UV photometric estimates (gray points; see 2025ApJ...980..138H and references therein).
  • Figure 2: Top panels: cumulative observed rate versus redshift for different GRB populations under various Pop III IMF models. Red lines show the rates of GRBII detected with EP/WXT (solid) and SVOM/ECLAIRs (dashed). Green and blue lines show the corresponding upper limits on GRBIII rates derived from $f_{\mathrm{GRBIII_{up2}}}$ and $f_{\mathrm{GRBIII_{up1}}}$, respectively. Bottom panels: redshift evolution of the GRBIII fraction, $\mathcal{R}_{\mathrm{GRBIII}_{\mathrm{up},i}}/(\mathcal{R}_{\mathrm{GRBII}}+\mathcal{R}_{\mathrm{GRBIII}_{\mathrm{up},i}})$, for various Pop III IMF models. Orange and black lines show the ratios for EP/WXT (solid) and SVOM/ECLAIRs (dashed), calculated with $f_{\mathrm{GRBIII_{up2}}}$ and $f_{\mathrm{GRBIII_{up1}}}$, respectively.