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

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.