2026ApJ...996..125W

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 (EP) and the coded-mask gamma-ray imager (ECLAIRs) aboard the Space-based multiband astronomical Variable Objects Monitor (SVOM). Our population synthesis model, calibrated to Swift data, predicts the following population II/I (Pop II/I) GRB detection rates at z > 6: ∼2.4 events yr^–1 for EP/WXT and ∼0.9 events yr^–1 for SVOM/ECLAIRs. For the IMF with very massive first stars (100 - 500 M_⊙), we derive upper limits on the Pop III GRB rate at z > 6 of <0.06 events yr^–1 (EP/WXT) and <0.13 events yr^–1 (SVOM/ECLAIRs), based on the absence of confirmed Pop III progenitors in 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 ∼8% (∼34%) at z > 10 and ∼28% (∼68%) at z > 16 for EP/WXT (SVOM/ECLAIRs). This trend is systematically enhanced in the other two IMF models, which adopt a lower stellar mass range of [0.1, 100] M_⊙. 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.