Extending the Short Gamma-Ray Burst Population from Subthreshold Triggers in Fermi/GBM and GECAM Data and Its Implications

Detection of short gamma-ray bursts (SGRBs) is critically important for the research of compact object mergers and multimessenger astrophysics, but a significant part of SGRBs fall below the trigger threshold of GRB detectors, and thus are often missed. Here we present a systematic search for and verification of missed SGRBs using Fermi/Gamma-ray Burst Monitor (GBM) subthreshold triggers, jointly analyzing data from GBM, Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM)-B, and GECAM-C. Among 466 Fermi/GBM subthreshold events (with reliability ≥5) from 2021 to 2024, 181 are within GECAM’s field of view. We find that 49 out of 181 are confirmed astrophysical transients, and 41 can be classified as SGRBs. Thus, the SGRB detection rate of Fermi/GBM is increased to about 50 per year. Additionally, a complete multi-instrument monitoring and systematic verification of GBM subthreshold events is expected to further increase the SGRB rate to about 80 per year, which is ∼100% improvement relative to the GBM-triggered SGRBs. These results may have important implications on the local formation rate of SGRBs and the binary neutron star merger rate. We also searched for potential temporal coincidences between these SGRBs and gravitational waves from the LIGO–Virgo–KAGRA O4 run resulting in no detection.