2025A&A...701A...3Z

Context. The Gravitational-wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) was primarily designed to detect gamma-ray bursts (GRBs), particularly those associated with gravitational-wave events. The energy response of detectors plays a crucial role in detection performance and scientific data analysis.
Aims. We applied the cross-calibration methodology to calibrate and optimize the energy response of the gamma-ray detectors (GRDs) on board GECAM-B, using eight GRBs that were jointly observed by GECAM-B and other well-calibrated instruments. These GRBs enable the comprehensive calibration of all GRDs with different incident angles.
Methods. Cross-calibration using well-calibrated instruments is the most efficient way to verify the energy response of the detectors. The basic approach involves performing a joint spectral analysis and introducing a constant factor to quantify the difference in effective area between instruments. This calibration method has been widely used in the cross-calibration of many high-energy astrophysical instruments.
Results. We completed the cross-calibration of all 25 GRDs, covering two electronics readout channels and different incidence angles. The results of the joint spectral fitting demonstrate that GECAM-B exhibits good consistency with the well-calibrated Fermi/GBM instrument. Furthermore, the GECAM-B energy response matrix accurately reproduces the observational data without requiring any corrections or modifications.
Conclusions. Thanks to the broad energy band, accurate energy response, and anti-saturation design, GECAM-B can constrain spectral models and measure spectral parameters for not only typical bursts but also exceptionally bright events, including GRB 221009A and GRB 230307A, the two brightest GRBs ever detected.