The localization method of GECAM and simulation analysis

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is scheduled to launch in late 2020. It will monitor the whole sky for gamma-ray transients, such as Gamma-Ray Bursts (GRBs) and high-energy electromagnetic counterpart of gravitational wave events. One of the key scientific capabilities of GECAM is to localize the GRB. In this study, we established a ground localization method for GECAM by simultaneously fitting the spectrum and location of GRB. In addition, with the instrument response and in-flight background simulated using the current GECAM mass model, we examined the localization capability of GECAM. As a result, we derived the relationship between localization accuracy and different parameters of the GRB, such as brightness, spectral hardness, and burst duration. Moreover, for typical brightness (flux 10^-6 erg s^-1 cm^-2 in 10-1000 keV energy range with duration of 10 s) spectrum of medium hardness, the localization accuracy was observed at about 0.6° (1-σ statistical error). We also performed localization for two special bursts: (1) the suspected high-energy counterpart of the Fast Radio Burst (FRB 131104) with the GECAM localization error approximately 5 deg and (2) the gamma-ray electromagnetic counterpart (GRB 170817A) of the first binary neutron star merger gravitational wave event GW170817, with GECAM localization error of about 10°.