GALI Localization of Off-Grid GRB Directions: A Comparison of Rectangular and Triangular Calibration Meshes

Abstract

Rapid localization of gamma-ray bursts (GRBs) is essential for multi-wavelength and multi-messenger follow-up, but combining wide sky coverage with accurate localization remains challenging. GALI (GAmma-ray burst Localizing Instrument) determines source directions from count-rate modulations caused by internal shadowing in a three-dimensional scintillator array. We investigate a triangular calibration mesh based on an icosphere tessellation of the hemisphere and compare it with the rectangular mesh used previously. The triangular mesh samples the hemisphere more isotropically and gives better interpolation between calibration points. Laboratory measurements with the full-scale GALI detector yield an average angular separation of $0.86^{\circ }\pm 0.14^{\circ }$ for random source directions, compared with $1.19^{\circ }\pm 0.28^{\circ }$ for the rectangular mesh. Monte Carlo simulations of 50,000 randomly distributed GRBs in realistic low-Earth-orbit background conditions, each with 1000 detected counts, give average angular separations of $1.91^{\circ }\pm 1.20^{\circ }$ and $2.73^{\circ }\pm 2.18^{\circ }$ for triangular and rectangular meshes, respectively. The better laboratory performance is partly attributed to its approximately 3-4 times higher detected counts. Additional simulations indicate sensitivity from 10 keV to $\sim$1 MeV, with a peak effective area of about 130 cm${}^2$ near 100-150 keV. A compact 1U GALI implementation also achieves degree-scale localization in simulations, demonstrating the concept's potential for CubeSat-based GRB and multi-messenger missions.

arXiv commentSPIE Proceedings 2026