GALI a gamma-ray burst localizing instrument

The detection of astrophysical Gamma-Ray Bursts (GRBs) has always been intertwined with the challenge of identifying the direction of the source. Accurate angular localization of better than a degree has been achieved to date only with heavy instruments on large satellites, and a limited field of view. The recent discovery of the association of GRBs with neutron star mergers gives new motivation for observing the entire γ-ray sky at once with high sensitivity and accurate directional capability. We present a novel γ-ray detector concept, which utilizes the mutual occultation between many small scintillators to reconstruct the GRB direction. We built an instrument with 90 (9 mm)³ CsI(Tl) scintillator cubes attached to silicon photomultipliers. Our laboratory prototype tested with a 60 keV source demonstrates an angular accuracy of a few degrees for ∼25 ph cm⁻² bursts. Simulations of realistic GRBs and background show that the achievable angular localization accuracy with a similar instrument occupying 1l volume is < 2^◦. The proposed concept can be easily scaled to fit into small satellites, as well as large missions.