GaLI: a Gamma-ray burst Localizing Instrument


  Roi Rahin  ,  Ehud Behar  ,  Shlomit Tarem  ,  Luca Moleri  ,  Alon Osovizky  ,  Max Ghelman  ,  Avner Keidar  ,  Alex Vdovin  ,  Amir Feigenboim  
Technion - Israel Institute of Technology

The recent association of gamma-ray bursts with neutron star mergers highlighted the need for sensitive gamma-ray detectors with high localization accuracy. Identification of the host galaxy of the gravitational wave event GW170817 and the gamma-ray burst GRB170817A was only achieved 11 hours after initial detection, due to difficulties in surveying the uncertainty region of the gravitational wave event. Since gamma-ray bursts are transient events, constant all sky observation is required. Additionally, any delay in the precise localization means loss of crucial data from the longer-wavelength afterglow following gamma-ray bursts. This problem can only be mitigated using detector systems with large viewing angles and far improved angular resolution of a few degrees, which is required for prompt follow-up.
We present the first gamma-ray burst detector concept developed in Israel for space applications. Our detector aims to improve the angular localization with respect to the current state of the art in the world. The presented detector system relies on a non-uniform pattern of small scintillators coupled to silicon photomultipliers. The pattern utilizes the mutual occultation between detectors to reconstruct the gamma-ray burst’s direction in the sky with good angular accuracy. Our simulations show that the achievable localization accuracy for such a configuration is considerably better than those obtained by larger scintillator assemblies while maintaining a field-of-view of the entire sky. We show that even when the total effective area is decreased, our detector system still shows better angular sensitivity compared to designs based on the current state of the art. We present laboratory experiments on a configuration containing 90 small detectors and compare them with a larger detector system built with a traditional approach. Both simulations and experiments clearly show our novel concept can achieve a considerable improvement in angular sensitivity, without compromising sensitivity or field-of-view. The proposed concept can be easily scaled to fit into small satellites, as well as larger missions.