Localization Capability of the LEAP Polarimeter on the ISS

The LargE Area Polarimeter (LEAP) is a proposed mission concept, composed of a large array of CsI and plastic scintillators, that would be deployed as an external payload on the ISS in 2027. LEAP’s 420 CsI scintillators can be used to localize transient sources on the sky in the same manner that Fermi GBM localizes gamma-ray bursts (GRBs) with its 12 NaI detectors. Although the LEAP detectors are all co-pointed in the same direction, each detector’s response on the sky is slightly different due to the occultation resulting from the rest of the LEAP instrument. These differences in the CsI responses can be leveraged to enable LEAP to localize GRBs and other high energy transients to a few degrees accuracy without requiring a separate dedicated instrument for localization. While localizations of GRBs are critical to the success of LEAP’s mission to measure the polarization of GRBs, LEAP will also promptly provide localizations to the community to enable and encourage multi-wavelength and multi-messenger studies.