LEAP — A Large Area Gamma-Ray Burst Polarimeter for the ISS

Polarization studies of the prompt emission from Gamma-ray bursts (GRBs) provide crucial information about the magnitude and coherence of magnetic fields carried by the astrophysical jets powering them. Polarization studies of GRBs to date present limited significance and conflicting results. The LargE Area burst Polarimeter (LEAP) will provide a large sample of high sensitivity polarization measurements, including time-resolved polarization measurements, required to understand GRB jets and their geometry. LEAP was proposed as an International Space Station external payload, launching in 2025, and was selected by NASA for a Phase A concept study. LEAP consists of an array of both plastic and CsI scintillation detectors to identify Compton scatter events from prompt GRB emission. The source polarization is measured from the azimuthal distribution of the Compton scattered photons. Since GRBs can come from any part of the sky, LEAP has a wide field-of-view and is a non-imaging instrument. It has a total geometric scintillator area of 3000 cm² and an effective area for polarization measurements (double scatter) of ~1000 cm². LEAP will provide high sensitivity polarization measurements from 50-500 keV, a range that covers the bulk of observed Epeak values. The large effective area will simultaneously provide broad-band spectral data from 20 keV to 5 MeV and enable time- and/or energy- resolved polarization measurements for the brightest GRBs. During its nominal 2.5 year mission lifetime, LEAP will trigger on ~400 GRBs, including ~320 long and ~80 short GRBs, and will provide polarization results on >100 GRBs. LEAP can self-sufficiently determine GRB localization, with an accuracy that depends on fluence. Typically localization errors (1-sigma) are 2-3 deg. LEAP enables exciting science beyond GRB polarization studies, including but not limited to, polarization studies of solar flares and observations of GRBs coincident with gravitational waves.