The LargE Area Burst Polarimeter (LEAP)- A NASA Mission of Opportunity for the ISS

The LargE Area burst Polarimeter (LEAP) will expose the underlying physics that governs astrophysical jets and the extreme environment surrounding newborn compact objects. LEAP will do this by making the highest fidelity polarization measurements to date of the prompt gamma-ray emission from a large sample of Gamma-Ray Bursts (GRBs). The LEAP science objectives are met with a single instrument — a wide FOV Compton polarimeter that measures GRB polarization over the energy range from 50–1000 keV, performs GRB spectroscopy from 20 keV to 6 MeV, and self-sufficiently determines the source direction. LEAP measures polarization using seven independent polarimeter modules, each with a 12 x 12 array of optically isolated high-Z (CsI(Tl)) and low-Z (plastic) scintillation detectors (including two plastic scintillators infused with ⁶⁰Co for calibration) read out by individual PMTs. Scatter events recorded by the scintillator array within each module are used to measure polarization. The distribution of azimuthal scatter angles for these events provides a polarization signature, which is further enhanced by measurement of the Compton (polar) scatter angle. The total effective area for polarimetry is ~1000 cm² at energies above 100 keV. To characterize the GRB parameters, spectroscopic measurements (20 keV - 6 MeV) are obtained using all event types (both multiple and singles events), with a total effective area that reaches >3000 cm² between 50 and 500 keV. If approved, LEAP will be deployed as an external payload on the International Space Station (ISS) in 2027 for a 32-month mission. Over its lifetime, LEAP will, for the first time, measure the level of polarization for a significant number of GRBs with sufficient sensitivity (defined as the Minimum Detectable Polarization, or MDP) to determine the magnetic field structure, the composition (whether dominated by matter or Poynting flux), and the energy dissipation mechanism of GRB jets. Specifically, LEAP will measure 135 GRBs with a 50-300 keV MDP of <30%, 224 GRBs with an MDP of <50%, and 23 GRBs with an MDP of <10%. For brighter events, LEAP will measure the energy and/or time dependence of the polarization. Finally, LEAP will provide burst alerts to the community, following procedures developed by Fermi-GBM.