LEAP – The LargE Area burst Polarimeter on the ISS

The LargE Area burst Polarimeter (LEAP) is proposed as an International Space Station external payload, launching in 2027, and was selected by NASA for a Phase A concept study. LEAP will make the fist high-fidelity polarization and spectroscopic measurements of gamma-ray bursts (GRBs). GRBs are highly luminous explosions, involving relativistic, jetted outflows and play an important role in the multimessenger era. Basic questions about the physics of GRBs have yet to be answered. These include: 1.) The jet composition: Is the energy in the jet carried by magnetic fields or baryons? 2.) The structure of the magnetic fields in the emission region: Are they ordered or oriented randomly? 3.) What is the emission mechanism: Is it synchrotron radiation or some modified thermal process? We argue that past spectral and temporal observations have been maximally exploited for addressing these questions and a fundamentally different approach is needed to achieve significant progress. Sensitive polarization measurements will open up a new dimension for GRBs and will naturally address the above questions. LEAP’s baseline mission requires observation of at least 65 GRBs with a sensitivity defined by a minimum detectable polarization (MDP) of 30%. The current LEAP design is expected to trigger on approximately 400 GRBs, with about 86 of those having an MDP < 30%. The LEAP design enables a broad range of secondary science such as polarization sensitive observations of short GRBs jointly with gravitational waves, magnetar bursts, accreting pulsars and Solar flares. LEAP will open a new window into the nature of the most energetic phenomena in the universe with gamma-ray polarization.