PETS - A GRB Polarimetry Mission on the International Space Station

Author: Dieter Hartmann

Abstract: Polarimetry of Energetic Transients in Space (PETS) is a gamma-ray polarimetry mission that was recently proposed as an NASA Astrophysics Mission of Opportunity. It will make the first definitive observations of the inner jets of GRBs, which cannot be probed with conventional non-polarization instruments. It will also observe, for the first time, the polarization signature from SGRs, revealing high-energy emission processes originating from the most intense magnetic field conditions known to exist. PETS will use gamma-ray polarimetry to uncover the energy release mechanism associated with the formation of stellar-mass black holes and investigate the physics of extreme magnetic fields in the vicinity of compact objects. The objectives are : 1) determine the structure and composition of GRB jets and uncover the mechanisms powering them; and 2) determine the emission geometry and mechanisms under the extreme magnetic field conditions found in SGRs. The PETS science objectives are met with two instruments. The primary instrument, the TRAnsient Polarimeter (TRAP), is a wide FOV non-imaging polarimeter that measures polarization over the energy range from 50-500 keV. Knowledge of the transient source location, required for the polarization analysis, is provided by the TRAnsient Location Experiment (TRALE). PETS will be mounted on the ISS with the two instruments pointed towards the zenith, scanning the sky as it orbits the Earth. During the two-year baseline mission, PETS will achieve its primary science objective with the polarization measurement of ∼ 100 GRBs with a minimum detectable polarization (MDP) better than 50%, ∼35 GRBs with an MDP of better than 30%, and ∼5 with an MDP of better than 15%. These data will be sufficient to distinguish amongst three basic models for the inner jet at a 90% confidence level. The secondary science objective will be achieved with the measurement of 3-4 SGRs with a minimum detectable polarization of 15-50%. PETS is a self-contained mission in that it will be able to achieve its objectives without relying on other sources for transient location data, while providing potentially important contextual data for other ongoing investigations. Scheduled launch date is 2018.