Introduction

GRIPS, Gamma-Ray Burst Investigation via Polarimetry and Spectroscopy, had been proposed in 2007 in response to the ESA Cosmic Vision call as a new-generation Compton-and-pair telescope. Though it was not selected for further study, a variety of investigations are being performed to improve the concept and to verify the performance.

With the Compton scattering being dependent on the polarization of the incoming photons, any Compton telescope is, per se, a decent polarimeter. Beyond this, such detectors can be tailored to have a particularly high polarization sensitivity by obeying some simple design principles.

Polarization is the last property of high-energy electromagnetic radiation which has not been utilized to its full extent, and promises to uniquely determine the emission processes of a variety of astrophysical sources, among them pulsars, anomalous X-ray pulsars (AXP) and soft-gamma repeaters (SGR), or gamma-ray bursts (GRB).

GRIPS would carry two major telescopes: the gamma-ray monitor (GRM) and the X-ray monitor. The GRM is a combined Compton-scattering and pair-creation telescope for the energy range 0.2–50 MeV. It will thus follow the successful concepts of imaging high-energy photons used in COMPTEL (0.7–30 MeV) as well as EGRET (>30 MeV) and Fermi (>100 MeV) but combines them into one instrument. The following deals exclusively with the GRM concept, and its capability to measure polarization at unprecedented sensitivity.