Gamma Ray polarimetry; a new window for the non-thermal Universe

Abstract: Over the past few decades impressive progress has been made in the field of photon polarimetry, especially in the hard X-ray and soft gamma-ray energy regime. Measurements of the linear degree of polarization for some of the most energetic astrophysical sources, such as Gamma Ray Bursts (GRBs) or Blazars, is now possible, at energies below the pair creation threshold. As such, a new window has been opened into understanding exact nature of the non-thermal emission mechanisms responsible for some of the most energetic phenomena in the Universe. There are still many open questions, and active debates, such as the discrimination between leptonic vs. hadronic models of emission for Blazars or ordered vs random field models for GRBs. Since the competing models predict different levels of linear photon polarization at energies above 1 MeV, gamma-ray polarimetry in that energy band could provide additional crucial insights. However, no polarimeter for gamma-rays with energies above 1MeV has been flown into space, as the sensitivity is severely limited by a quick degradation of the angular resolution and by multiple Coulomb scatterings in the detector. Over the past few years a series of proposals and demonstrator instruments that aim to overcome those inherent difficulties have been put forth, and the prospects look promising.