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