2019PASP..131k1001I

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, are now possible, at energies below the pair creation threshold. As such, a new window has been opened for understanding the exact nature of the nonthermal 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 versus hadronic models of emission for Blazars or ordered versus random field models for GRBs. Because 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 ∼1 MeV 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. The paper is organized as follows: Section 1, briefly reviews the history and principles of gamma-ray polarimetry, emphasizing its challenges and successes; Section 2 is dedicated the discussion of gamma-ray polarization and polarimetry, and Section 3 discusses the past and current instruments with which measurements of linear polarization for hard X-rays and soft gamma rays were successfully obtained for astrophysical sources; Section 4 outlines the scientific questions that could be solved by using gamma-ray polarimetry measurements. A summary and outlook are provided in Section 5.