Gamma-Ray Polarimetry: A New Window for the Nonthermal 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 ∼ 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: in Sec. 1 I briefly review the history and principles of gamma-ray polarimetry, emphasizing its challenges and successes; Sec. 2 is dedicated the discussion of gamma-ray polarization and polarimetry, whereas in Sec. 3.1 I discuss 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; Sec. 4 outlines the scientific questions that could be solved by using gamma-ray polarimetry measurements. We end with a summary and outlook in Sec. 5

High-energy γ -photon polarization in nonlinear Breit-Wheeler pair production and γ polarimetry

“…Moreover, the resolution does not vary much and remains well below 1% with a shorter or longer laser pulse, a larger energy spread ε γ /ε γ = 0.1, a larger angular divergence θ γ = 1 mrad, and a different colliding angle θ γ = 175 • [85]. The proposed polarimetry based on the BW process avoids limitations of γ -ray polarimetry via the BH process [68][69][70][71][72] with respect to the γ -ray flux and angular resolution, imposed by the convertor material in the latter.…”

“…Note that the high-resolution polarimetry of high-energy γ rays is an important problem in astrophysics and high-energy physics, which can be employed, e.g., to determine the nature of the emission mechanisms responsible for blazars, γ -ray bursts (GRBs), pulsars, and magnetars and to address problems in fundamental physics [67][68][69][70]. Current polarimetries for high-energy γ photons mainly employ the principles of Compton scattering and Bethe-Heitler pair production by the Coulomb fields of atoms, with an accuracy of about several percents [69,70].…”

“…Note that the high-resolution polarimetry of high-energy γ rays is an important problem in astrophysics and high-energy physics, which can be employed, e.g., to determine the nature of the emission mechanisms responsible for blazars, γ -ray bursts (GRBs), pulsars, and magnetars and to address problems in fundamental physics [67][68][69][70]. Current polarimetries for high-energy γ photons mainly employ the principles of Compton scattering and Bethe-Heitler pair production by the Coulomb fields of atoms, with an accuracy of about several percents [69,70]. The former is not efficient at photon energies larger than 100 MeV because of the kinematic suppression of the Compton rate at large scattering angles; in the latter the photon flux and polarimetry angular resolution are restricted by the convertor material (damage threshold and multiple scattering), because, in particular, multiple scattering in a converter material decreases significantly the angular resolution of polarimetry [68][69][70][71][72].…”

“…Current polarimetries for high-energy γ photons mainly employ the principles of Compton scattering and Bethe-Heitler pair production by the Coulomb fields of atoms, with an accuracy of about several percents [69,70]. The former is not efficient at photon energies larger than 100 MeV because of the kinematic suppression of the Compton rate at large scattering angles; in the latter the photon flux and polarimetry angular resolution are restricted by the convertor material (damage threshold and multiple scattering), because, in particular, multiple scattering in a converter material decreases significantly the angular resolution of polarimetry [68][69][70][71][72]. Our polarimetry concept via nonlinear BW pair production working in a small duty cycle (determined by the laser pulse) is specifically designed for high-flux GeV γ photons and provides a competitive resolution.…”

High-energy gamma-photon polarization in nonlinear Breit-Wheeler pair production and gamma-polarimetry

“…Moreover, our results suggest an interesting application in high-resolution polarimetry of high-energy high-flux LP γ rays through the pair yield. Note that the high-resolution polarimetry of high-energy γ rays is an important problem in astrophysics and high-energy physics [54][55][56][57]. Current polarimetries for high-energy γ photons mainly employ the principles of Compton scattering and Bethe-Heitler pair production by the Coulomb fields of atoms, with an accuracy of about several percents [56,57].…”