Linear Polarimetry with γ→e+e− Conversions

Abstract

γ-rays are emitted by cosmic sources by non-thermal processes that yield either non-polarized photons, such as those from π 0 decay in hadronic interactions, or linearly polarized photons from synchrotron radiation and the inverse-Compton up-shifting of these on high-energy charged particles. Polarimetry in the MeV energy range would provide a powerful tool to discriminate among "leptonic" and "hadronic" emission models of blazars, for example, but no polarimeter sensitive above 1 MeV has ever been flown into space. Low-Z converter telescopes such as silicon detectors are developed to improve the angular resolution and the point-like sensitivity below 100 MeV. We have shown that in the case of a homogeneous, low-density active target such as a gas time-projection chamber (TPC), the single-track angular resolution is even better and is so good that in addition the linear polarimetry of the incoming radiation can be performed. We actually characterized the performance of a prototype of such a telescope on beam. Track momentum measurement in the tracker would enable calorimeter-free, large effective area telescopes on low-mass space missions. An optimal unbiased momentum estimate can be obtained in the tracker alone based on the momentum dependence of multiple scattering, from a Bayesian analysis of the innovations of Kalman filters applied to the tracks.

HARPO, a gas TPC active target for high-performance γ-ray astronomy; demonstration of the polarimetry of MeV γ-rays converting to Bernard1 2019Nuclear Instruments and Methods in Physics Research Section A: 10040View full textAdd to dashboardCite

“…In the case of the full 5D differential cross section, an additional gain in the precision of the measurement of A × P of a factor of two-to-three is obtained [11,27]. The two "maps", that is, the two x, t and y, t projections of a conversion event of a 3.93 MeV γ-ray provided by the NewSUBARU BL01 beam line converting to an e + e − pair in the 2.1 bar argon-isobutane (95-5 %) gas of the HARPO TPC prototype [34]. The vertical dashed lines denote the physical limits of the detector, i.e.…”

“…Figure 1:The two "maps", that is, the two x, t and y, t projections of a conversion event of a 3.93 MeV γ-ray provided by the NewSUBARU BL01 beam line converting to an e + e − pair in the 2.1 bar argon-isobutane (95-5 %) gas of the HARPO TPC prototype[34]. The vertical dashed lines denote the physical limits of the detector, i.e.…”