Linear Polarimetry with γ→e+e− Conversions

Denis Bernard

doi:10.3390/galaxies5040072

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.

Highlights

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
MeV γ-ray Astronomy γ-ray astronomy is suffering from a huge sensitivity gap between the sub-MeV energy range for which Compton telescopes are very efficient and the energy range above 100 MeV for which pair telescopes are very efficient [1]
From pair creation threshold (1 MeV) to 100 MeV, the main issue is the difficulty of rejecting true-photon backgrounds due to the bad single-photon angular resolution: the Fermi-Large Area Telescope (LAT), for example, has published results mainly above 100 MeV [2]

Summary

Polarimetry with Pairs

In contrast to low-energy (radio waves, optics) polarimetry that formed the core of this conference, and for which polarimetry is performed with detectors that measure electric fields or light intensities, at high energies photons are observed individually: a photon conversion in the detector is named an “event”. The segmentation consists of two series of orthogonal strips along x and y, which enables a number of electronics channels that scales as 2 × n so as to fit into the limited electrical power available on a space mission (a pad-based system would scale as n2 ) This reduction comes at the cost of a track-assignment ambiguity in multi-track events. This issue is solved thanks to the wild variation of the energy deposition along each track: track matching is performed by comparing the deposited-charge time profiles of the tracks [15,16] These chips include a self-trigger facility and provide real-time information of the channels that have seen signal while the drifing electrons from the TPC volume are raining on the collecting strips: a space-grade autonomous TPC trigger system is being studied

Gamma-Ray Astronomy with an Autonoumous Active Target

Conclusions

A TPC as high performance gamma-ray telescope and polarimeter