MeV-GeV Polarimetry with $\gamma \to e^+e^-$: Asserting the Performance of Silicon Strip Detectors-Based Telescopes

TL;DR

This paper evaluates the potential of silicon strip detector-based telescopes for gamma-ray polarimetry, aiming to enable polarization measurements of high-energy gamma-ray sources with improved sensitivity and guide future mission designs.

Contribution

It provides the first characterization of SSD-based polarimeters' performance and develops methods to enhance their sensitivity for gamma-ray polarimetry.

Findings

SSD-based detectors can effectively measure gamma-ray polarization.

New methods improve the sensitivity of SSD polarimeters.

Potential to analyze existing Fermi LAT and AGILE data for polarization signals.

Abstract

The polarimetry of gamma rays converting to an $e^+e^-$ pair would open a new window on the high-energy gamma-ray sky by, among other things, providing insight into the radiation mechanism in pulsars (curvature or synchrotron) or deciphering the composition of the gamma-ray emitting jets in blazars (leptonic or lepto-hadronic). The performance of polarimeters based on homogeneous active targets (gas detectors (MeV, HARPO) or emulsions (GeV, GRAINE) has been studied both with simulation and by the analysis of data collected with telescope prototypes on linearly-polarised gamma-ray beams, and found to be excellent. The present (Fermi LAT), AGILE and future project (AMEGO, ASTROGAM) gamma-ray missions, though, are using active targets based on silicon strip detectors (SSD). No demonstration of a non-zero effective polarisation asymmetry with SSDs has been published to date, be it only with simulated data, and sensitivity estimations were obtained from an assumed value of the effective polarisation asymmetry. I present a characterisation of the potential of SSD-based active targets for polarimetry with gamma-ray conversions to pairs and the development of various methods to improve on the sensitivity. This work could pave the way to providing the polarimetry of the brightest gamma-ray sources of the sky from the decade of data collected by the Fermi LAT and by AGILE, and to guiding the design of future missions.