HARPO, a gas TPC active target for high-performance $\gamma$-ray astronomy; demonstration of the polarimetry of MeV $\gamma$-rays converting to $e^+ e^-$ pair

TL;DR

HARPO developed a gas TPC active target and demonstrated for the first time the polarimetry of linearly polarized MeV gamma rays through pair conversion, opening new possibilities for space-based gamma-ray astronomy.

Contribution

This work introduces a novel gas TPC active target capable of gamma-ray polarimetry above 1 MeV, a capability not previously achieved in space.

Findings

First demonstration of MeV gamma-ray polarimetry using a gas TPC

Successful analysis of gamma-ray conversions to electron-positron pairs

Potential for space-based gamma-ray polarization measurements

Abstract

No $\gamma$-ray polarimeter sensitive above 1 MeV has ever flown space. $\gamma$-ray polarimetry would be a new window on the radiative processes at work in cosmic sources, processes that produce linearly polarised emission, each of which with different polarisation fractions. The HARPO Collaboration has designed, built and characterised on beam a gas-TPC active target with which we have demonstrated for the first time the polarimetry of a linearly polarised MeV $\gamma$-ray beam, from the analysis of the conversions to $e^+ e^-$ pairs.