# Imaging Polarimeter for a Sub-MeV Gamma-Ray All-Sky Survey using an Electron-Tracking Compton Camera

## TL;DR

This paper introduces an electron-tracking Compton camera (ETCC) as a highly-sensitive gamma-ray polarimeter capable of wide-field, all-sky surveys, demonstrating stable modulation factors under realistic conditions for studying celestial gamma-ray sources.

## Contribution

The paper presents the first demonstration of an ETCC's stable modulation factor under off-axis and high-background conditions, enabling effective all-sky gamma-ray polarization surveys.

## Key findings

- Measured modulation factor of 0.65 at 150 keV for off-axis incidence
- ETCC can survey weak persistent sources down to 13 mCrab with 10% polarization
- ETCC can detect GRBs with fluence down to 6×10⁻⁶ erg cm⁻² with 10% polarization

## Abstract

X-ray and gamma-ray polarimetry is a promising tool to study the geometry and the magnetic configuration of various celestial objects, such as binary black holes or gamma-ray bursts (GRBs). However, statistically significant polarizations have been detected in few of the brightest objects. Even though future polarimeters using X-ray telescopes are expected to observe weak persistent sources, there are no effective approaches to survey transient and serendipitous sources with a wide field of view (FoV). Here we present an electron-tracking Compton camera (ETCC) as a highly-sensitive gamma-ray imaging polarimeter. The ETCC provides powerful background rejection and a high modulation factor over a FoV of up to 2$\pi$ sr thanks to its excellent imaging based on a well-defined point spread function. Importantly, we demonstrated for the first time the stability of the modulation factor under realistic conditions of off-axis incidence and huge backgrounds using the SPring-8 polarized X-ray beam. The measured modulation factor of the ETCC was 0.65 $\pm$ 0.01 at 150 keV for an off-axis incidence with an oblique angle of 30$^\circ$ and was not degraded compared to the 0.58 $\pm$ 0.02 at 130 keV for on-axis incidence. These measured results are consistent with the simulation results. Consequently, we found that the satellite-ETCC proposed in Tanimori et al. (2015) would provide all-sky surveys of weak persistent sources of 13 mCrab with 10% polarization for a 10$^{7}$ s exposure and over 20 GRBs down to a $6\times10^{-6}$ erg cm$^{-2}$ fluence and 10% polarization during a one-year observation.