Imaging Polarimeter for a Sub-MeV Gamma-Ray All-sky Survey Using an Electron-tracking Compton Camera

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 2π 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 ± 0.01 at 150 keV for an off-axis incidence with an oblique angle of 30• and was not degraded compared to the 0.58 ± 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 × 10 −6 erg cm −2 fluence and 10% polarization during a one-year observation.

Snowmass2021 Cosmic Frontier: The landscape of cosmic-ray and high-energy photon probes of particle dark matter

“…The ETCC measures all parameters of the Compton kinematic equation, including the energy loss rate (dE/dx) of a recoil electron and the event topology, while providing a Point Spread Function (PSF) to identify the direction of the incident γ-rays with a linear imaging system [150,151]. The SMILE-3 ETCC, consisting of four 50cm-cubes filled with 3 atm CF4 gas, can achieve 2 • of PSF (Scatter Plane Deviation (SPD) = 10 • and Angular Resolution Measure (ARM) = 5 • ), providing a sensitivity of ¡1 m-Crab with a 200 cm 2 effective area in 10 6 s observation time [154].…”

Science prospects for SPHiNX – A small satellite GRB polarimetry mission

“…The short observing times, along with the relatively large mean angle of incidence (∼40 • , derived from a uniform distribution of GRBs on the sky), make this approach impractical. Instead, the response to an unpolarised beam must be determined and the measured polarisation corrected for the residual modulation which results from the detector geometry and response variations [40]. This is achieved using computer simulations validated using both polarised and unpolarised beams in the laboratory [41].…”

A Space-based All-sky MeV gamma-ray Survey with the Electron Tracking Compton Camera

“…ETCC can also make the best polarization measurements in the MeV band. Since it constrains the scattering direction of every γ-ray photon, it can naturally measure polarizations of any γ-ray source in its large FOV, unlike conventional Compton γ-ray polarimeters, which need to narrow incoming γ-ray radiation with collimators (Komura et al, 2017). Furthermore, ETCC's powerful background rejection capability will bring high-quality polarization data in intense background conditions in space, which should help detect the polarization of faint and/or weakly polarized sources.…”