AN ELECTRON-TRACKING COMPTON TELESCOPE FOR A SURVEY OF THE DEEP UNIVERSE BY MeV GAMMA-RAYS

Abstract

Photon imaging for MeV gammas has serious difficulties due to huge backgrounds and unclearness in images, which are originated from incompleteness in determining the physical parameters of Compton scattering in detection, e.g., lack of the directional information of the recoil electrons. The recent major mission/instrument in the MeV band, Compton Gamma Ray Observatory/COMPTEL, which was Compton Camera (CC), detected mere ∼ 30 persistent sources. It is in stark contrast with ∼2000 sources in the GeV band. Here we report the performance of an Electron-Tracking Compton Camera (ETCC), and prove that it has a good potential to break through this stagnation in MeV gamma-ray astronomy. The ETCC provides all the parameters of Compton-scattering by measuring 3D recoil electron tracks; then the Scatter Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx), which CCs cannot measure, is also obtained, and is found to be indeed helpful to reduce the background under conditions similar to space. Accordingly the significance in gamma detection is improved severalfold. On the other hand, SPD is essential to determine the point-spread function (PSF) quantitatively. The SPD resolution is improved close to the theoretical limit for multiple scattering of recoil electrons. With such a well-determined PSF, we demonstrate for the first time that it is possible to provide reliable sensitivity in Compton imaging without utilizing an optimization algorithm. As such, this study highlights the fundamental weak-points of CCs. In contrast we demonstrate the possibility of ETCC reaching the sensitivity below 1×10 −12 erg cm −2 s −1 at 1 MeV.

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

“…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. …”

Radioactive Gamma-Ray Emissions from Neutron Star Mergers

“…However, the astrophysics in the photon energy range 0.2-100 MeV is very rich, including GRBs, blazars, neutron stars, supernovae, etc; particularly the radioactive decay emissions from various sources. To study the spectacular astrophysics in the MeV gamma-ray range, a few missions and telescopes have been launched or proposed, for instance, the Satellite-ETCC (Electron-Tracking Compton Experiments, Tanimori et al 2015), the e-ASTROGAM space mission (Tatischeff et al 2016), and the AMEGO (All-sky Medium Energy Gamma-ray Observatory, Moiseev 2017; Rando 2017).…”

Possibility of Systematic Study of Supernova Explosions by Nuclear Imaging Spectroscopy

“…A well-defined PSF is the most effective tool for background suppression similar to optical/X-ray telescopes, which suppresses for the most part of gamma rays coming from the outside of the PSF in particular major components, and it enables us to estimate the remaining contamination quantitatively. A particle identification using energy-loss rate (dE/dx) selects events in which the electron stopped in the gaseous volume [8]. The dE/dx rejects non-gamma-ray particles and electron-escaped events.…”

“…Both SPI and HXD suffer from intense background radiations [2,6]. We have developed an electrontracking Compton camera (ETCC), which is the first MeV gamma-ray telescope to provide a welldefined point spread function (PSF) within a few degrees and efficient background rejection [7,8].We note the PSF is defined with two directional angles similar to optics, whereas in standard Compton cameras, the angular resolution is defined with only one directional angle. We have revealed a satellite equipped with four modules of (50 cm) 3 -sized ETCCs, that will reach a sensitivity of 1 mCrab [8].…”