S. Kishimoto

Basic Performance of a Polarimeter for Gamma-Ray Bursts Using Segmented Scintillators

We have been developing a Compton-scattering-type hard-X-ray polarimeter sensitive over the energy range from ~ 60 keV to ~ 300 keV for Gamma-Ray Burst (GRB) studies. The polarimeter consists of four identical modules each with a field of view of ±45°. To estimate the instrument performance accurately, we have investigated the instruments response for gamma rays incident off-axis. We report here results of computer simulations coupled with basic experiments using polarized X-ray beams. As a result of this work we estimate that our polarimeter can detect the polarization for ~ 100 GRBs in 5 years at 99% confidence level if the degree of polarization is higher than 40%.

Low energy response of a prototype detector array for the PoGO astronomical hard X-ray polarimeter

The Polarized Gamma-ray Observer (PoGO) is a new balloon-borne instrument designed to measure polarization from astrophysical objects in the 30-200 keV range. It is under development for the first flight anticipated in 2008. PoGO is designed to minimize the background by an improved phoswich configuration, which enables a detection of 10 % polarization in a 100 mCrab source in a 6--8 hour observation. To achieve such high sensitivity, low energy response of the detector is important because the source count rate is generally dominated by the lowest energy photons. We have developed new PMT assemblies specifically designed for PoGO to read-out weak scintillation light of one photoelectron (1 p.e.) level. A beam test of a prototype detector array was conducted at the KEK Photon Factory, Tsukuba in Japan. The experimental data confirm that PoGO can detect polarization of 80-85 % polarized beam down to 30 keV with a modulation factor 0.25 ± 0.05.

Expected performance of the PHENEX polarimeter by enlargement of detection area

We have been developing a hard X-ray polarimeter with high sensitivity, called as a PHENEX (Polarimetry for High ENErgy X rays) polarimeter. We constructed prototype PHENEX polarimeter and carried out a preliminary observation of the Crab Nebula on Jun. 13th 2006 as a balloon-borne experiment. Though we confirmed from the data that PHENEX polarimeter detected hard X rays from the Crab Nebula with a significance of 8σ, the degree and the direction of polarization with high accuracy could not be determined because of the trouble for attitude control system (ACS) and the small detection area of the prototype polarimeter. Now we are fixing the ACS trouble and improving the detector to realize the observation of the Crab Nebula with higher accuracy in the next balloon-borne experiment. In this paper, we will mainly explain about the improvement of the PHENEX polarimeter and present the expected performance for the next observation.

Basic performance of the polarimeter for gamma-ray bursts using MAPMTs and segmented scintillators

We have been designing a polarimeter for Gamma-Ray Bursts (GRBs) sensitive to the energy range from 60 keV to 300 keV. It is a Compton-scattering-type polarimeter and it consists of four modules. The field of view for one module is ±45°. To estimate the performance, it is important to investigate the response for the irradiation from off-axis. So we had basic experiments with polarized beam and computer simulation. As the results, we recognized that it can measure the polarization for 19 GRBs per year at 99% confidence level if the degree of polarization is higher than 40 %.

Preliminary observation of polarization for Crab Nebula by PHENEX polarimeter

We have been developing a hard X-ray polarimeter called “PHENEX” (Polarimetry for High ENErgy X rays). The PHENEX is Compton-scattering-type polarimeter. It is constructed from an array of identical small polarimeter called “unit counter”. In Jun. 13 2006, we had carried out balloon-borne experiment to study the polarization of hard X rays from the Crab Nebula, loading the PHENEX polarimeter with four unit counters surrounded by active shields of CsI (Tl). The PHENEX accomplished the level flight at the altitude of 37.5 km over 6 hours and the detector system operated well over the duration. However, attitude control system did not function correctly and the line of the sight wandered around the Crab Nebula. Even so the PHENEX polarimeter can observe the Crab Nebula for about one hour and also a blank region of the sky for about one hour. Calculating the effect of the wandering by computer simulation, we carried out the analysis about the polarization of the Crab Nebula. Though the statistical error is large and the significance is low, the modulation to indicate the polarization can be found for the data of Crab Nebula. In near future, we will carry out balloon borne experiment again, improving the PHENEX polarimeter and fixing the trouble of the attitude control system.