Gamma-Ray Burst Polarimeter (GAP)
There are almost no previous studies of polarization observation in gamma-ray band. The polarization was once discussed using the data from the orbiting solar observatory, RHESSI, and the gamma-ray observatory, INTEGRAL. However, there was some doubt since completely different results were given by independent groups using the same data. Therefore, group founded by Kanazawa University, Yamagata University and the Institute of Physical and Chemical Research (RIKEN) considered to realize the first observation in the world by a small but a brand new instrument which is designed and calibrated for the main purpose of gamma-ray polarization observation. Installed on IKAROS, GAP has been the first detector in the world to carry out gamma-ray polarization observation in space.
Figure 1 shows an inner structure and a photograph of the flight model of GAP. It is 17 cm in diameter, 16 cm in height, 3.7 kg in weight and 5 watts of electrical power consumption. This is a very small size for a gamma-ray detector. The internal sensors consist of a dodecagonal plastic scintillator and 12 pieces of CsI scintillators surrounding it. Since gamma-rays tend to scatter toward the direction perpendicular to the polarization direction, GAP makes it possible to measure the distribution of the scattering angles. For example, if the incident gamma-ray is polarized horizontally, the scattering intensity will be higher in the vertical direction. Therefore the intensity distribution of the scattering angles shows a modulation as a function of scattering angle like two mountains in the shape of M (or W) from 0 to 360 degrees.
We developed the most part of GAP on a very narrow clean bench in our laboratory. We used several radiation tolerant devices, for example the power relay circuit connecting to the bus system of IKAROS, as well as the LVDS device for communication, the FPGA and the CPU (8051 core burnt inside the FPGA), which are also main parts inside the GAP, and a part of the data storage memory. However the most of the other circuit components are all consumer products selected by radiation tolerance experiments by ourselves. The vibration test at the Industrial Research Institute of Ishikawa and the drop impact tests (the pyro shock test) in the ISAS made us shed tears on my pillow again and again. But every time after that, we would start again and repeat this trial-and-error job. We also disassembled and remade the flight model. Toshio Murakami, the team leader from the Kanazawa University, implemented the policy that if we could not assure 2 issues, GAP should not be installed on IKAROS. The 2 issues were making there to be no unknown pointsEand handling all anxious factorsE I myself made the development with the spirits as well. GAP worked in space for 1.5 years and there were no troubles at all. I am very impressed by this fact although I am one of the party concerned.
The First Polarization Observation
On August 26, 2010, GAP detected GRB 100826A, a very bright burst. The brightness of this GRB could easily be in the top 1% compared to other cases in the past, and GAP was able to observe it in a very good condition with the incident angle of only 20 degrees from GAPs detector axis. This GRB lasted about 100 seconds and showed several pulses. We analyzed the polarization data by dividing the entire data into two intervals, and found that the phase of the polarization angle changed drastically with a significance level of 3.5σ (Figure 2: Left). We have also measured that the average degree of polarization of the both two intervals was 27±11% (with a significance level of 2.9σ, shown in Figure 2: Right).
Further in the next year, we detected two bright bursts GRB 110301A and GRB 110721A, and also succeeded in measuring their polarization degree as 70±22% (significance 3.7σ) and 84 (+16, -28)% (significance 3.3σ), respectively. Both of the two cases of GRB had a high level of polarization. We have also studied the time variation of the polarization angles of the two cases but found no significant changes. Their time duration was as short as 10 seconds, so there are some different situations from the GRB 100826A mentioned above. GAP has detected 30 cases of GRB so far, and only in the 3 cases above, we detected significant polarization. From other cases we obtained the upper limit of the polarization degree, and we consider the average value is about 30% for all cases.