Gamma-ray burst observation requiring continuous watchingE
In a gamma-ray burst, information on distance from the burst is as important as the high-energy gamma-ray. We use afterglowEto determine distance. Recent observations revealed that gamma-ray bursts shine for more than one day in lower energy such as X-ray, visible light, and radiowave. It is possible to measure distance to the burst by observing the bursts afterglow and then identifying the accompanying galaxy. The chance of determining distance increases as we observe the afterglow when it is brightest. Therefore, we need to provide information on detection of gamma-ray bursts as soon as possible to other observatories with X-ray and/or visible light telescopes.
To this end, researchers from Japan, the U.S. and Europe continuously monitor satellite data by rotation. Once a gamma-ray burst is detected and data received, the researchers in charge analyze the data quickly to report information, such as its location determined by LAT, to researchers around the world. In Japan, ISAS, Tokyo Institute of Technology and Hiroshima University participate in the monitoring team.
As of January 2010, the Fermi satellite has detected high-energy gamma-ray of over 100 million electron volts from 14 gamma-ray bursts. Four of these were reported by Japanese researchers as the principal analyzer. Due to the LATs high location-determination accuracy and quick data analysis, the distance was successfully determined for seven out of 14 cases. The number of detections has already exceeded that of EGRET onboard the Compton satellite. In addition, nine papers (including those under peer review) have been published in the journals Nature, Science, etc. Thus, the Fermi satellite has achieved much more than expected in little more than one year.
Now I would like to introduce in more detail the case of GRB 090510, a gamma-ray burst observed by the Fermi satellite.
GRB 090510 observed by the Fermi satellite
At 9:23 a.m. on May 10, 2009 (Japan time), we received notice that the Fermi satellite had detected gamma-ray burst GRB 090510. Partly because the Japan team was monitoring, the detection data were quickly analyzed and released to the world, and the distance was successfully determined. The result indicated that the burst occurred 7.3 billion light years away. Fig. 2 shows the temporal changes in the brightness of GRB 090510, which were produced over several energy ranges. They are shown in order of low to high energy from the top. From this graph, we found a tendency that the onset of the high-energy gamma-ray in the burst is delayed as its energy is high.
From past data from the Compton satellite, we never imagined such a tendency. This was first discovered because of the high gamma-ray detection performance of the Fermi satellite. Fortunately, the X-ray afterglow of GRB 090510 was simultaneously observed by the Swift satellite. As a result, it was revealed that X-ray afterglow and high-energy gamma ray show similar darkening behavior.