Study of the formation and the evolution of the Universe with prompt observations of gamma-ray bursts

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The goal of this study is to investigate the production mechanism of the relativistic jets in gamma-ray bursts (GRBs) and the history of the massive star formation in the ancient Universe using GRBs by prompt observations of early optical and near-infrared afterglows. We succeeded in measuring the optical spectrum of the afterglow of GRB 050904 with the Subaru Telescope. We found that it was the most distant gamma-ray burst ever recorded, at a redshift of 6.3, or 12.8 billion years ago, and that the Universe was largely ionized at that age, and measured the metallicity of the host galaxy. We built two 50cm optical telescopes equipped with specially designed tricolor cameras. These two telescopes respond automatically to the GRB alerts from the satellites to perform observations of the GRB afterglows. The obtained images are automatically analyzed and stored in the database. As an example, the afterglow of GRB 060115 was imaged at Okayama first, and led the redshift measurement (z=3.5) by a foreign team. We also modified the existing 91cm telescope in Okayama into an automatic near-infrared telescope with a very wide field of view. Furthermore, we installed our tri-color camera on the 1-m telescope at the Ishigakijima Observatory to increase the observing opportunities.As a first step towards a small satellite that promptly notifies the location of GRBs, we developed a light-weight radiation detector using APDs, which we mounted on the student-built very small satellite "Cute 1.7+APD". The satellite was successfully launched into an orbit as a piggy back payload with the JAXA M-V 8 rocket in February 2006, but the data of the detector was not obtained due to a trouble in the communication system of the satellite. With the second satellite "Cute 1.7+APD II" launched in April 2008, we succeeded in the first in-orbit verification of radiation detector using APDs by measuring the low-energy charged particles in the radiation zone.