Study of Gamma-ray Bursts by the Follow-up Observations Cooperated with the HETE2 Satellite

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HETE2 satellite has started the static observation since early 2001. It has already found about 300 gamma-ray bursts(grbs). For about 70 bursts among them, HETE2 made a multi-wavelength follow up observation possible by delivering their precise positions to all over the world within a few dozens of second via Internet. Almost half of these grbs were found to have the x-ray, optical, or radio afterglow. The distance to the burst was also determined for about half of them. According to these results, the relation between grbs and type Ic supernovae becomes fairly certain.Grbs found and observed by HETE2 can be divided into three categories : typical grb, x-ray rich grb, or x-ray flash by their spectra. But there is no clear boundary among them, and their spectra found to be distributed continuously. Furthermore the relation between the peak energy and the isotropic energy found for typical grbs is also valid for x-ray flashes. So these results show the origin of three kinds of burst is possibly identical.We have successfully observed the optical afterglows of GRB030329 and GRB041006 using the automatic follow up observatory by receiving the HETE2 alert. Especially for GRB041006,we got the optical afterglow at only 70 seconds after the burst. This was the earliest observation among the world. We found that two breaks in its light curve are well explained by synchrotron shock model and a forward shock plays an important roll in generating the optical afterglow.