Hironobu Kodaira

Spectral Evolution of GRB060904A Observed with Swift and Suzaku- Possibility of Inefficient Electron Acceleration

We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from

Development of polarimeter for gamma-ray bursts onboard the solar-powered sail mission

\Gamma = 1.51^{+0.04}_{-0.03}

Swift and Suzaku observations of the X-ray afterglow from the GRB 060105

to

Suzaku and Swift observations for X‐ray afterglows; Investigation into the electron acceleration in the internal/external shocks

\Gamma = 5.30^{+0.69}_{-0.59}

The Suzaku-Swift Joint Observation of the Early X-Ray Afterglow of GRB060105

within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of

The Suzaku-Swift Joint Observation of the Early X-Ray Afterglow of GRB060105(Chapter 1. Gamma Ray Bursts, Cosmic Rays and Neutrino, The Extreme Universe in the Suzaku Era)

\propto t^{-3} \sim t^{-4}

Strategy of the Suzaku gamma-ray burst observations

while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.

GRB 060801: Kanazawa optical observation.

The solar powered sail spacecraft using a huge sail is a next Japanese engineering verification satellite planned to launch in 2012. It has a hybrid propulsion system with ion engines and a huge solar sail panel of 50 m in diameter. Based on the present mission plan, it will take about 6 years to cruise to Jupiter via Earth swing-bys and 5 more years to reach the Jovian L4 Trojan asteroids. During its cruising phase, we plan to mount a gamma-ray burst (GRB) detector with polarization detection capability which also works as one of the interplanetary network (IPN) to determine the GRB positions. The emission mechanism of GRB is thought to be the synchrotron radiation from the relativistic outflows. If the emission mechanism of GRBs is really synchrotron radiation, the emitted gamma-rays should be strongly polarized. The detection principle is the anisotropy of the Compton scattering. The Compton-scattered gamma-ray photons show the strongly biased distribution toward the vertical direction against the oscillating electric field vector. The design concept of our detector is simple but carefully avoid a fake modulation. The plastic scintillator in one Compton-length as the scattering body is placed at the center, and 12 CsI scintillators are allocated around it. To avoid a fake modulation through the satellite body scattering, these detectors work in coincidence mode. The coincidence also helps to reduce the particle background. We will use the VA-TA ASIC and FPGA as the analog readout and the digital event processing, respectively, to make the detector weight of almost 2.0 kg. In this paper, we introduce the solar sail mission and show the overview of gamma-ray polarimeter.

GRB 060807: kanazawa r-band observation.

Results are presented of early X-ray afterglow observations of GRB 060105 by Swift and Suzaku. The bright, long gamma-ray burst GRB 060105 triggered the Swift Burst Alert Telescope (BAT) at 06:49:28 on 5 January 2006. The Suzaku team commenced a pre-planned target of opportunity observation at 19 ks (5.3 hr) after the Swift trigger. Following the prompt emission and successive very steep decay, a shallow decay was observed from T_0+187 s to T_0+1287 s. After an observation gap during T_0 +(1.5-3) ks, an extremely early steep decay was observed in T_0+(4-30) ks. The lightcurve flattened again at T_0+30 ks, and another steep decay followed from T_0+50 ks to the end of observations. Both steep decays exhibited decay indices of 2.3 - 2.4. This very early break, if it is a jet break, is the earliest case among X-ray afterglow observations, suggesting a very narrow jet whose opening angle is well below 1 degree. The unique Suzaku/XIS data allow us to set very tight upper limits on line emission or absorption in this GRB. For the reported pseudo-redshift of z=4.0+/-1.3 the upper limit on the iron line equivalent width is 50 eV.

GRB 060108: ISAS early observations and near IR upper limit.

We have performed ToO observations for three bright GRBs with the Japanese X‐ray satellite “Suzaku”. Suzaku strongly supports the follow‐up observations by Swift, especially in the late time afterglow phase, with its capabilities of larger effective area and wider energy band compared with Swift/XRT. In this presentation, mainly focusing on the X‐ray afterglow of GRB 060904A, we introduce these ToO observation results. In GRB 060904A, we found rapid spectral softening with the photon indices from 1.5 to 5.3 during the steep decay (prompt tail) phase in the BAT and XRT data. This ultra soft spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. To explain this ultra soft spectra, we introduce a spectral cutoff model instead of power‐law, and investigate the possibility of the maxi...