Daisuke Yonetoku

Gamma-Ray Burst Formation Rate Inferred from the Spectral Peak Energy-Peak Luminosity Relation

We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (Ep) and the peak luminosity. The new relation is derived by combining the data of Ep and the peak luminosities by BeppoSAX and BATSE, and it looks considerably tighter and more reliable than the relations suggested by the previous works. Using the new Ep-luminosity relation, we estimate redshifts of the 689 GRBs without known distances in the BATSE catalog and derive a GRB formation rate as a function of the redshift. For the redshift range of 0 ≤ z ≤ 2, the GRB formation rate increases and is well correlated with the star formation rate, while it keeps constant toward z ~ 12. We also discuss the luminosity function and the redshift dependence of the intrinsic luminosity (luminosity evolution).

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

In-orbit performance of the hard X-ray detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600keV, energy resolutions of ∼ 4keV (FWHM) at 40keV and ∼ 11% at 511keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

DETECTION OF GAMMA-RAY POLARIZATION IN PROMPT EMISSION OF GRB 100826A

We report the polarization measurement in prompt γ-ray emission of GRB 100826A with the Gamma-Ray Burst Polarimeter on board the small solar-power-sail demonstrator IKAROS. We detected the firm change of polarization angle (PA) during the prompt emission with 99.9% (3.5σ) confidence level, and the average polarization degree (Π) of 27% ± 11% with 99.4% (2.9σ) confidence level. Here the quoted errors are given at 1σ confidence level for the two parameters of interest. The systematic errors have been carefully included in this analysis, unlike other previous reports. Such a high Π can be obtained in several emission models of gamma-ray bursts (GRBs), including synchrotron and photospheric models. However, it is difficult to explain the observed significant change of PA within the framework of axisymmetric jet as considered in many theoretical works. The non-axisymmetric (e.g., patchy) structures of the magnetic fields and/or brightness inside the relativistic jet are therefore required within the observable angular scale of ~Γ–1. Our observation strongly indicates that the polarization measurement is a powerful tool to constrain the GRB production mechanism, and more theoretical works are needed to discuss the data in more detail.

Development of the HXD-II wide-band all-sky monitor onboard Astro-E2

The hard X-ray detector (HXD-II) is one of the three scientific instruments onboard Japanese X-ray astronomy satellite Astro-E2 scheduled to be launched in 2005. This mission is very unique in a point of having a lower background than any other past missions in the 10-600 keV range. In the HXD-II, the large and thick BGO crystals are used as active shields for particle and gamma-ray background to the main detector. They have a wide field of view of ~2pi and a large effective area of 400 cm2 even at 1 MeV. Hence, the BGO shields have been developed as a wide-band all-sky monitor (WAM) with a broadband coverage of 50-5000 keV. In this paper, overall design and performance of the HXD-II/WAM based on the results of preflight calibration tests carried out in June 2004 are described. By irradiating various radio isotopes with the WAM flight model, we verified that it had comparable capabilities with other gamma-ray burst detectors

THE QUIESCENT COUNTERPART OF THE SOFT GAMMA-RAY REPEATER SGR 0526 66

It is now commonly believed that soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are magnetars—neutron stars powered by their magnetic fields. However, what differentiates these two seemingly dissimilar objects is, at present, unknown. We present Chandra observations of RX J052600.3� 660433, the quiescent X-ray counterpart of SGR 0526� 66, famous for the intense burst of 1979 March 5. The source is unresolved at the resolution of Chandra. Restricting observations to a period range around 8 s, the period noted in the afterglow of the burst of 1979 March 5, we find evidence for a similar periodicity in two epochs of data obtained 20 months apart. The secular period derivative based on these two observations is 6:6ð5 Þ� 10 � 11 ss � 1 , similar to the period derivatives of the magnetars. As is the case with other magnetars, the spectrum is best fitted by a combination of a blackbody and a power law. However, quite surprisingly, the photon index of the power-law component is � � 3—intermediate to those of AXPs and SGRs. This continuum of C led us to suggest that the underlying physical parameter that differentiates SGRs from AXPs is manifested in the power-law component. Two decades ago, SGR 0526� 66 was a classical SGR, whereas now it behaves like an AXP. Thus, it is possible that the same object cycles between the SGR and AXP states. We speculate that the main difference between AXPs and SGRs is the geometry of the B fields, and this geometry is time dependent. Finally, given the steep spectrum of RX J052600.3� 660433, the total radiated energy of RX J052600.3� 660433 can be much higher than traditionally estimated. If this energy is supplied by the decay of the magnetic field, then the inferred B field of RX J052600.3� 660433 is in excess of 10 15 G, the traditional value for magnetars. Independent of this discussion, there could well be a class of neutron stars, 10 14 GdBd10 15 G, which are neither radio pulsars nor magnetars. Subject headings: pulsars: individual (SGR 0526� 66) — stars: neutron — X-rays: stars

Magnetic Structures in Gamma-Ray Burst Jets Probed by Gamma-Ray Polarization

We report polarization measurements in two prompt emissions of gamma-ray bursts, GRB 110301A and GRB 110721A, observed with the gamma-ray burst polarimeter (GAP) on borad the IKAROS solar sail mission. We detected linear polarization signals from each burst with polarization degree of {Pi} = 70 {+-} 22% with statistical significance of 3.7{sigma} for GRB 110301A, and {Pi} = 84{sup +16}{sub -28}% with 3.3{sigma} confidence level for GRB 110721A. We did not detect any significant change of polarization angle. These two events had shorter durations and dimmer brightness compared with GRB 100826A, which showed a significant change of polarization angle, as reported in Yonetoku et al. Synchrotron emission model can be consistent with the data of the three GRBs, while the photospheric quasi-thermal emission model is not favored. We suggest that magnetic field structures in the emission region are globally ordered fields advected from the central engine.

Improvements of the astro-E2 hard X-ray detector (HXD-II)

We summarize significant improvements which have been achieved in the development of Astro-E2 Hard X-ray Detector (HXD-II). An expanded energy range and better energy resolution have been achieved from progresses in device materials and redesigning of the front-end electronics. An improved estimation for the detector background in orbit has also been conducted based upon results from our proton irradiation experiment. The sensitivity of HXD-II can be expected to reach an order of 10/sup -6/ [cs/sup -1/ keV/sup -1/ cm/sup -2/].

Development and qualification of the HXD-II onboard Astro-E2

The Hard X-ray Detector (HXD-II), one of instruments onboard the Astro-E2 satellite to be launched in February 2005, is in the final stage of its development. The HXD-II probes the universe in the energy range of 10-600 keV with a sensitivity by an order of magnitude better than those of previous missions. The assembly of the HXD-II completed in January 2004, followed by a series of pre-launch qualification tests. As a result, the design goals of the HXD-II have been met. These include; a background level of 5 x 10-6 counts/s/keV/cm2 at 200 keV for GSO and 1 x 10-5 counts/s/keV/cm2 at 30 keV for PIN; energy resolutions of 2.9 keV (PIN diode, at 59.5 keV) and 10% (GSO scintillator, at 662 keV); and low energy thresholds of 10 keV for PIN diodes and 30 keV for GSO scintillators. The measured background predicts a continuum sensitivity of a few x 10-6 photons/s/keV/cm2. Anti-Counter units surrounding the HXD-II provide 50 keV-5 MeV information on gamma-ray bursts and bright X-ray transients.

Soft gamma-ray detector for the ASTRO-H mission

The Soft Gamma-ray Detector (SGD) on board ASTRO-H (Japanese next high-energy astrophysics mission) is a Compton telescope with narrow fleld-of-view, which utilizes Compton kinematics to enhance its background rejection capabilities. It is realized as a hybrid semiconductor detector system which consists of silicon and CdTe (cadmium telluride) detectors. It can detect photons in a wide energy band (50-600 keV) at a background level 10 times better than that of the Suzaku Hard X-ray Detector, and is complimentary to the Hard X-ray Imager on board ASTRO-H with an energy coverage of 5-80 keV. Excellent energy resolution is the key feature of the SGD, allowing it to achieve good background rejection capability taking advantage of good angular resolution. An additional capability of the SGD, its ability to measure gamma-ray polarization, opens up a new window to study properties of gamma-ray emission processes. Here we describe the instrument design of the SGD, its expected performance, and its development status.