Toru Tamagawa

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.

An optical spectrum of the afterglow of a γ-ray burst at a redshift of z = 6.295

The prompt γ-ray emission from γ-ray bursts (GRBs) should be detectable out to distances of z > 10 (ref. 1), and should therefore provide an excellent probe of the evolution of cosmic star formation, reionization of the intergalactic medium, and the metal enrichment history of the Universe. Hitherto, the highest measured redshift for a GRB has been z = 4.50 (ref. 5). Here we report the optical spectrum of the afterglow of GRB 050904 obtained 3.4 days after the burst; the spectrum shows a clear continuum at the long-wavelength end of the spectrum with a sharp cut-off at around 9,000 Å due to Lyman α absorption at z ≈ 6.3 (with a damping wing). A system of absorption lines of heavy elements at z = 6.295 ± 0.002 was also detected, yielding the precise measurement of the redshift. The Si ii fine-structure lines suggest a dense, metal-enriched environment around the progenitor of the GRB.

Discovery of the short gamma-ray burst GRB 050709.

Gamma-ray bursts (GRBs) fall into two classes: short-hard and long-soft bursts. The latter are now known to have X-ray and optical afterglows, to occur at cosmological distances in star-forming galaxies, and to be associated with the explosion of massive stars. In contrast, the distance scale, the energy scale and the progenitors of the short bursts have remained a mystery. Here we report the discovery of a short-hard burst whose accurate localization has led to follow-up observations that have identified the X-ray afterglow and (for the first time) the optical afterglow of a short-hard burst; this in turn led to the identification of the host galaxy of the burst as a late-type galaxy at z = 0.16 (ref. 10). These results show that at least some short-hard bursts occur at cosmological distances in the outskirts of galaxies, and are likely to be caused by the merging of compact binaries.

Global Characteristics of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by HETE-2

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first 3 years of its mission, focusing on the properties of X-ray flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable, and that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy E of the burst spectrum in νFν, the peak energy flux Fpeak, and the energy fluence SE of XRFs are much smaller (and those of XRRs are smaller) than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the [SE(2-30 keV), SE(30-400) keV] plane, the [SE(2-400 keV), Epeak] plane, and the [Fpeak(50-300 keV), Epeak] plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon.

Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE

We present a spectral analysis of 35 GRBs detected with the HETE-2 gamma-ray detectors (the FREGATE instru- ment) in the energy range 7-400 keV. The GRB sample analyzed is made of GRBs localized with the Wide Field X-ray Monitor onboard HETE-2 or with the GRB Interplanetary Network. We derive the spectral parameters of the time-integrated spectra, and present the distribution of the low-energy photon index, alpha, and of the peak energy, Ep. We then discuss the existence and nature of the recently discovered X-Ray Flashes and their relationship with classical GRBs.

SUZAKU DISCOVERY OF THE STRONG RADIATIVE RECOMBINATION CONTINUUM OF IRON FROM THE SUPERNOVA REMNANT W49B

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant (SNR) W49B obtained with the Suzaku satellite. The spectrum exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This bump cannot be explained by any combination of high-temperature plasmas in ionization equilibrium. We firmly conclude that this bump is caused by the strong radiative recombination continuum (RRC) of iron, detected for the first time in a SNR. The electron temperature derived from the bremsstrahlung continuum shape and the slope of the RRC is ~1.5 keV. On the other hand, the ionization temperature derived from the observed intensity ratios between the RRC and Kα lines of iron is ~2.7 keV. These results indicate that the plasma is in a highly overionized state. Volume emission measures independently determined from the fluxes of the thermal and RRC components are consistent with each other, suggesting the same origin of these components.

On the Spectrum and Spectropolarimetry of Type Ic Hypernova SN 2003dh/GRB 030329

Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB 030329 obtained in 2003 May using the Subaru 8.2 m Telescope are presented. The properties of the supernova (SN) are investigated through a comparison with spectra of the Type Ic hypernovae SN 1997ef and SN 1998bw (hypernovae being a tentatively defined class of SNe with very broad absorption features: these features suggest a large velocity of the ejected material and possibly a large explosion kinetic energy). Comparison with spectra of other hypernovae shows that the spectrum of SN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame days after the gamma-ray burst (GRB; for z = 0.1685), are similar to those of SN 1997ef obtained ~34-42 days after the fiducial time of explosion of that SN. The match with SN 1998bw spectra is not as good (at rest 7300-8000 A), but again spectra obtained ~33-43 days after GRB 980425 are preferred. This indicates that the SN may have intermediate properties between SNe 1997ef and 1998bw. On the basis of the analogy with the other hypernovae, the time of explosion of SN 2003dh is then constrained to be between -8 and +2 days of the GRB. The Si and O P Cygni lines of SN 2003dh seem comparable to those of SN 1997ef, which suggests that the ejected mass in SN 2003dh may match that in SN 1997ef. Polarization was marginally detected at optical wavelengths. This is consistent with measurements of the late afterglow, implying that it originated mostly in the interstellar medium of the host galaxy.

The Astro-H High Resolution Soft X-Ray Spectrometer

We present the overall design and performance of the Astro-H (Hitomi) Soft X-Ray Spectrometer (SXS). The instrument uses a 36-pixel array of x-ray microcalorimeters at the focus of a grazing-incidence x-ray mirror Soft X-Ray Telescope (SXT) for high-resolution spectroscopy of celestial x-ray sources. The instrument was designed to achieve an energy resolution better than 7 eV over the 0.3-12 keV energy range and operate for more than 3 years in orbit. The actual energy resolution of the instrument is 4-5 eV as demonstrated during extensive ground testing prior to launch and in orbit. The measured mass flow rate of the liquid helium cryogen and initial fill level at launch predict a lifetime of more than 4 years assuming steady mechanical cooler performance. Cryogen-free operation was successfully demonstrated prior to launch. The successful operation of the SXS in orbit, including the first observations of the velocity structure of the Perseus cluster of galaxies, demonstrates the viability and power of this technology as a tool for astrophysics.

Soft x-ray spectrometer (SXS): The high-resolution cryogenic spectrometer onboard ASTRO-H

We present the development status of the Soft X-ray Spectrometer (SXS) onboard the ASTRO-H mission. The SXS provides the capability of high energy-resolution X-ray spectroscopy of a FWHM energy resolution of < 7eV in the energy range of 0.3 – 10 keV. It utilizes an X-ray micorcalorimeter array operated at 50 mK. The SXS microcalorimeter subsystem is being developed in an EM-FM approach. The EM SXS cryostat was developed and fully tested and, although the design was generally confirmed, several anomalies and problems were found. Among them is the interference of the detector with the micro-vibrations from the mechanical coolers, which is the most difficult one to solve. We have pursued three different countermeasures and two of them seem to be effective. So far we have obtained energy resolutions satisfying the requirement with the FM cryostat.

When do internal shocks end and external shocks begin? Early-time broadband modeling of GRB 051111

Even with the renaissance in gamma-ray burst (GRB) research fostered by the Swift satellite, few bursts have both contemporaneous observations at long wavelengths and exquisite observations at later times across the electromagnetic spectrum. We present here contemporaneous imaging with the KAIT robotic optical telescope, dense optical sampling with Lulin, supplemented with infrared data from PAIRITEL and radio to gamma-ray data from the literature. For the first time, we can test the constancy of microphysical parameters in the internal-external shock paradigm and carefully trace the flow of energy from the GRB to the surrounding medium. KAIT data taken ≤1 minute after the start of GRB 051111 and coinciding with the fading gamma-ray tail of the prompt emission indicate a smooth reinjection of energy into the shock. No color change is apparent in observations beginning ∼ 1.5 minutes after the GRB and lasting for the first hour after the burst. There are achromatic flux modulations about the best-fit model at late (t ≈ 104 s) times, possibly due to variations in the external density. We find that the host galaxy extinction is well fit by a curve similar to that of the Small Magellanic Cloud. Low visual extinction, AV ≈ 0.2 mag, combined with high column densities determined from the X-ray and optical spectroscopy (NH > 10 21 cm-2), indicate a low dust-to-metals ratio and a possible overabundance of the light metals. An apparent small ratio of total to selective extinction (RV ≈ 2) argues against dust destruction by the GRB. Time constancy of both the IR/optical/UV spectral energy distribution and the soft X-ray absorption suggests that the absorbing material is not local to the GRB.