Takao Kitaguchi

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.

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

Long-duration γ ray emissions from 2007 and 2008 winter thunderstorms

The Gamma-Ray Observation of Winter Thunderclouds (GROWTH) experiment, consisting of two radiation detection subsystems, has been operating since 2006 on the premises of Kashiwazaki-Kariwa nuclear power plant located at the coastal area of Japan Sea. By February 2010, GROWTH detected seven long-duration γ ray emissions associated with winter thunderstorms. Of them, two events, obtained on 13 December 2007 and 25 December 2008, are reported. On both occasions, all inorganic scintillators (NaI, CsI, and BGO) of the two subsystems detected significant γ ray signals lasting for >1 min. Neither of these two events were associated with any lightning. In both cases, the γ ray energy spectra extend to 10 MeV, suggesting that the detected γ rays are produced by relativistic electrons via bremsstrahlung. Assuming that the initial photon spectrum at the source is expressed by a power law function, the observed photons can be interpreted as being radiated from a source located at a distance of 290–560 m for the 2007 event and 110–690 m for the 2008 one, both at the 90% confidence level. Employing these photon spectra, the number of relativistic electrons is estimated as 10^(9)–10^(11). The estimation generally agrees with those calculated on the basis of the relativistic runaway electron avalanche model. A GROWTH photon spectrum, summed over three individual events including the present two events and another reported previously, has similar features including a cutoff energy, to an averaged spectrum of terrestrial γ ray flashes.

X-Ray Study of Temperature and Abundance Profiles of the Cluster of Galaxies Abell 1060 with Suzaku

We carried out observations of the central and 20 0 east offset regions of the cluster of galaxies Abell 1060 with Suzaku. Spatially resolved X-ray spectral analysis has revealed temperature and abundance profiles of Abell 1060 out to 27 0 380 h � 1 70 kpc, which corresponds to � 0.25 r180 .T emperature decrease of the intra-cluster medium from 3.4 keV at the center to 2.2 keV in the outskirt region was clearly observed. The abundances of Si, S, and Fe also decrease by more than 50% from the center to the outer region, while Mg shows a fairly constant abundance distribution at � 0:7 solar within r . 17 0 .O shows al owe ra bundance of � 0:3 solar in the central region (r . 6 0 ), and indicates a similar feature with Mg; however, it is sensitive to the estimated contribution of the Galactic components of kT1 � 0.15 keV and kT2 � 0.7 keV in the outer annuli (r& 13 0 ). Systematic effects due to the point-spread function tails, contamination on the XIS filters, instrumental background, cosmic and/or Galactic X-ray background, and the assumed solar abundance tables were carefully examined. The results on the temperature and abundances of Si, S, and Fe are consistent with those derived by XMM-Newton at r . 13 0 .T he formation and metal-enrichment process of the cluster are discussed based on the present results.

Hard X-ray properties of the merging cluster Abell 3667 as observed with Suzaku

Abstract : Wide-band Suzaku data on the merging cluster Abell 3667 were examined for hard X-ray emission in excess to the known thermal component. Suzaku detected X-ray signals in a wide energy band from 0.5 to 40 keV. The hard X-ray (>10 keV) flux observed by the HXD around the cluster center cannot be explained by a simple extension the thermal emission with an average temperature of approx. 7 keV. The emission is most likely to be from a very hot kT > 13.2 keV) thermal component around the cluster center, produced via a strong heating process in a merger. In the north-west radio relic, no signature of non-thermal emission was observed. Using the HXD, the overall upperlimit flux within a 34 x 34 field-of-view around the relic was derived to be 5.3 x 10(exp -12) ergs/s/sq cm in the 10-40 keV band, after subtracting the ICM contribution estimated using the XIS or the XMM-Newton spectra. Directly on the relic region, the upper limit is further tightened by the XIS data to be less than 7.3 x 10(exp -13) erg/s/sq cm, when converted into the 10?40 keV band. The latter value suggest that the average magnetic field within the relic is higher than 1.6 microG. The non-thermal pressure due to magnetic fields and relativistic electrons may be as large as 20% of the thermal pressure in the region.

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

Suzaku Observation of the Metallicity Distribution in the Intracluster Medium of the Fornax Cluster

The metallicity distribution in the Fornax cluster was studied with the XIS instrument on board the Suzaku satellite. The K-shell lines of O and Mg were resolved clearly, and the abundances of O, Mg, Si, S, and Fe were measured with great accuracy. The region within a 4 � radius of NGC 1399 shows approximately solar abundances of Fe, Si, and S, while the O/Fe and Mg/Fe abundance ratios are about 0.4–0.5 and 0.7 in solar units, respectively. In the outer region of the radius range 6 � <r< 23 � , the Fe and Si abundances drop to 0.4–0.5 solar and show no significant gradient within this region. The abundance ratios, O/Fe and Mg/Fe, are consistent with those in the central region. We also measured the Fe abundance around NGC 1404 to be approximately solar, and the O, Ne, and Mg abundances to be 0.5–0.7 times the Fe level. A significant relative enhancement of Fe within 130kpc of NGC 1399 and in NGC 1404 indicates an origin in SN Ia, in contrast to the species O, Ne, and Mg, which reflect the stellar metallicity. The mass-to-light ratios for O and Fe within 130kpc of NGC 1399 are over an order of magnitude lower than those in rich clusters, reflecting the metal enrichment history of this poor cluster.

Suzaku Observations of the Centaurus Cluster : Absence of Bulk Motions in the Intracluster Medium

The Centaurus cluster (z=0.0104) was observed with the X-ray Imaging Spectrometer (XIS) onboard the Suzaku X-ray satellite in three pointings, one centered on the cluster core and the other two offset by +-8 in declination. To search for possible bulk motions of the intracluster medium, the central energy of He-like Fe-K line (at a rest-frame energy of 6.7 keV) was examined to look for a positional dependence. Over spatial scales of 50 kpc to 140 kpc around the cluster core, the central line energy was found to be constant within the calibration error of 15 eV. The 90% upper limit on the line-of-sight velocity difference is |Delta_v|< 1400 km/s, giving a tighter constraint than previous measurements. The significant velocity gradients inferred from a previous Chandra study were not detected between two pairs of rectangular regions near the cluster core. These results suggest that the bulk velocity does not largely exceed the thermal velocity of the gas in the central region of the Centaurus cluster. The mean redshift of the intracluster medium was determined to be 0.0097, in agreement with the optical redshift of the cluster within the calibration uncertainty. Implications of the present results for the estimation of the cluster mass are briefly discussed.

Improvements in Calibration of GSO Scintillators in the Suzaku Hard X-Ray Detector

Improvements of the in-orbit calibration of GSO scintillators in the Hard X-ray Detector aboard Suzaku are reported. To resolve an apparent change in the energy scale of GSO, which appeared across the launch for unknown reasons, consistent and thorough re-analyses of both pre-launch and in-orbit data have been performed. With laboratory experiments using spare hardware, the pulse-height offset, corresponding to zero energy input, was found to change by ~0.5% of the full analog voltage scale, depending on the power supply. Furthermore, by carefully calculating all of the light outputs of secondaries from activation lines used in the in-orbit gain determination, their energy deposits in GSO were found to be effectively lower, by several percent, than their nominal energies. Taking both of these effects into account, the in-orbit data agree with the on-ground measurements within ~5%, without employing the artificial correction introduced in previous work (Kokubun et al. 2007, PASJ, 59, S53). With this knowledge, we updated the data processing, the response, and the auxiliary files of GSO, and reproduced the HXD-PIN and HXD-GSO spectra of the Crab Nebula over 12–300 keV by a broken power-law with a break energy of ~110 keV.

Constraint of Non-Thermal X-Ray Emission from the On-Going Merger Cluster Abell 3376 with Suzaku

Clusters of galaxies are among the best candidates for particle acceleration sources in the universe, a signature of which is non-thermal hard X-ray emission from the accelerated relativistic particles. We present early results on Suzaku observations of non-thermal emission from Abell 3376, which is a nearby on-going merger cluster. Suzaku observed the cluster twice, focusing on the cluster center containing the diffuse radio emission to the east, and cluster peripheral region to the west. For both observations, we detect no excess hard X-ray emission above the thermal cluster emission. An upper limit on the non-thermal X-ray flux of