Shunsaku Okada

Restoring the Suzaku Source Position Accuracy and Point-Spread Function

We present an empirical correction of sky coordinates of X-ray photons obtained with the X-ray Imaging Spectrometer (XIS) aboard the Suzaku satellite to improve the source position accuracy and restore the point-spread function (PSF). The XIS images are known to have an uncertainty in position of up to 1 0 , and to show considerable degradations of the PSF. These problems are caused by a drifting of the satellite attitude due to thermal distortion of the side panel #7, where the attitude control system is mounted. We found that the position error averaged over a pointing observation can be largely reduced by using the relation between the deviation of the source position in the DETX direction and the ecliptic latitude of the pointing target. We parameterized the wobbling of the source position synchronized with the 96-minute satellite orbital period with temperatures of onboard radiators and elapsed time since the night–day transition of the spacecraft. We developed software, aeattcor, to correct the image drift using these parameters, and applied it to 27 point-source images taken in the Suzaku initial science-operation phase. We show that the radius of the 90% error circle of the source position was reduced to 19 00 and the PSF was sharpened. These improvements have enhanced the scientific capability of the Suzaku XIS.

Suzaku Discovery of Hard X-Ray Pulsations from a Rotating Magnetized White Dwarf, AEAquarii

The fast rotating magnetized white dwarf, AE Aquarii, was observed with Suzaku, in October 2005 and October 2006 with exposures of 53.1 and 42.4 ks, respectively. In addition to clear spin modulation in the 0.5–10 keV band of the XIS data at the barycentric period of 33.0769 ±0.0001 s, the 10–30 keV HXD data in the second half of the 2005 observation also showed statistically significant periodic signals at a consistent period. On that occasion, the spin-folded HXD light curve exhibited two sharp spikes separated by ∼ 0.2 cycles in phase, in contrast to approximately sinusoidal profiles observed in energies below ∼ 4 keV. The folded 4–10 keV XIS light curves are understood as a superposition of those two types of pulse profiles. The phase averaged 1.5–10 keV spectra can be reproduced by two thermal components with temperatures of 2.90 +0.20 −0.16 keV and 0.53 +0.14 −0.13 keV, but the 12-25 keV HXD data show a significant excess above the extrapolated model. This excess can be explained by either a power-law model with photon index of 1.12 +0.63 −0.62 or a third thermal component with a temperature of 54 +26 −47 keV. At a distance of 102 pc, the 4–30 keV luminosities of the thermal and the additional components become 1.7 +1.3 −0.6 and 5.3 +15.3 −0.3 ×10 29 erg s −1 , respectively. The latter corresponds to 0.09% of the spin down energy of the object. Possible emission mechanisms of the hard pulsations are discussed, including in particular non-thermal ones.

Suzaku X-Ray Imaging and Spectroscopy of Cassiopeia A

Suzaku X-ray observations of a young supernova remnant, Cassiopeia A, were carried out. K-shell transition lines from highly ionized ions of various elements were detected, including Chromium (Cr-K˛ at 5.61 keV). The X-ray continuum spectra were modeled in the 3.4–40 keV band, summed over the entire remnant, and were fitted with a simplest combination of the thermal bremsstrahlung and the non-thermal cut-off power-law models. The spectral fits with this assumption indicate that the continuum emission is likely to be dominated by non-thermal emission with a cut-off energy at >1 keV. The thermal-to-nonthermal fraction of the continuum flux in the 4–10 keV band is best estimated as 0.1. Non-thermal-dominated continuum images in the 4–14 keV band were made. The peak of the non-thermal X-rays appears at the western part. The peak position of the TeV -rays measured with HEGRA and MAGIC is also shifted at the western part with the 1-sigma confidence. Since the location of the X-ray continuum emission was known to be presumably identified with the reverse shock region, the possible keV–TeV correlations give a hint that the accelerated multi-TeV hadrons in Cassiopeia A are dominated by heavy elements in the reverse shock region.

Density Diagnostics of the Hot Plasma in AE Aquarii with XMM-Newton

We report on XMM-Newton observations of the magnetic CV AE Aqr. High-resolution spectroscopy of the He-like triplet of N and O with the RGS has enabled us to measure the electron number density of the plasma as ~1011 cm-3. Incorporating this with the emission measure, we estimate the geometrical scale of the plasma emitting these lines to be lp (2-3) × 1010 cm. Since the density and the scale are both incompatible with the standard postshock accretion column of an mCV, the plasma cannot be a product of mass accretion onto the white dwarf. The widths of the H-like Kα emission lines of N and O (=1250-1600 km s-1) are of similar magnitude to the thermal velocity dispersion of the hottest part of the plasma, where kTmax is 4.6 keV. It is known that Balmer series and UV emission lines also show a similar velocity dispersion. In addition, like these lines, the X-ray emission lines are found to show dramatic flaring activity. These facts strongly suggest that all these broad emission lines from X-ray to optical wave bands are produced in the course of adiabatic cooling of the plasma once heated up to Tmax in the deep gravitational potential of the white dwarf. This interpretation can resolve the problem of the absence of the high-velocity component in the Hα emission line spectrum by a scenario where the plasma that is expelled due to the propeller action is still too hot to emit the Hα line within a region of r < lp (roughly equal to the Roche lobe size) from the white dwarf where the high-velocity component is expected to originate.

Characterization of a Hard X-ray Telescope at Synchrotron Facility SPring-8

Space-borne astronomical instruments require extensive characterization on the ground before launch. In the hard X-ray region however, it is difficult for a laboratory-based beamline using a conventional X-ray source to provide a capability sufficient for pre-flight high-precision calibration. In this paper, we describe an experiment to characterize a hard X-ray telescope at a synchrotron facility, mainly on the basis of experimental setup and examples of measured results. We have developed hard X-ray telescopes consisting of Wolter-I grazing incidence optics and platinum-carbon multilayer supermirror coatings. The telescopes have been characterized at the synchrotron facility SPring-8 beamline BL20B2. The measurements at BL20B2 have great advantages such as extremely high flux, large-sized and less-divergent beam, and monochromatic beam covering the entire hard X-ray region from 8 to over 100 keV. The telescope was illuminated by monochromatic hard X-rays, and the focused image was measured by high resolution hard X-ray imagers. The entire telescope aperture was mapped by a small beam, and the effective area and the point spread function were obtained as well as local optical properties for further diagnostics of the characteristics of the telescope.

Suzaku Observations of the Dwarf Nova SS Cyg in Quiescence and Outburst

We present results from the Suzaku observations of the dwarf nova SS Cyg in quiescence and outburst in 2005 November. High sensitivity of the HXD PIN and high spectral resolution of the XIS enable us to determine plasma parameters with unprecedented precision. The maximum temperature of the plasma in quiescence 20.4+4.0 −2.6 keV is significantly higher than that in outburst 6.0+0.2 −1.3 keV. The elemental abundances of oxygen and iron are both subsolar (0.46+0.04 −0.03Z and 0.37 +0.01 −0.03Z , respectively). The solid angle of cold reflecting matter subtending over an optically thin thermal plasma is Ω/2π = 1.7 ± 0.2 in quiescence. A 6.4 keV iron Kα line is resolved into narrow and broad components. These facts indicate that both the white dwarf and the accretion disk contribute to the reflection. We consider the standard optically thin boundary layer as the most plausible picture for the plasma configuration in quiescence. The solid angle of the reflector in outburst Ω/2π = 0.9+0.5 −0.4 and a broad 6.4 keV iron line indicate that the reflection in outburst occurs at the surface of the accretion disk. The broad 6.4 keV line suggests that the optically thin thermal plasma is distributed over the accretion disk like solar coronae.

Ground-based X-ray calibration of the Astro-E2 X-ray telescope. II. With diverging geam at PANTER

We report a ground-based X-ray calibration of the Astro-E2 X-ray telescope at the PANTER test facility. Astro-E2, to be launched in February 2005, has five X-Ray Telescopes (XRTs). Four of them focus on the X-Ray Imaging Spectrometers (XIS) while the other on the X-Ray Spectrometer (XRS). They are designed with a conical approximation of Wolter-I type optics, nested with thin foil mirrors to enhance their throughput. A calibration test of the first Astro-E2 flight XRT for XIS was carried out at the PANTER facility in August 2003. This facility has an 130 meter long diverging beam from X-ray generator to XRT. Owing to the small X-ray spot size of about 2 mm dia., we verified that the focal position of each quadrant unit converged within 10 arcsec. The energy band around Au-M edge structures was scanned with a graphite crystal. The edge energy (Au M5) is consistent with that listed in Henke et al. 1997. Owing to the large area coverage of the PSPC detector which is a spare of the ROSAT satellite, off-axis images including stray lights at large off-axis angle (up to 6 degree) were obtained with a large field of view. We also compared the results with those measured with the parallel pencil beam at ISAS which is in detail reported in our companion paper by Itoh A. et al..

X-ray polarimeter with a multilayer-coated CCD

We report a new type X-ray imaging polarimeter: a multilayer-coated CCD. When the X-rays are detected by the CCD, with the incident angle of 45 deg, through the coated multi-layer, the transmissions of the P and S polarized photons are different from each other and we can get an image with a selected position angle of the polarization. By the simulation of the transmission of the multi-layer, we designed an optimal number of the layer-pair and their thickness. The target wave length is 135Å, because the Mo/Si multi-layer has a good performance in this energy range. If the dead layer of the back-side CCD is 1000Å, nine layer-pairs make the largest difference between the P and S transmission. We deposited the Mo/Si multi-layer directly on a back-side CCD. The CCD was exposed to the polarized photons from synchrotron radiation with 45 deg incident angle. The detected intensity is measured as a function of the photon energy and of the rotation angle around the photon beam. The detection of the polarization is confirmed. However the measured performance is lower than expected. Some possibilities of the cause are discussed.

Systematic surveys of the non thermal emission from white dwarfs with Suzaku and INTEGRAL

Continuous searches for other possible white dwarf (WD) pulsars like AE Aquarii[12] with Suzaku and INTEGRAL have been performed. After picking up WDs with known magnetic field strengths and spin periods from catalogs of CVs and isolated WDs, objects whose induced electric potentials exceed 1012 volts and dipole radiations over 1029 erg s−1 are selected; AM Her, EUVE J0317–85, PG1031+234, LHS1734, PG1015+014 etc. Their X‐rays were studied with INTEGRAL archive data and/or Suzaku follow‐up observations. A promising non‐thermal emission from an object, AM Her in a very low state, has been found with Suzaku at the X‐ray luminosity of 6.6×1029 erg s−1.

Suzaku Observation of a White Dwarf as a new Candidate of Cosmic‐ray Origin

Strongly magnetized white dwarfs (WDs) are able to accelerate particles like rotating neutron‐star pulsars. If magnetized WDs are a new Cosmic‐ray origin, they should be a quiet but numerous accelerators, giving important contributions of low‐energy cosmic‐rays. We performed a high sensitive search for possible hard X‐ray emission from the fast rotating WD, AE Aquarii with Suzaku. The object has a magnetic field with 105 Gauss and a rapid spin period of 33 sec. From 100 ksec dataset of Suzaku XIS and HXD, we have discovered spiky pulsations like pulsars in the hard X‐ray band of over 4 keV, in addition to the well‐known thermal modulation in the softer band. In order to account for the hard X‐ray signals with the HXD, the X‐ray spectrum requires an additional hard X‐ray component on the well‐known thermal emissions with temperatures of 0.5 and 2.9 keV. Combined with results from timing analyses, spectral shapes and flux, we concluded that the hard X‐ray pulsations should be a non‐thermal origin, for examp...