Kumi Yoshita

New Technique of the X-Ray Efficiency Measurement of a Charge-Coupled Device with a Subpixel Resolution

We report the use of a new technique to measure the X-ray detection efficiency of a charge-coupled device (CCD) with subpixel resolution. The new technique makes use of a parallel X-ray beam and metal mesh placed just in front of the CCD. The CCD camera we used is a conventional system using the TC213 (Texas Instrument Japan (TIJ)) whose pixel size is 12 µ m ×12 µ m with one million pixels. The mesh has 4 µm diameter holes spaced at 12 µm intervals. We produced an efficiency map within a typical pixel showing the gate structure in detail: a virtual gate, a clock gate and an antiblooming gate. The gate structure we measured is consistent with the manufacturers design value. By selecting single pixel events, we detected a pixel boundary. Additional plans for application will also be discussed.

How Big Are Charge Clouds inside the Charge-Coupled Device Produced by X-Ray Photons?

We report here the charge cloud shape produced by an X-ray photon inside the charge-coupled device (CCD) as well as a method to measure it. The measurement is carried out by using a multi-pitch mesh which enables us to specify the interaction position of X-rays with subpixel resolution not only for single events but also for split events. Split events are generated when the X-ray interaction position is close to the pixel boundary. The width of this area depends on the apparent charge size. Finally, we measured the signal output from the pixel according to the interaction position of X-rays. By differentiating this function, we obtain, in detail, the charge cloud shape which can be well represented by an asymmetric Gaussian function. The charge cloud size for Al-K X-rays is 0.7×1.4 µm2 while that for Mo-L X-rays is 0.8×1.4 µm2. The size of the photoelectron in Si produced by these X-rays is about 0.04 µm. Taking into account the mean absorption length for these X-rays in Si, diffusion process in the depletion region cannot explain the charge cloud size. The asymmetry of the charge cloud probably arises from the asymmetry of the electric field in the CCD.

Improvement of the position resolution of the CCD for X-ray use

The authors report here experimental results relating X-ray interaction location and event splitting. The X-ray interaction location can be localized at subpixel scale using the mesh technique. The authors found that the center of gravity of the split event is well-correlated with the X-ray interaction location. They analyzed the data using two models for the charge cloud shape: one is the rectangular model and the other is the Gaussian model. Although the authors could not distinguish between these models, they measured a root mean square charge cloud size of 1 to 2 /spl mu/m for X-rays of Y-L (1.9 keV), Ag-L (3.0 keV), and Ti-K (4.5 keV). When the X-rays enter near the pixel boundary, the charge splits into adjacent pixels, allowing determination of the X-ray interaction location with an accuracy of 1.5 to 2.2 /spl mu/m. The authors, therefore, expect that the X-ray CCD can function as an X-ray imager with subpixel resolution, which will be especially useful in applications involving very high spatial resolution optics.

The X-Ray Structure of the Supernova Remnant 3C 400.2

We present here the results of an X-ray study of the supernova remnant 3C 400.2 (G53.6−2.2) using the ASCA data. 3C 400.2 has an unusual morphology at radio wavelengths, suggesting two SNRs superposed along the same line of sight, whereas its X-ray emission is known to be centrally peaked. We investigated the X-ray spectral variation across the remnant using the ASCA GIS and the ROSAT PSPC data. The X-ray spectra can be well fitted by thin thermal plasma models. However, there is no significant variation in the temperature and the ionization parameter across the remnant. We conclude that it is a single SNR rather than two overlapping SNRs. The centrally peaked X-ray morphology and the thin thermal emission with nearly cosmic abundances indicate that 3C 400.2 belongs to a class of “mixed-morphology SNRs”. We found that the physical parameters of 3C 400.2 are similar to those of other mixed-morphology SNRs. The morphology of 3C 400.2 can be explained by a supernova explosion occurring near to the edge of an interstellar cloud.

Discovery of a New Supernova Remnant in the Direction of G69.7+1.0

We discovered a middle-aged supernova remnant (SNR) in the vicinity of G69.7+1.0 using the ASCA satellite. G69.7+1.0 was identified in the 2.7 GHz survey and classified as a shell-type SNR with a diameter of 16 arcmin. During the ROSAT all-sky survey, the X-ray emission was detected in the direction of G69.7+1.0. However, it extends beyond the radio shell, and an X-ray bright region was located outside of the radio shell. A spectral study with the ASCA and ROSAT shows a thin thermal plasma with an electron temperature of ~0.4 keV. There is no significant variation of the spectral parameters over the field of view, except for the lower column density of the eastern part. We also found a large shell structure which surrounds the X-ray bright region in both optical and radio images. We suggest that the observed X-ray emission is associated with the large optical and radio shell, and that they are part of a new SNR, different from the radio SNR G69.7+1.0, which we have named AX J2001+3235 or G69.4+1.2. The large shell and the electron temperature of ~0.4 keV indicate that AX J2001+3235 is an evolved SNR. From a comparison with the column density of CTB 80 (G69.0+2.7), we estimate that the distance of the SNR is about 2.5 kpc.

Soft x-ray response of the prototype CCD camera (XIS) for Astro-E

The x-ray imaging spectrometers (XIS) are x-ray CCD cameras on-board the Astro-E satellite launched in 2000. The XIS consists of 4 cameras, each of them will be installed on a focal plane of the Astro-E X-ray Telescope (XRT). The XIS not only have a higher sensitivity, which comes from a larger effective area of the XRT and thicker depletion layers of the XIS CCDs, than ASCA SIS. But also have several features that will overcome the radiation damage effects anticipated in the orbit. The calibration experiment at Osaka focuses on the soft x-ray response of the XIS. The calibration system employs a grating spectrometer which irradiates the CCD with dispersed x-rays. We have obtained preliminary results on the XIS proto model, including the energy-pulse-height relation, the energy-resolution relation, and the quantum efficiency at the energy range of 0.25-2.2 keV.

X-ray imaging spectrometers for Astro-E: ground calibration in the soft x-ray range

Soft X-ray response of X-ray Imaging Spectrometers (XIS) for the Astro-E satellite is measured with a grating spectrometer system at Osaka. First, relation between incident X-ray energy and output pulse height peak (E-PH relation) is examined with an SX grating. It is found that jump in the E-PH relation around Si-K edge is at most 2.7 eV. Second, quantum efficiency (QE) of the XIS in 0.4 - 2.2 keV range is measured relatively to the reference CCD of which absolute QE was calibrated with a gas proportional counter. The QE is fitted with a model in which CCD gate structures are considered. Systematic error on the QE results is estimated by referring an independent measurement. Third, tuning and improvement of the response function is performed. We employ six components to reproduce the response profile of the XIS. In this paper, improvement of one component which is originated in the events absorbed in the channel-stop is presented. Nevertheless, Astro-E was lost due to the launch failure. We overview the XIS project in its flight model phase, modified points of the design, problems and solutions etc., in order to be utilized in a possible recovery of the satellite.

MAXI (Monitor of All-sky X-ray Image) for JEM on the International Space Station

Abstract MAXI has been approved as one of the first payloads on JEM-EF (Japanese Experiment Module - Exposed Facility) on the International Space Station. It will be developed in collaboration with NASDA (NAtional Space Development Agency of Japan) to be launched in 2003. The objective of MAXI is to monitor X-ray sources over the whole sky with unprecedented sensitivity (less than 3 mCrab in 1 day). Two instruments are planned. The GSC (Gas Slit Cameras) are 1-dim position sensitive proportional counters. The SSC (Solid-state Slit Cameras) contain X-ray CCDs. The GSC covers 2–30 keV with a large window size (more than 5000cm 2 ) and a low background rate. The SSC has a smaller window (∼ 200cm 2 ), but has a higher energy resolution ( E Δ E ∼ 50) and a sensitivity in the lower energies (0.5–10 keV). MAXI will be the first spectroscopic all-sky monitor. Objects to be observed are X-ray transients, AGNs, γ-ray burst afterglows, CXB and so on.

DOES AN X-RAY PHOTON REALLY GENERATE A CHARGE CLOUD WITH AN ASYMMETRIC SHAPE?

An X-ray photon produces a finite-sized primary charge cloud inside a charge-coupled device (CCD). Its shape is determined to be asymmetric using a mesh experiment. It is elongated in the direction perpendicular to the charge transfer direction. We checked whether or not the asymmetry of the charge cloud shape was due to the experimental setup. The geometrical shape of the mesh holes is confirmed to be symmetric by a Scanning Electron Microscope (SEM). A new method is introduced to quantitatively measure the effective size of the mesh hole for the X-rays. The geometrical diameter of the mesh hole is 3.4 µm while the effective diameter of the mesh hole is 3.9± 0.3 µm for Al-K X-ray photons (1.5 keV) and 4.0±0.3 µm for Mo-L X-ray photons (2.3 keV), respectively. These values are consistent with that taking into account the diffraction of X-rays. We conclude that the asymmetry of the primary charge cloud generated inside the CCD is due not to the experimental setup but to the asymmetry of the electric field inside the CCD.

Observation of Kes 27: A Typical Mixed-Morphology SNR

We report here on the observation of Kes 27, a proto-typical mixed-morphology SNR, using ASCA. It clearly shows a filled-center structure in the X-ray region while a shell structure in the radio region. There are two radio bright regions: one is in the center, while the other is in the east rim. The X-ray intensity peak coincides well with the radio bright region at the center. The X-ray spectrum was well-fitted by a collisional ionization equilibrium model with solar abundances. Taking into account the ionization parameter (>10^{12} cm^{-3} s) and the plasma density (0.39 cm^{-3}), we found that the age of the SNR is longer than 8x10^4 yr. The hardness ratio map indicates that the inner region shows a harder spectrum than that in the outer region, which does not come from the heavier interstellar absorption feature, but from the higher temperature. There is a temperature gradient from the innner region (0.84 keV) toward outer region (0.59 keV), indicating that the thermal conduction does not play an important role.