Emi Miyata

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of � 20 mCrab (� 7 � 10 � 10 erg cm � 2 s � 1 in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), � 4.5 mCrab for one day, and � 2 mCrab for one week. The final detectability of MAXI could be � 0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

RX J0852.0–4622: Another Nonthermal Shell-Type Supernova Remnant (G266.2–1.2)

The newly discovered supernova remnant G266.2-1.2 (RX J0852.0-4622), along the line of sight to the Vela supernova remnant (SNR), was observed with ASCA for 120 ks. We find that the X-ray spectrum is featureless and well described by a power law, extending to three of the class of shell-type SNRs dominated by nonthermal X-ray emission. Like G347.3-0.5, this low-latitude remnant displays discrete regions of enhanced emission along the rim as well as faint nonthermal emission from the interior. We derive limits on the thermal content of the remnant emission, although the presence of the Vela SNR compromises our ability to seriously constrain a low-temperature component. Limits placed on the amount of Sc-K emission are compared with the expected flux based on the reported 44Ti emission from G266.2-1.2. We also report on an unresolved X-ray source surrounded by diffuse emission near the center of the remnant. The properties of the source are not well determined but appear consistent with the interpretation that the source is a neutron star surrounded by a synchrotron nebula. Alternatively, the source may be associated with one of two stars located within the positional error circle, but this appears somewhat unlikely.

Detection of X-ray polarization with a charge coupled device☆

Abstract We report here the results of the detection of polarized X-rays with a charge coupled device (OCD) whose pixel size is 12×12 μm2. The primary charge clouds produced in the silicon extends in the direction of the initial momentum of the photoelectron that is parallel to the electric vector e of the incident X-ray. Using a synchrotron radiation facility, we studied the structure of spread events and found that the CCD could detect the polarization of incident X-rays for 15–37 keV.

Characterization of the Advanced Satellite for Cosmology and Astrophysics x-ray telescope: preflight calibration and ray tracing

The x-ray properties of multinested thin-foil mirror x-ray telescopes (XRTs) on board ASCA, the Advanced Satellite for Cosmology and Astrophysics, were fully evaluated with an x-ray pencil beam.Scanning over the telescope aperture of 35 cm in diameter with an x-ray pencil beam, we found the effective area of a set of XRTs to be 325, 200, and 113 cm(2) at energies of 1.5, 4.5, and 8.0 keV, respectively. We derive the point-spread functions (PSFs) of the XRTs by measuring the image profile at the focal plane with an x-ray CCD. The PSF is found to exhibit a sharp core concentrated within 30 arcsec and a broad wing extended to 3 arcmin in half-power diameter. We also evaluate the contribution of stray light, which is caused by the single reflection of x rays by primary or secondary mirrors and by the backside reflection of the mirrors. To obtain the characteristics of the XRT in the energy region of 0.5-10.0 keV, incorporated with the measurements at discrete energies, we develop a ray-tracing method with the telescope design parameter, the PSF, and optical constants. In particular, we obtain the optical constants around the gold-atom M shell (Au-M) absorption-edge energies by measuring the reflectivity of our mirror sample, with monochromatized x-rays in the energy range of 2.0-3.5 keV from synchrotron radiation. Taking into account the PSFs and optical constants, we find that our ray-tracing program can reproduce all these XRT performances.

The Radial Structure of the Cygnus Loop Supernova Remnant: Possible Evidence of a Cavity Explosion

We observed the northeast limb toward the center region of the Cygnus Loop with the ASCA Observatory. In our previous paper (Miyata et al.), we analyzed the data obtained with the X-ray CCD cameras (SISs). We found a radial variation of electron temperature (kTe) and ionization timescale [log(τ)], whereas no variation could be found for the abundances of heavy elements. In this paper, we reanalyzed the same data set and new observations with the latest calibration files. Then we constructed the precise spatial variations of kTe, log(τ), and abundances of O, Ne, Mg, Si, and Fe over the field of view (FOV). We found a spatial variation not only in kTe and in log(τ) but also in most of heavy elements. As described in Miyata et al., values of kTe increase and those of log(τ) decrease toward the inner region. We found that the abundance of heavy elements increases toward the inner region. The radial profiles of O, Ne, and Fe show clear jump structures at a radius of 0.9Rs, where Rs is the shock radius. Outside of 0.9Rs, abundances of all elements are constant. On the contrary, inside of 0.9Rs, abundances of these elements are 20%-30% larger than those obtained outside of 0.9Rs. The radial profile of kTe also shows the jump structure at 0.9Rs. This means that the hot and metal-rich plasma fills the volume inside of 0.9Rs. We concluded that this jump structure was the possible evidence for the pre-existing cavity produced by the precursor. If the ejecta fills inside of 0.9Rs, the total mass of the ejecta was roughly 4.1 M☉. We then estimated the main-sequence mass to be roughly 15 M☉, which supports the massive star in origin of the Cygnus Loop supernova remnant and the existence of a pre-existing cavity.

Suzaku Observations of Tycho's Supernova Remnant

Tychos supernova remnant was observed by the XIS and HXD instruments onboard the Suzaku satellite on 2006 June 26-29 for 92 ks. The spectrum up to 30 keV was well fitted with a two-component model, consisting of a power-law with photon index of 2.7 and a thermal bremsstrahlung model with temperature of 4.7 keV. The former component can alternatively be modeled as synchrotron emission from a population of relativistic electrons with an estimated roll-off energy of around 1 keV. In the XIS spectra, in addition to the prominent Fe K_alpha line (6.445 keV), we observe for the first time significant K_alpha line emission from the trace species Cr and Mn at energies of 5.48 keV and 5.95 keV, respectively. Faint K_beta lines from Ca (4.56 keV) and Fe (7.11 keV) are also seen. The ionization states of Cr and Mn, based on their line centroids, are estimated to be similar to that of Fe K_alpha (Fe XV or XVI).

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.

Measurement of the subpixel structure of AXAF CCD's

The authors present a method to measure the subpixel structure of a charge-coupled device (CCD), information necessary to accurately determine (<1% uncertainty) the absolute detection efficiency of the device. Their approach uses a thin metal film with periodically spaced holes (small, compared to the pixel size) to localize incident X-rays to a particular region of the pixel. The mesh is rotated to create a small angular misalignment between the grid holes and the CCD pixels, producing a moire effect in the data. The resultant moire pattern is compared to a CCD model, and a best fit minimization technique is used to constrain the parameters that describe the subpixel structure. This technique was developed to measure and calibrate the X-ray CCDs that will comprise one of the two focal plane instruments on-board AXAF, but it is applicable for measuring the structure of any pixelated solid state device.

Chandra X-Ray Observatory Study of Vela Shrapnel A

We present high-resolution Chandra observations of shrapnel A in the vicinity of the Vela supernova remnant. The Chandra image reveals a bright X-ray region at the head position of shrapnel A and a fainter extended tail. The spectral analysis shows that the plasma is in a condition of nonequilibrium ionization. The abundance of O is 0.34 times that of the solar value, whereas that of Si is 3 times solar, showing an anomalous abundance ratio for the two elements. The ionization timescale indicates that the shrapnel had been heated up about 104 yr ago. We confirm that the pressure in the head region is 10 times higher than that in the tail region, which is not explained by a simple blast-wave model. These facts strongly support the view that shrapnel A is a fragment of the progenitor star created in the supernova explosion.

Overabundance of Calcium in the Young SNR RX J0852–4622: Evidence of Over-Production of 44Ti

Recently, COMPTEL has detected