arXiv subjects

H. Tsunemi

Publications and source records attributed to H. Tsunemi.

At least 37 records · Page 2Linked to original sources

arXiv2008

Discovery of Fast-Moving X-Ray--Emitting Ejecta Knots in the Oxygen-Rich Supernova Remnant Puppis A

We report on the discovery of fast-moving X-ray--emitting ejecta knots in the Galactic Oxygen-rich supernova remnant Puppis A from XMM-Newton observations. We find an X-ray knotty feature positionally coincident with an O-rich fast-moving optical filament with blue-shifted line emission located in the northeast of Puppis A. We extract spectra from northern and southern regions of the feature. Applying a one-component non-equilibrium ionization model for the two spectra, we find high metal abundances relative to the solar values in both spectra. This fact clearly shows that the feature originates from metal-rich ejecta. In addition, we find that line emission in the two regions is blue-shifted. The Doppler velocities derived in the two regions are different with each other, suggesting that the knotty feature consists of two knots that are close to each other along the line of sight. Since fast-moving O-rich optical knots/filaments are believed to be recoiled metal-rich ejecta, expelled to the opposite direction against the high-velocity central compact object, we propose that the ejecta knots disclosed here are also part of the recoiled material.

astro-ph↗

arXiv2008

The Slow X-Ray Expansion of the Northwestern Rim of the Supernova Remnant RX J0852.0-4622

The detection of radioactive decay line of 44Ti provides a unique evidence that the gamma-ray source is a young (< 1,000 yr) supernova remnant because of its short lifetime of about 90 yr. Only two Galactic remnants, Cassiopeia A and RX J0852.0-4622, are hitherto reported to be the 44Ti line emitter, although the detection from the latter has been debated. Here we report on an expansion measurement of the northwestern rim of RX J0852.0-4622 obtained with X-ray observations separated by 6.5 yr. The expansion rate is derived to be 0.023+/-0.006% that is about five times lower than those of young historical remnants. Such a slow expansion suggests that RX J0852.0-4622 is not a young remnant as has been expected. We estimate the age of 1,700-4,300 yr of this remnant depending on its evolutionary stage. Assuming a high shock speed of about 3000 km/sec, which is suggested by the detection of non-thermal X-ray radiation, the distance of about 750 pc to this remnant is also derived.

astro-ph↗

arXiv2008

Asymmetric Ejecta Distribution of the Cygnus Loop revealed with Suzaku

We observed a linearly sliced area of the Cygnus Loop from the north-east to the south-west with Suzaku in seven pointings. After dividing the entire fields of view (FOV) into 119 cells, we extracted spectra from all of the cells and performed spectral analysis for them. We then applied both one- and two-component non-equilibrium ionization (NEI) models for all of the spectra, finding that almost all were significantly better fitted by the two-component NEI model rather than the one-component NEI model. Judging from the abundances, the high-kT_e component must be the ejecta component, while the low-kT_e component comes from the swept-up matter. Therefore, the ejecta turn out to be distributed inside a large area (at least our FOV) of the Cygnus Loop. We divided the entire FOV into northern and southern parts, and found that the ejecta distributions were asymmetric to the geometric center: the ejecta of Si, S, and Fe seem to be distributed more in the south than in the north of the Cygnus Loop by a factor of about 2. The degree of ejecta-asymmetry is consistent with that expected by recent supernova explosion models.

astro-ph↗

arXiv2008

Abundance Inhomogeneity in the Northeastern Rim of the Cygnus Loop Revealed by Suzaku Observatory

We present the results of a spatially resolved spectral analysis from four Suzaku observations covering the northeastern rim of the Cygnus Loop. A two-kT_e non-ionization equilibrium (NEI) model fairly well represents our data, which confirms the NEI condition of the plasma there. The metal abundances are depleted relative to the solar values almost everywhere in our field of view. We find abundance inhomogeneities across the field: the northernmost region (Region A) has enhanced absolute abundances compared with other regions. In addition, the relative abundances of Mg/O and Fe/O in Region A are lower than the solar values, while those in the other regions are twice higher than the solar values. As far as we are concerned, neither a circumstellar medium, fragments of ejecta, nor abundance inhomogeneities of the local interstellar medium around the Cygnus Loop can explain the relatively enhanced abundance in Region A. This point is left as an open question for future work.

astro-ph↗

arXiv2007

Performance of the Charge Injection Capability of Suzaku XIS

A charge injection technique is applied to the X-ray CCD camera, XIS (X-ray Imaging Spectrometer) onboard Suzaku. The charge transfer inefficiency (CTI) in each CCD column (vertical transfer channel) is measured by the injection of charge packets into a transfer channel and subsequent readout. This paper reports the performances of the charge injection capability based on the ground experiments using a radiation damaged device, and in-orbit measurements of the XIS. The ground experiments show that charges are stably injected with the dispersion of 91eV in FWHM in a specific column for the charges equivalent to the X-ray energy of 5.1keV. This dispersion width is significantly smaller than that of the X-ray events of 113eV (FWHM) at approximately the same energy. The amount of charge loss during transfer in a specific column, which is measured with the charge injection capability, is consistent with that measured with the calibration source. These results indicate that the charge injection technique can accurately measure column-dependent charge losses rather than the calibration sources. The column-to-column CTI correction to the calibration source spectra significantly reduces the line widths compared to those with a column-averaged CTI correction (from 193eV to 173eV in FWHM on an average at the time of one year after the launch). In addition, this method significantly reduces the low energy tail in the line profile of the calibration source spectrum.

astro-ph↗

arXiv2006

Spatially Resolved X-ray Spectroscopy of Vela Shrapnel A

We present the detailed X-ray spectroscopy of Vela shrapnel A with the XMM-Newton satellite. Vela shrapnel A is one of several protrusions identified as bullets from Vela supernova explosion. The XMM-Newton image shows that shrapnel A consists of a bright knot and a faint trailing wake. We extracted spectra from various regions, finding a prominent Si Ly$_α$ emission line in all the spectra. All the spectra are well represented by the non-equilibrium ionization (NEI) model. The abundances are estimated to be O$\sim$0.3, Ne$\sim$0.9, Mg$\sim$0.8, Si$\sim$3, Fe$\sim$0.8 times their solar values. The non-solar abundance ratio between O and Si indicates that shrapnel A originates from a deep layer of a progenitor star. We found that the relative abundances between heavy elements are almost uniform in shrapnel A, which suggests that the ejecta from supernova explosion are well mixed with swept-up interstellar medium.

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arXiv2006

Discovery of a Featureless X-Ray Spectrum in the Supernova Remnant Shell of G330.2+1.0

We report here on the first pointed X-ray observation of the supernova remnant (SNR) G330.2+1.0. The X-ray morphology is characterized by an extended shell. Its X-ray spectrum is well represented by a single power-law function with a photon index of $γ\simeq 2.8$ and interstellar absorption of $n_{\rm H}\simeq2.6\times 10^{22}$[cm$^{-2}$]. We interpret this emission as synchrotron radiation from accelerated electrons at the SNR shock, as seen in SN 1006. The surface brightness of the X-ray emission is anti-correlated with the radio emission, and the power-law spectrum is dominated at the western shell where the radio emission is weak. The co-existence of two distinct (radio bright/X-ray faint and radio faint/X-ray bright) shells in a single supernova remnant challenges our understanding of the particle acceleration and radiation mechanisms in different interstellar environments. The object may be a good target for searching TeV gamma-rays and molecular gas surrounding the blast shock. We also report on the nature of a bright point-like source (AX J1601-5143) to the south of the SNR.

astro-ph↗

arXiv2004

Wide band X-ray Imager (WXI) and Soft Gamma-ray Detector (SGD) for the NeXT Mission

The NeXT mission has been proposed to study high-energy non-thermal phenomena in the universe. The high-energy response of the super mirror will enable us to perform the first sensitive imaging observations up to 80 keV. The focal plane detector, which combines a fully depleted X-ray CCD and a pixellated CdTe detector, will provide spectra and images in the wide energy range from 0.5 keV to 80 keV. In the soft gamma-ray band up to ~1 MeV, a narrow field-of-view Compton gamma-ray telescope utilizing several tens of layers of thin Si or CdTe detector will provide precise spectra with much higher sensitivity than present instruments. The continuum sensitivity will reach several times 10^(-8) photons/s/keV/cm^(2) in the hard X-ray region and a few times10^(-7) photons/s/keV/cm^(2) in the soft gamma-ray region.

astro-ph↗

arXiv2004

Spatial Variation of the X-ray spectrum of the Crab Nebula

We present spectral analysis of the Crab Nebula obtained with the {\it Chandra} X-ray observatory. The X-ray spectrum is characterized by a power-law whose index varies across the nebula. The variation can be discussed in terms of the particle injection from the pulsar in two different directions: the equatorial plane containing the torus and the symmetry axis along the jet. In the equatorial plane, spectra within the torus are the hardest, with photon index $α\approx 1.9$, and are almost independent of the surface brightness. At the periphery of the torus, the spectrum gradually softens in the outer, lower surface brightness regions, up to $α\approx 3.0$. This indicates that synchrotron losses become significant to X-ray emitting particles at the outer boundary of the torus. We discuss the nature of the torus, incorporating information from observations at other wavelengths. Spectral variations are also seen within the southern jet. The core of the jet is the hardest with $α\approx$ 2.0, and the outer sheath surrounding the core becomes softer with $α$ up to 2.5 at the outermost part. Based on the similarity between the spectra of the jet core and the torus, we suggest that the electron spectra of the particles injected from the pulsar are also similar in these two different directions. The brightness ratio between the near and far sides of the torus can be explained by Doppler boosting and relativistic aberration; however, the observed ratio cannot be derived from the standard weakly magnetized pulsar wind model. We also found a site where an optical filament comprised of supernova ejecta is absorbing the soft X-ray emission ($<$ 2 keV).

astro-ph↗

arXiv2004

An X-ray measurement of Titan's atmospheric extent from its transit of the Crab Nebula

Saturn's largest satellite, Titan, transited the Crab Nebula on 5 January 2003. We observed this astronomical event with the {\it Chandra} X-ray Observatory. An ``occultation shadow'' has clearly been detected and is found to be larger than the diameter of Titan's solid surface. The difference gives a thickness for Titan's atmosphere of 880 $\pm$ 60 km. This is the first measurement of Titan's atmospheric extent at X-ray wavelengths. The value measured is consistent with or slightly larger than those estimated from earlier Voyager observations at other wavelengths. We discuss the possibility of temporal variations in the thickness of Titan's atmosphere.

astro-ph↗

arXiv2003

Year-scale morphological variation of the X-ray Crab Nebula

We present year-scale morphological variations of the Crab Nebula revealed by Chandra X-ray observatory. Observations have been performed about every 1.7 years over the 3 years from its launch. The variations are clearly recognized at two sites: the torus and the southern jet. The torus, which had been steadily expanding until 1.7 years ago, now appears to have shrunk in the latest observation. Additionally, the circular structures seen to the northeast of the torus have decayed into several arcs. On the other hand, the southern jet shows the growth of its overall kinked-structure. We discuss the nature of these variations in terms of the pulsar wind nebula (PWN) mechanism.

astro-ph↗

arXiv2002

Proton Irradiation Experiment for the X-ray Charge-Coupled Devices of the Monitor of All-sky X-ray Image mission onboard the International Space Station: I. Experimental Setup and Measurement of the Charge Transfer Inefficiency

We have investigated the radiation damage effects on a CCD to be employed in the Japanese X-ray astronomy mission including the Monitor of All-sky X-ray Image (MAXI) onboard the International Space Station (ISS). Since low energy protons release their energy mainly at the charge transfer channel, resulting a decrease of the charge transfer efficiency, we thus focused on the low energy protons in our experiments. A 171 keV to 3.91 MeV proton beam was irradiated to a given device. We measured the degradation of the charge transfer inefficiency (CTI) as a function of incremental fluence. A 292 keV proton beam degraded the CTI most seriously. Taking into account the proton energy dependence of the CTI, we confirmed that the transfer channel has the lowest radiation tolerance. We have also developed the different device architectures to reduce the radiation damage in orbit. Among them, the ``notch'' CCD, in which the buried channel implant concentration is increased, resulting in a deeper potential well than outside, has three times higher radiation tolerance than that of the normal CCD. We then estimated the charge transfer inefficiency of the CCD in the orbit of ISS, considering the proton energy spectrum. The CTI value is estimated to be 1.1e-5 per each transfer after two years of mission life in the worse case analysis if the highest radiation-tolerant device is employed. This value is well within the acceptable limit and we have confirmed the high radiation-tolerance of CCDs for the MAXI mission.

physics.ins-det↗

arXiv2002

Application of the Mesh Experiment for the Back-Illuminated CCD: I. Experiment and the Charge Cloud Shape

We have employed a mesh experiment for back-illuminated (BI) CCDs. BI CCDs possess the same structure to those of FI CCDs. Since X-ray photons enter from the back surface of the CCD, a primary charge cloud is formed far from the electrodes. The primary charge cloud expands through diffusion process until it reaches the potential well that is just below the electrodes. Therefore, the diffusion time for the charge cloud produced is longer than that in the FI CCD, resulting a larger charge cloud shape expected. The mesh experiment enables us to specify the X-ray point of interaction with a subpixel resolution. We then have measured a charge cloud shape produced in the BI CCD. We found that there are two components of the charge cloud shape having different size: a narrow component and a broad component. The size of the narrow component is $2.8-5.7 μ$m in unit of a standard deviation and strongly depends on the attenuation length in Si of incident X-rays. The shorter the attenuation length of X-rays is, the larger the charge cloud becomes. This result is qualitatively consistent with a diffusion model inside the CCD. On the other hand, the size of the broad component is roughly constant of $\simeq 13 μ$m and does not depend on X-ray energies. Judging from the design value of the CCD and the fraction of each component, we conclude that the narrow component is originated in the depletion region whereas the broad component is in the field-free region.

physics.ins-det↗

arXiv2002

Direct X-ray Imaging of $μ$m precision using Back-Illuminated CCD

A charge-coupled device (CCD) is a standard imager in optical region in which the image quality is limited by its pixel size. CCDs also function in X-ray region but with substantial differences in performance. An optical photon generates only one electron while an X-ray photon generates many electrons at a time. We developed a method to precisely determine the X-ray point of interaction with subpixel resolution. In particular, we found that a back-illuminated CCD efficiently functions as a fine imager. We present here the validity of our method through an actual imaging experiment.

astro-ph↗

arXiv2001

Chandra reveals the Dynamic Structure of the Inner Crab Nebula

We present a series of monitoring observations of the Crab Nebula with the Chandra X-ray Observatory, focusing on the temporal evolution of the structure. This series of 8 observations, spanning a period of approximately six months, shows the dynamic nature of the inner X-ray structures. We detected outward moving ``wisps'' from the recently discovered inner ring seen in optical observations. We also find that the inner ring itself shows temporal variations in structure. The torus also appears to be expanding. Such temporal variations generally match the canonical scenario that an expanding synchrotron nebula injected from the pulsar is confined by the supernova ejecta.

astro-ph↗

arXiv2001

SSC of MAXI experiment

Monitor of All-sky X-ray Image (MAXI) on the International Space Station (ISS) has two kinds of X-ray detectors: the Gas Slit Camera (GSC) and the Solid-state Slit Camera (SSC). SSC is an X-ray CCD array, consisting of 16 chips, which has the best energy resolution as an X-ray all-sky monitor in the energy band of 0.5 to 10 keV. Each chip consists of 1024x1024 pixels with a pixel size of 24$μ$m, thus the total area is ~200 cm^2. We have developed an engineering model of SSC, i.e., CCD chips, electronics, the software and so on, and have constructed the calibration system. We here report the current status of the development and the calibration of SSC.

astro-ph↗

arXiv2001

Subpixel Spatial Resolution of the X-Ray Charge-Coupled Device Based on the Charge Cloud Shape

When an X-ray photon lands into a pixel (event pixel), the primary charge is mainly collected into the event pixel. If the X-ray landing position is sufficiently close to the pixel boundary, the primary charge spills over to the adjacent pixel forming split events. We can easily understand that there are three parameters coupled together; the X-ray landing position inside the pixel, the X-ray event pattern and the primary charge cloud shape. We can determine any one of them from the other two parameters. Since we know the charge cloud shape using the multi-pitch mesh experiment, we can calculate the X-ray landing position with subpixel resolution using the event pattern. We applied our method to Ti-K X-rays for the charge-coupled device with $12 μ$m square pixel. Once the primary charge splits into the adjacent pixel, we can determine the X-ray landing position with subpixel resolution. Using three- or four-pixel split events, we can determine the X-ray landing position with an accuracy of less than $1 μ$m. For a two-pixel split event, we obtained a similar position accuracy in the split direction with no improvement in the direction perpendicular to it. We will discuss the type of CCD which can achieve the subpixel resolution for the entire area of the CCD.

physics.ins-det↗

arXiv2000

Direct measurement of sub-pixel structure of the EPIC MOS CCD on-board th e XMM/NEWTON satellite

We have used a mesh experiment in order to measure the sub-pixel structure of the EPIC MOS CCDs on-board the XMM/NEWTON satellite. The EPIC MOS CCDs have 40 $μ$m-square pixels which have an open electrode structure in order to improve the detection efficiency for low-energy X-rays. We obtained restored pixel images for various X-ray event grades (e.g. split-pixel events, single pixel events, etc.) at various X-ray energies. We confirmed that the open electrode structure results in a distorted horizontal pixel boundary. The open electrode region generates both single pixel events and vertically split events, but no horizontally split events. Because the single pixel events usually show the best energy resolution, we discuss a method of increasing the fraction of single pixel events from the open electrode region. Furthermore, we have directly measured the thickness of the electrodes and dead-layers by comparing spectra from the open electrode region with those from the other regions: electrodes, electrode finger and channel stop. We can say that EPIC MOS CCDs are more radiation hard than front-illumination chips of ACIS on-board Chandra X-ray Observatory because of their extra absorption thickness above the charge transfer channel. We calcurated the mean pixel response and found that our estimation has a good agreement with that of the ground calibration of EPIC MOS CCD.

physics.ins-det↗