Ryoko Nakamura

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.

The Nature of a Cosmic-Ray Accelerator, CTB 37 B, Observed with Suzaku and Chandra

We report on Suzaku and Chandra observations of the young supernova remnant CTB 37 B, from which TeV � -rays were detected by the H.E.S.S. Cherenkov telescope. The 80 ks Suzaku observation provided us with a clear image of diffuse emission and high-quality spectra. The spectra revealed that the diffuse emission is comprised of thermal and non-thermal components. The thermal component can be represented by an NEI model with a temperature, a pre-shock electron density and an age of 0.9˙0.2 keV, 0.4˙0.1 cm � 3 , and 650 +2500 � 300 yr, respectively. This suggests that the explosion of CTB 37 B occurred in a low-density space. A non-thermal power-law component was found from the southern region of CTB 37 B. Its photon index of � 1.5 and a high roll-off energy (& 15 keV) indicate efficient cosmic-ray acceleration. A comparison of this X-ray spectrum with the TeV � -ray spectrum leads us to conclude that the TeV � -ray emission seems to be powered by either multi-zone Inverse Compton scattering or the decay of neutral pions. The point source resolved by Chandra near the shell is probably associated with CTB 37 B, because of the common hydrogen column density with the diffuse thermal emission. Spectral and temporal characteristics suggest that this source is a new anomalous X-ray pulsar.

EVOLUTION OF SYNCHROTRON X-RAYS IN SUPERNOVA REMNANTS

A systematic study of the synchrotron X-ray emission from supernova remnants (SNRs) has been conducted. We selected a total of 12 SNRs whose synchrotron X-ray spectral parameters are available in the literature with reasonable accuracy and studied how their luminosities change as a function of radius. It is found that the synchrotron X-ray luminosity tends to drop especially when the SNRs become larger than {approx}5 pc, despite large scatter. This may be explained by the change of spectral shape caused by the decrease of the synchrotron roll-off energy. A simple evolutionary model of the X-ray luminosity is proposed and is found to reproduce the observed data approximately, with reasonable model parameters. According to the model, the total energy of accelerated electrons is estimated to be 10{sup 47-48} erg, which is well below the supernova explosion energy. The maximum energies of accelerated electrons and protons are also discussed.

Identification of CXOU J171405.7-381031 as a New Magnetar with XMM-Newton

We have observed the 3.8 s pulsar CXOU J171405.7-381031 with XMM-Newton, and discovered the significant dP/dt of 6.40+/-0.14X10^-11 s/s from this source for the first time, with the aid of archival Chandra data. The characteristic age (950 yr), the magnetic field strength (5X10^14 G), and the spin-down luminosity (4.5X10^34 erg/s) derived from P and dP/dt lead us to conclude that CXOU J171405.7-381031 should be identified as a new magnetar. The obtained characteristic age indicates that CXOU J171405.7-381031 is youngest among all known anomalous X-ray pulsars, which is consistent with the age estimation from the thermal X-rays of the associated supernova remnant. The ratio between 2-10 keV luminosity and spin-down luminosity is almost unity, which implies that CXOU J171405.7-381031 is the key source to connect magnetars and traditional radio pulsars.

X-ray spectroscopy of the mixed morphology supernova remnant W 28 with XMM-Newton

We report on spatially resolved X-ray spectroscopy of the north-eastern part of the mixed morphology supernova remnant (SNR) W28 with XMM-Newton. The observed field of view includes a prominent and twisted shell emission forming the edge of this SNR as well as part of the center-filled X-ray emission brightening toward the southwest edge of the field of view. The shell region spectra are in general represented by an optically thin thermal plasma emission in collisional ionization equilibrium with a temperature of ∼0.3 keV and a density of ∼10 cm, which is much higher than the density obtained for inner parts. In contrast, we detected no significant X-ray flux from one of the TeV γ-ray peaks with an upper-limit flux of 2.1×10 erg cm s in the 2–10 keV band. The large flux ratio of TeV to X-ray, larger than 16, and the spatial coincidence of the molecular cloud and the TeV γ-ray emission site indicate that the TeV γ-ray of W28 is π-decay emission originating from collisions between accelerated protons and molecular cloud protons. Comparing the spectrum in the TeV band and the X-ray upper limit, we obtained a weak upper limit on the magnetic field strength B < ∼ 1500 μG.

Chandra HETG Line Spectroscopy of the Nonmagnetic Cataclysmic Variable SS Cygni

We present Chandra HETG observations of SS Cygni in quiescence and outburst. The spectra are characterized by He-like and H-like Kα emission lines from O to Fe, as well as L-shell emission lines from Fe. In quiescence, the spectra are dominated by the H-like Kα lines, whereas in outburst the He-like lines are as intense as the H-like lines. In outburst, the H-like Kα lines from O to Si are broad, with widths of 4-14 eV in Gaussian σ (1800-2300 km s−1). The large line widths, together with line profiles, indicate that the line-emitting plasma is associated with the Keplerian disk and still retains the azimuthal bulk motion. In quiescence, the emission lines are narrower, with a Gaussian σ of 1-3 eV (420-620 km s−1). A slightly larger velocity for lighter elements suggests that the lines in quiescence are emitted from an ionizing plasma at the entrance of the boundary layer, where the bulk motion of the optically thick accretion disk is converted into heat due to friction. Using the line intensity ratio of He-like and H-like Kα lines for each element, we have also investigated the temperature distribution in the boundary layer both in quiescence and outburst. The distribution of SS Cyg is found to be consistent with other dwarf novae investigated systematically with ASCA data.

Development of the Compact Infrared Camera (CIRC) for earth observation

We have developed Compact Infrared Camera (CIRC) with an uncooled infrared array detector (microbolometer) for space applications. The main mission of the CIRC is to demonstrate technology for wildfire detection. Wildfires are a major and chronic disaster that affects many countries, especially those in the Asia-Pacific region, and the situation may get worse with global warming and climate change. The CIRC detector has the largest format (640 × 480 pixels) ever used for observations of Earth from space. Microbolometers have the advantage of not requiring cooling systems such as a mechanical cooler and are suitable for resource-limited sensor systems or small satellites. In addition, the CIRC employs athermal optics and a shutter-less system, and hence, it is of a small size, is lightweight, and consumes low electrical power. The CIRC design was based on a commercial infrared camera and employs commercial-off-the-shelf (COTS) parts to reduce the cost and time for development. The CIRC will be carried as a technology demonstration payload of ALOS-2 and ISS/JEM, which will be launched in 2013 and 2014. We have developed the CIRC Proto Flight Model (PFM) and performed experiments for calibration in January 2012. In this paper, we present the verification results of the athermal characteristics and the calibration of the shutter-less system.

Tandem-L instrument design and SAR performance overview

Tandem-L is a proposal for an innovative interferometric radar mission to monitor the Earth system and its intricate dynamics. Important mission objectives are global inventories of forest height and above-ground biomass, largescale measurements of Earth surface deformations due to plate tectonics, erosion and anthropogenic activities, observations of glacier movements and 3-D structure changes in land and sea ice, and the monitoring of ocean surface currents. A detailed description of the mission goals can be found in [1]. The mission concept is based on co-flying two fully-polarimetric L-band SAR satellites in a close formation. Tandem-L employs new techniques and advanced technologies to achieve its ambitious mission goals. The feasibility of a joint DLR/JAXA mission is currently being investigated in the scope of a pre-phase A study. This paper provides an overview of the Tandem-L instrument design concept and its performance predictions. Innovative aspects like the employment of advanced digital beamforming techniques and operation in a variety of imaging modes are detailed.

X-band interferometric SAR sensor for the Japanese altimetry mission, COMPIRA

COMPIRA (Coastal and Ocean Measurement mission with Precise and Innovative Radar Altimeter) is a new Japanese altimetry mission with X-band interferometric synthetic aperture radar (SHIOSAI; SAR Height Imaging Oceanic Sensor with Advanced Interferometry). We will use a wide-swath altimeter to maximize the spatial coverage of the altimetry measurements and to reduce errors in the tidal model in shallow marginal seas. We are now working on a conceptual study of the X-band interferometric synthetic aperture radar. In parallel, to demonstrate the sea surface height measurement with X-band interferometric SAR and obtain some parameters for design of SHIOSAI, we conducted aircraft experiments of the sea surface height measurement with an airborne interferometric SAR. In the paper, we will present current status of conceptual study of the SHIOSAI sensor and the airborne experiments.