arXiv subjects

Koji Mukai

Publications and source records attributed to Koji Mukai.

At least 109 records · Page 6Linked to original sources

arXiv2010

The ASTRO-H Mission

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe by performing high-resolution, high-throughput spectroscopy with moderate angular resolution. ASTRO-H covers very wide energy range from 0.3 keV to 600 keV. ASTRO-H allows a combination of wide band X-ray spectroscopy (5-80 keV) provided by multilayer coating, focusing hard X-ray mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3-12 keV) provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD camera as a focal plane detector for a soft X-ray telescope (0.4-12 keV) and a non-focusing soft gamma-ray detector (40-600 keV) . The micro-calorimeter system is developed by an international collaboration led by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of Delta E ~7 eV provided by the micro-calorimeter will enable a wide variety of important science themes to be pursued.

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arXiv2010

Deriving an X-ray luminosity function of dwarf novae based on parallax measurements

We have derived an X-ray luminosity function using parallax-based distance measurements of a set of 12 dwarf novae, consisting of Suzaku, XMM-Newton and ASCA observations. The shape of the X-ray luminosity function obtained is the most accurate to date, and the luminosities of our sample are concentrated between ~10^{30}-10^{31} erg s^{-1}, lower than previous measurements of X-ray luminosity functions of dwarf novae. Based on the integrated X-ray luminosity function, the sample becomes more incomplete below ~3 x 10^{30} erg s^{-1} than it is above this luminosity limit, and the sample is dominated by X-ray bright dwarf novae. The total integrated luminosity within a radius of 200 pc is 1.48 x 10^{32} erg s^{-1} over the luminosity range of 1 x 10^{28} erg s^{-1} and the maximum luminosity of the sample (1.50 x 10^{32} erg s^{-1}). The total absolute lower limit for the normalised luminosity per solar mass is 1.81 x 10^{26} erg s^{-1} M^{-1}_{solar} which accounts for ~16 per cent of the total X-ray emissivity of CVs as estimated by Sazonov et al. (2006).

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arXiv2009

Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)

We report optical spectroscopy and high speed photometry and polarimetry of the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry, polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1) hours. Orbital circularly polarized modulations are seen from 0 to -18 per cent, unambiguously identifying IGRJ14536-5522 as a polar. Some of the high speed photometric data show modulations that are consistent with quasi-periodic oscillations (QPOs) on the order of 5-6 minutes. Furthermore, for the first time, we detect the (5-6) minute QPOs in the circular polarimetry. We discuss the possible origins of these QPOs. We also include details of HIPPO, a new high-speed photo-polarimeter used for some of our observations.

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arXiv2009

Distribution and Structure of Matter in and around Galaxies

Understanding the origins and distribution of matter in the Universe is one of the most important quests in physics and astronomy. Themes range from astro-particle physics to chemical evolution in the Galaxy to cosmic nucleosynthesis and chemistry in an anticipation of a full account of matter in the Universe. Studies of chemical evolution in the early Universe will answer questions about when and where the majority of metals were formed, how they spread and why they appar today as they are. The evolution of matter in our Universe cannot be characterized as a simple path of development. In fact the state of matter today tells us that mass and matter is under constant reformation through on-going star formation, nucleosynthesis and mass loss on stellar and galactic scales. X-ray absorption studies have evolved in recent years into powerful means to probe the various phases of interstellar and intergalactic media. Future observatories such as IXO and Gen-X will provide vast new opportunities to study structure and distribution of matter with high resolution X-ray spectra. Specifically the capabilities of the soft energy gratings with a resolution of R=3000 onboard IXO will provide ground breaking determinations of element abundance, ionization structure, and dispersion velocities of the interstellar and intergalactic media of our Galaxy and the Local Group

astro-ph.HE↗

arXiv2009

Structure and Evolution of Pre-Main Sequence Stars

Low-mass pre-main sequence (PMS) stars are strong and variable X-ray emitters, as has been well established by EINSTEIN and ROSAT observatories. It was originally believed that this emission was of thermal nature and primarily originated from coronal activity (magnetically confined loops, in analogy with Solar activity) on contracting young stars. Broadband spectral analysis showed that the emission was not isothermal and that elemental abundances were non-Solar. The resolving power of the Chandra and XMM X-ray gratings spectrometers have provided the first, tantalizing details concerning the physical conditions such as temperatures, densities, and abundances that characterize the X-ray emitting regions of young star. These existing high resolution spectrometers, however, simply do not have the effective area to measure diagnostic lines for a large number of PMS stars over required to answer global questions such as: how does magnetic activity in PMS stars differ from that of main sequence stars, how do they evolve, what determines the population structure and activity in stellar clusters, and how does the activity influence the evolution of protostellar disks. Highly resolved (R>3000) X-ray spectroscopy at orders of magnitude greater efficiency than currently available will provide major advances in answering these questions. This requires the ability to resolve the key diagnostic emission lines with a precision of better than 100 km/s.

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arXiv2008

The Suzaku Observations of SS Cygni in Quiescence and Outburst

We present results from the Suzaku observations of the dwarf nova SS Cyg in quiescence and outburst in 2005 November. Owing to high sensitivity of the HXD PIN detector and high spectral resolution of the XIS, we have determined parameters of the plasma with unprecedented precision. The maximum temperature of the plasma in quiescence 20.4 +4.0-2.6 (stat.) +/- 3.0 (sys.) keV is significantly higher than that in outburst 6.0 +0.2-1.3 keV. The elemental abundances are close to the solar ones for the medium-Z elements (Si, S, Ar) whereas they decline both in lighter and heavier elements. Those of oxygen and iron are 0.46 and 0.37 solar, respectively. That of carbon is exceptionally high and 2 solar at least. The solid angle of the reflector subtending over the optically thin thermal plasma is Omega/2π= 1.7+/-0.2 (stat.) +/-0.1 (sys.) in quiescence. A 6.4 keV iron Ka line is resolved into a narrow and broad components. These facts indicate that both the white dwarf and the accretion disk contribute to the continuum reflection and the 6.4 keV iron Ka line. 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 Omega/2π= 0.9 +0.5-0.4 and a broad 6.4 keV iron line indicates that the reflection in outburst originates from the accretion disk and an equatorial accretion belt. From the energy width of the 6.4 keV line, we consider the optically thin thermal plasma in outburst as being distributed on the accretion disk like solar coronae.

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arXiv2008

RS Ophiuchi in Quiescence: Why is it X-ray Faint?

The short interval between successive outbursts of RS Oph strongly suggests that it has a high mass white dwarf accreting at a high rate. This, in turn, suggests the possibility of prominent X-ray emission from RS Oph in quiescence. However, archival quiescent X-ray observations of RS Oph show it to be a modest soft X-ray source but not a strong 2-10 keV X-ray source. In this aspect, RS Oph differs markedly from T CrB. We speculate on the possible mechanisms that could significantly suppress the 2-10 keV X-ray emission in RS Oph.

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arXiv2008

The Intermediate Polar EI UMa: A Pre-Polar Cataclysmic Variable

We present optical and X-ray time-series photometry of EI UMa that reveal modulation at 746 and 770 s, which we interpret as the white dwarf spin and spin-orbit sidebands. These detections, combined with previous X-ray studies, establish EI UMa as an intermediate polar. We estimate the mass accretion rate to be ~ 3.6 x 10^{17} g s^{-1}, which is close to, and likely greater than, the critical rate above which dwarf nova instabilities are suppressed. We also estimate the white dwarf to have a large magnetic moment mu > (3.4 +/- 0.2) x 10^{33} G cm^3. The high mass accretion rate and magnetic moment imply the existence of an accretion ring rather than a disk, and along with the relatively long orbital period, these suggest that EI UMa is a rare example of a pre-polar cataclysmic variable.

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arXiv2008

Suzaku Discovery of Hard X-ray Pulsations from the Rotating Magnetized White Dwarf, AE Aquarii

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 \pm 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 about 0.2 cycles in phase, in contrast to approximately sinusoidal profiles observed in energies below about 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 $2.90_{-0.16}^{+0.20}$ keV and $0.53_{-0.13}^{+0.14}$ 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 $1.12_{-0.62}^{+0.63}$ or a third thermal component with a temperature of $54_{-47}^{+26}$ keV. At a distance of 102 pc, the 4--30 keV luminosities of the thermal and the additional components become $1.7_{-0.6}^{+1.3}$ and $5.3_{-0.3}^{+15.3} \times 10^{29}$ 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.

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arXiv2007

High energy X-ray emission from recurrent novae in quiescence: T CrB

We present Suzaku X-ray observations of the recurrent nova T CrB in quiescence. T CrB is the first recurrent nova to be detected in the hard-X-ray band (E ~ 40.0 keV) during quiescence. The X-ray spectrum is consistent with cooling-flow emission emanating from an optically thin region in the boundary layer of an accretion disk around the white dwarf. The detection of strong stochastic flux variations in the light curve supports the interpretation of the hard X-ray emission as emanating from a boundary layer.

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arXiv2007

The Symbiotic System SS73 17 Seen with Suzaku

We observed with Suzaku the symbiotic star SS73 17, motivated by the discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard X-rays. Our observations showed a highly-absorbed X-ray spectrum with NH > 10^23 cm-2, equivalent to A_V > 26, although the source has B magnitude 11.3 and is also bright in UV. The source also shows strong, narrow iron lines including fluorescent Fe K as well as Fe xxv and Fe xxvi. The X-ray spectrum can be fit with a thermal model including an absorption component that partially covers the source. Most of the equivalent width of the iron fluorescent line in this model can be explained as a combination of reprocessing in a dense absorber plus reflection off a white dwarf surface, but it is likely that the continuum is partially seen in reflection as well. Unlike other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB, GX1+4), SS73 17 is not known to have shown nova-like optical variability, X-ray flashes, or pulsations, and has always shown faint soft X-ray emission. As a result, although it is likely a white dwarf, the nature of the compact object in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the recently discovered and relatively small class of hard X-ray emitting symbiotic systems.

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arXiv2007

XMM-Newton Observations of the Cataclysmic Variable GW Lib

XMM-Newton observations of the accreting, pulsating white dwarf in the quiescent dwarf nova GW Librae were conducted to determine if the non-radial pulsations present in previous UV and optical data affect the X-ray emission. The non-radial pulsations are evident in the simultaneous Optical Monitor data but are not detected in X-ray with an upper limit on the pulsation amplitude of 0.092 mags. The best fits to the X-ray spectrum are with a low temperature diffuse gas model or a multi-temperature cooling flow model, with a strong OVIII line, similar to other short period dwarf novae, but with a lower temperature range than evident in normal short period dwarf novae. The lack of pulsations and the spectrum likely indicate that the boundary layer does not extend to the surface of the white dwarf.

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arXiv2006

Suzaku Observations of the Local and Distant Hot ISM

Suzaku observed the molecular cloud MBM12 and a blank field less than 3 degrees away to separate the local and distant components of the diffuse soft X-ray background. Towards MBM12, a local (D< 275 pc) O VII emission line was clearly detected with an intensity of 3.5 ph cm^{-2} s^{-1} sr^{-1}$ (or line units, LU), and the O VIII flux was <0.34 LU. The origin of this O VII emission could be hot gas in the Local Hot Bubble (LHB), charge exchange between oxygen ions in the solar wind (SWCX) and geocoronal or interplanetary material, or a combination of the two. If entirely from the LHB, the implied temperature and emission measure would predict 1/4 keV emission in excess of observations. There is no evidence in the X-ray light curve or solar wind data for a significant contribution from geocoronal SWCX. In any case, the observed O VII flux represents an upper limit to both the LHB emission and interplanetary SWCX in this direction. The off-cloud observation was performed immediately following the on-cloud. The net off-cloud O VII and O VIII intensities were (respectively) 2.34\pm0.33 and 0.77\pm0.16 LU, after subtracting the on-cloud foreground emission. Assuming the LHB and SWCX components did not change, these increases can be attributed to more distant Galactic disk, halo, or extragalactic emission. If the distant O VII and O VIII emission is from a thermal plasma in collisional equilibrium beyond the Galactic disk, a temperature of (2.1\pm0.1)\times10^6 K with an emission measure of (4\pm0.6)\times10^-3} cm^{-6}pc is inferred.

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arXiv2006

An Apparent Hard X-ray Decline of CH Cygni

CH Cygni is a symbiotic star consisting of an M giant and an accreting white dwarf, which is known to be a highly variable X-ray source with a complex, two-component, spectra. Here we report on two Suzaku observations of CH Cyg, taken in 2006 January and May, during which the system was seen to be in a soft X-ray bright, hard X-ray faint state. Based on the extraordinary strength of the 6.4 keV fluorescent Fe K-alpha line, we show that the hard X-rays observed with Suzaku are dominated by scattering.

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arXiv2004

On the iron Kalpha complex in magnetic cataclysmic variables

We present a compilation of spectra of the iron Kalpha region in magnetic cataclysmic variables, using data from the Chandra-HETG. The H-like, He-like and fluorescent components are clearly resolved, and there are hints of the structure within each component. The different shape of the He-like component in AM Her might be related to greater cyclotron cooling in this star. A surprising absence of Doppler shifts in the H-like and He-like components implies that the X-ray emission is predominantly from the denser, lower-velocity base of the accretion columns. This absence will allow ASTRO-E2 to resolve the structure in each component, leading to temperature diagnostics. We do not confirm the report that the H-like and He-like components of AO Psc are Compton broadened; however we do detect a Compton-downshifted shoulder to the fluorescent line of GK Per. Further, a Doppler-shifted wing of this line arises in the high-velocity, pre-shock flow.

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arXiv2003

X-ray observations of 4 Draconis: symbiotic binary or cataclysmic triple?

We present the first X-ray observations of the 4 Draconis system, consisting of an M3III giant with a hot ultraviolet companion. It has been claimed that the companion is itself an AM Her-type binary system, an identification that places strong constraints on the evolution of cataclysmic variables. We find that the X-ray properties of 4 Draconis are consistent with the presence of an accreting white dwarf, but not consistent with the presence of an AM Her system. We conclude that 4 Dra is therefore most-likely a symbiotic binary containing a white dwarf accreting material from the wind of the red giant. The X-ray spectrum of 4 Dra is sometimes dominated by partially-ionised photoelectric absorption, presumably due to the wind of the red giant. We note that X-ray monitoring of such systems would provide a powerful probe of the wind and mass-loss rate of the giant, and would allow a detailed test of wind accretion models.

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arXiv2003

Far-UV FUSE spectra of peculiar magnetic cataclysmic variables

We present far-UV spectra of the three magnetic cataclysmic variables (MCVs) BY Cam, V1309 Ori and AE Aqr obtained with the FUSE satellite. These MCVs have revealed strongly unusual NV and CIV UV resonance lines. The FUSE spectra exhibit broad OVI lines as well as a strong NIII line at 991A, while the CIII 1175A line is nearly absent, supporting non-solar CNO abundances of the accreting matter in these sources. The spectrum of BY Cam shows molecular H2 lines which might be of circumstellar nature. The flaring activity of AE Aqr is also observed in the far-UV range. The radial velocities of the broad OVI components in AE Aqr are orbitally modulated and would indicate an emission region close to the magnetosphere.

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arXiv2002

The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System

A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026 using observations made with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. The mass function obtained of ~16Mo together with the detection of an extended near-total eclipse confirm that the primary star is a supergiant as predicted. The orbital eccentricity is found to be very low with a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be 6.0724 +/- 0.0009 days using an improved and extended light curve obtained with RXTE's All Sky Monitor. Observations with the ASCA satellite provide an improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0" (2000) with an estimated systematic uncertainty of less than 12". A serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA observations.

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