Sei-ichiro Watanabe

MULTIWAVELENGTH OBSERVATIONS OF THE POWERFUL GAMMA-RAY QUASAR PKS 1510 089: CLUES ON THE JET COMPOSITION

We present the results from a multiwavelength campaign conducted in August 2006 of the powerful {gamma}-ray quasar PKS 1510--089 (z = 0.361). This campaign commenced with a deep Suzaku observation lasting three days for a total exposure time of 120 ks, and continued with Swift monitoring over 18 days. Besides Swift observations, which sampled the optical/UV flux in all 6 UVOT filters as well as the X-ray spectrum in the 0.3--10 keV energy range, the campaign included ground-based optical and radio data, and yielded a quasi-simultaneous broad-band spectral energy distribution from 109 Hz to 1019 Hz. Thanks to its low instrumental background, the Suzaku observation provided a high S/N X-ray spectrum, which is well represented by an extremely hard power-law with photon index {Gamma}{approx_equal}1.2, augmented by a soft component apparent below 1 keV, which is well described by a black-body model with temperature kT {approx_equal}0.2 keV. Monitoring by Suzaku revealed temporal variability which is different between the low and high energy bands, again suggesting the presence of a second, variable component in addition to the primary power-law emission. We model the broadband spectrum of PKS 1510--089 assuming that the high energy spectral component results from Comptonization of infrared radiation produced by hot dust located in the surrounding molecular torus. In the adopted internal shock scenario, the derived model parameters imply that the power of the jet is dominated by protons but with a number of electrons/positrons exceeding a number of protons by a factor {approx} 10. We also find that inhomogeneities responsible for the shock formation, prior to the collision may produce bulk-Compton radiation which can explain the observed soft X-ray excess and possible excess at {approx} 18 keV. We note, however, that the bulk-Compton interpretation is not unique, and the observed soft excess could arise as well via some other processes discussed briefly in the text.

The Suzaku Observation of the Nucleus of the Radio-loud Active Galaxy Centaurus A: Constraints on Abundances of the Accreting Material

A Suzaku observation of the nucleus of the radio-loud AGN Centaurus A in 2005 has yielded a broadband spectrum spanning 0.3 to 250 keV. The net exposure times after screening were: 70 ks per X-ray Imaging Spectrometer (XIS) camera, 60.8 ks for the Hard X-ray Detector (HXD) PIN, and 17.1 ks for the HXD-GSO. The hard X-rays are fit by two power-laws of the same slope, absorbed by columns of 1.5 and 7 x 10{sup 23} cm{sup -2} respectively. The spectrum is consistent with previous suggestions that the power-law components are X-ray emission from the sub-pc VLBI jet and from Bondi accretion at the core, but it is also consistent with a partial covering interpretation. The soft band is dominated by thermal emission from the diffuse plasma and is fit well by a two-temperature vapec model, plus a third power-law component to account for scattered nuclear emission, jet emission, and emission from X-ray Binaries and other point sources. Narrow fluorescent emission lines from Fe, Si, S, Ar, Ca and Ni are detected. The Fe K{alpha} line width yields a 200 light-day lower limit on the distance from the black hole to the line-emitting gas. Fe, Ca, and S K-shell absorption edges are detected. Elemental abundances are constrained via absorption edge depths and strengths of the fluorescent and diffuse plasma emission lines. The high metallicity ([Fe/H]=+0.1) of the circumnuclear material suggests that it could not have originated in the relatively metal-poor outer halo unless enrichment by local star formation has occurred. Relative abundances are consistent with enrichment from Type II and Ia supernovae.

Gas Accretion Flows onto Giant Protoplanets: High-Resolution Two-dimensional Simulations

We performed two-dimensional local hydrodynamic simulations of the accretion flows onto protoplanets (with different masses Mp and semimajor axis a) from a protoplanetary disk with extremely high resolution in order to clarify the fine structure of circumplanetary flows after the onset of the nucleated instability of the planetary atmosphere. We find that two types of shocks are formed: a pair of bow shocks extended outside the planetary gravitational sphere and a pair of spiral shocks inside the sphere winding toward the planet. The disk gas within narrow bands on both sides of the planetary orbit flows into the planetary gravitational sphere. The offset of the bands is determined by the energy dissipation across the bow shock on a streamline toward a stagnant point, and the width is determined by the energy dissipation across the spiral shock on another streamline toward the stagnant point of the opposite side of the planet. This means that the mass accretion rate onto the planet is also determined by the energy dissipation across the spiral shock. With the assumption that the gas is isothermal, we obtain the mass accretion rate as a function of normalized sound speed iso, which corresponds to the ratio of the disk scale height to the Hill radius, as = 8.0 × 10-3 M⊕(a/5.2 AU)-1.5 (Mp/10 M⊕)1.3 (Σ/Σmin)yr-1, where Σ is the surface density of the disk, Σmin is that for the minimum-mass solar nebula model. Note that the slower contraction of the planetary atmosphere (when Mp < 120 M⊕) as well as heating due to the gas accretion luminosity may make the accretion rate smaller. We also find that the circumplanetary spiral shocks could strongly affect the torques exerted on the planet by the disk gas.

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

Sublimation temperature of circumstellar dust particles and its importance for dust ring formation

Dust particles in orbit around a star drift toward the central star by the Poynting-Robertson effect and pile up by sublimation. We analytically derive the pile-up magnitude, adopting a simple model for optical cross sections. As a result, we find that the sublimation temperature of drifting dust particles plays the most important role in the pile-up rather than their optical property does. Dust particles with high sublimation temperature form a significant dust ring, which could be found in the vicinity of the sun through in-situ spacecraft measurements. While the existence of such a ring in a debris disk could not be identified in the spectral energy distribution (SED), the size of a dust-free zone shapes the SED. Since we analytically obtain the location and temperature of sublimation, these analytical formulae are useful to find such sublimation evidences.

Hugoniot equation of state of basalt

Abstract Shock compression experiments on Kinosaki basalt using an in-material gauge were carried out in the interests of elucidation of planetesimal collisions in the early solar nebula. Hugoniot equation of state is necessary for estimating impact induced pressure. Hugoniot of Kinosaki basalt is also useful to analyze previous impact disruption results. Using in-material pressure gauge the shock pressure profile and shock wave velocity (Us) is measured to establish continuous Hugoniot equation of state in the range 1–45 GPa. Although no substantial step was detected in the shock pressure profiles, Hugoniot elastic limit (HEL) was identified at 5 GPa based on the Us-up relationship where up is the particle velocity and is calculated by the impedance matching method. The shock profile showed long rise time and no substantial step at HEL. These features can be explained by the constitution of multigrained basalt.

Cooling and Quasi-Static Contraction of the Primitive Solar Nebula After Gas Accretion

The evolution of the primitive solar nebula in the quasi-static contraction phase where the nebula cools down toward the thermal steady state is studied. The solar irradiation onto the nebula keeps the surface temperature constant, so that the convective ozone retreats from the surface as the nebula cools. Thus if thermal convection is the only source of turbulence, convection will quiet down in an early time of the cooling. Afterward, the nebula evolves toward an isothermal structure in a time scale of 1000 yr. The cooling rates in the vicinity of the midplate at 1 AU are 0.003 K/hr at T(c) = 1000 K and 3 x 10 to the -5th K/hr at T(c) = 300 K for the standard model. If some turbulence exists irrespective of convection, convection may continue for sufficiently strong turbulent heating. 39 refs.

Fine-Pitch Semiconductor Detector for the FOXSI Mission

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket mission which will study particle acceleration and coronal heating on the Sun through high sensitivity observations in the hard X-ray energy band (5-15 keV). Combining high-resolution focusing X-ray optics and fine-pitch imaging sensors, FOXSI will achieve superior sensitivity; two orders of magnitude better than that of the RHESSI satellite. As the focal plane detector, a Double-sided Si Strip Detector (DSSD) with a front-end ASIC (Application Specific Integrated Circuit) will fulfill the scientific requirements of spatial and energy resolution, low energy threshold and time resolution. We have designed and fabricated a DSSD with a thickness of 500 μm and a dimension of 9.6 mm × 9.6 mm, containing 128 strips with a pitch of 75 μm, which corresponds to 8 arcsec at the focal length of 2 m. We also developed a low-noise ASIC specified to FOXSI. The detector was successfully operated in the laboratory at a temperature of -20°C and with an applied bias voltage of 300 V. Extremely good energy resolutions of 430 eV for the p-side and 1.6 keV for the n-side at a 14 keV line were achieved for the detector. We also demonstrated fine-pitch imaging successfully by obtaining a shadow image. Hence the implementation of scientific requirements was confirmed.

THERMAL WAVES IN IRRADIATED PROTOPLANETARY DISKS

Protoplanetary disks are mainly heated by radiation from the central star. Since the incident stellar flux at any radius is sensitive to the disk structure near that location, destabilizing feedback may be present. Previous investigations have shown that the disk will be stable to finite-amplitude temperature perturbations if the vertical height of the optical surface is everywhere directly proportional to the gas scale height and if the intercepted fraction of stellar radiation is determined from the local grazing angle. We show that these assumptions may not be generally applicable. Instead, we calculate the quasi-static thermal evolution of irradiated disks by directly integrating the global optical depth to determine the optical surface and the total emitting area filling factor of surface dust. We show that in disks with modest mass accretion rates, thermal waves are spontaneously and continually excited in the outer disk, propagate inward through the planet-forming domains, and dissipate at small radii where viscous dissipation is dominant. This state is quasi-periodic over several thermal timescales, and its pattern does not depend on the details of the opacity law. The viscous dissipation resulting from higher mass accretion stabilizes this instability such that an approximately steady state is realized throughout the disk. In passive protostellar disks, especially transitional disks, these waves induce significant episodic changes in spectral energy distributions, on timescales of years to decades, because the midplane temperatures can vary by a factor of 2 between the exposed and shadowed regions. The transitory peaks and troughs in the potential vorticity distribution may also lead to baroclinic instability and excite turbulence in the planet-forming regions.

A self‐organized model of earthquakes with constant stress drops and the b‐value of 1

The magnitude-frequency relation and the constant stress drop are fundamental features of earth-quakes, to which a full physical explanation has yet to be given. We present a model that can reproduce the above two fundamental features simultaneously and spontaneously. The model is two-dimensionally configured spring-loaded blocks with a velocity-weakening friction law. We change widely the dynamic friction parameter, which results in the frequency distributions showing the critical, subcritical and supercritical behaviors. Seismicity near the critical state is characterized by almost constant stress drops and the b-value of 1, in which a self-healing pulse maintains its frontal dynamic stress at a level near the static friction in an environmental stress heterogeneity that has evolved through the healing process itself.