arXiv subjects

K. Takahashi

Publications and source records attributed to K. Takahashi.

At least 55 records · Page 3Linked to original sources

arXiv2014

Comparison of algorithms for determination of rotation measure and Faraday structure I. 1100 - 1400 MHz

(abridged) We run a Faraday structure determination data challenge to benchmark the currently available algorithms including Faraday synthesis (previously called RM synthesis in the literature), wavelet, compressive sampling and $QU$-fitting. The frequency set is similar to POSSUM/GALFACTS with a 300 MHz bandwidth from 1.1 to 1.4 GHz. We define three figures of merit motivated by the underlying science: a) an average RM weighted by polarized intensity, RMwtd, b) the separation $Δϕ$ of two Faraday components and c) the reduced chi-squared. Based on the current test data of signal to noise ratio of about 32, we find that: (1) When only one Faraday thin component is present, most methods perform as expected, with occasional failures where two components are incorrectly found; (2) For two Faraday thin components, QU-fitting routines perform the best, with errors close to the theoretical ones for RMwtd, but with significantly higher errors for $Δϕ$. All other methods including standard Faraday synthesis frequently identify only one component when $Δϕ$ is below or near the width of the Faraday point spread function; (3) No methods, as currently implemented, work well for Faraday thick components due to the narrow bandwidth; (4) There exist combinations of two Faraday components which produce a large range of acceptable fits and hence large uncertainties in the derived single RMs; in these cases, different RMs lead to the same Q, U behavior, so no method can recover a unique input model.

astro-ph.IM↗

arXiv2014

Excitation of Giant Monopole Resonance in $^{208}$Pb and $^{116}$Sn Using Inelastic Deuteron Scattering

The excitation of the isoscalar giant monopole resonance (ISGMR) in $^{116}$Sn and $^{208}$Pb has been investigated using small-angle (including $0^\circ$) inelastic scattering of 100 MeV/u deuteron and multipole-decomposition analysis (MDA). The extracted strength distributions agree well with those from inelastic scattering of 100 MeV/u $α$ particles. These measurements establish deuteron inelastic scattering at E$_d \sim$ 100 MeV/u as a suitable probe for extraction of the ISGMR strength with MDA, making feasible the investigation of this resonance in radioactive isotopes in inverse kinematics.

nucl-ex↗

arXiv2013

Radio-transparent multi-layer insulation for radiowave receivers

In the field of radiowave detection, enlarging the receiver aperture to enhance the amount of light detected is essential for greater scientific achievements. One challenge in using radio transmittable apertures is keeping the detectors cool. This is because transparency to thermal radiation above the radio frequency range increases the thermal load. A technology that maintains cold conditions while allowing larger apertures has been long-awaited. We propose radio-transparent multi-layer insulation (RT-MLI), composed from a set of stacked insulating layers. The insulator is transparent to radio frequencies, but not transparent to infrared radiation. The basic idea for cooling is similar to conventional multi-layer insulation. It leads to a reduction in thermal radiation while maintaining a uniform surface temperature. The advantage of this technique over other filter types is that no thermal links are required. As insulator material, we used foamed polystyrene; its low index of refraction makes an anti-reflection coating unnecessary. We measured the basic performance of RT-MLI to confirm that thermal loads are lowered with more layers. We also confirmed that our RT-MLI has high transmittance to radiowaves, but blocks infrared radiation. For example, RT-MLI with 12 layers has a transmittance greater than 95% (lower than 1%) below 200 GHz (above 4 THz). We demonstrated its effects in a system with absorptive-type filters, where aperture diameters were 200 mm. Low temperatures were successfully maintained for the filters. We conclude that this technology significantly enhances the cooling of radiowave receivers, and is particularly suitable for large-aperture systems. This technology is expected to be applicable to various fields, including radio astronomy, geo-environmental assessment, and radar systems.

astro-ph.IM↗

arXiv2013

Active Galactic Nuclei under the scrutiny of CTA

Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other "exotic" phenomena.

astro-ph.HE↗

arXiv2012

Well-developed deformation in 42Si

Excited states in 38,40,42Si nuclei have been studied via in-beam gamma-ray spectroscopy with multi-nucleon removal reactions. Intense radioactive beams of 40S and 44S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled gamma-gamma coincidence measurements. A prominent gamma line observed with an energy of 742(8) keV in 42Si confirms the 2+ state reported in an earlier study. Among the gamma lines observed in coincidence with the 2+ -> 0+ transition, the most probable candidate for the transition from the yrast 4+ state was identified, leading to a 4+_1 energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2+_1 and 4+_1 states indicates well-developed deformation in 42Si at N=28 and Z=14. Also for 38,40Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for 42Si, the results show a rapid deformation development of Si isotopes from N=24 to N=28.

nucl-ex↗

arXiv2008

3He Spin-Dependent Cross Sections and Sum Rules

We present a measurement of the spin-dependent cross sections for the \vec{^3He}(\vec{e},e')X} reaction in the quasielastic and resonance regions at four-momentum transfer 0.1 < Q^2< 0.9 GeV^2. The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt--Cottingham and extended GDH sum rules for the first time. Impulse approximation and exact three-body Faddeev calculations are also compared to the data in the quasielastic region.

nucl-ex↗

arXiv2007

The r-process of stellar nucleosynthesis: Astrophysics and nuclear physics achievements and mysteries

The r-process, or the rapid neutron-capture process, of stellar nucleosynthesis is called for to explain the production of the stable (and some long-lived radioactive) neutron-rich nuclides heavier than iron that are observed in stars of various metallicities, as well as in the solar system. A very large amount of nuclear information is necessary in order to model the r-process. This concerns the static characteristics of a large variety of light to heavy nuclei between the valley of stability and the vicinity of the neutron-drip line, as well as their beta-decay branches or their reactivity. The enormously challenging experimental and theoretical task imposed by all these requirements is reviewed, and the state-of-the-art development in the field is presented. Nuclear-physics-based and astrophysics-free r-process models of different levels of sophistication have been constructed over the years. We review their merits and their shortcomings. For long, the core collapse supernova of massive stars has been envisioned as the privileged r-process location. We present a brief summary of the one- or multidimensional spherical or non-spherical explosion simulations available to-date. Their predictions are confronted with the requirements imposed to obtain an r-process. The possibility of r-nuclide synthesis during the decompression of the matter of neutron stars following their merging is also discussed.

astro-ph↗

arXiv2005

Anisotropic gaseous models of tidally limited star clusters -- comparison with other methods

We present new models of the evolution and dissolution of star clusters evolving under the combined influence of internal relaxation and external tidal fields, using the anisotropic gaseous model based on the Fokker-Planck approximation, and a new escaper loss cone model. This model borrows ideas from loss cones of stellar distributions near massive black holes, and describes physical processes related to escaping stars by a simple model based on two timescales and a diffusion process. We compare our results with those of direct $N$-body models and of direct numerical solutions of the orbit-averaged Fokker-Planck equation. For this comparative study we limit ourselves to idealized single point mass star clusters, in order to present a detailed study of the physical processes determining the rate of mass loss, core collapse and other features of the system's evolution. With the positive results of our study the path is now open in the future to use the computationally efficient gaseous models for future studies with more realism (mass spectrum, stellar evolution).

astro-ph↗

arXiv2005

BRUSLIB and NETGEN: the Brussels nuclear reaction rate library and nuclear network generator for astrophysics

Nuclear reaction rates are quantities of fundamental importance in astrophysics. Substantial efforts have been devoted in the last decades to measure or calculate them. The present paper presents for the first time a detailed description of the Brussels nuclear reaction rate library BRUSLIB and of the nuclear network generator NETGEN so as to make these nuclear data packages easily accessible to astrophysicists for a large variety of applications. BRUSLIB is made of two parts. The first one contains the 1999 NACRE compilation based on experimental data for 86 reactions with (mainly) stable targets up to Si. The second part of BRUSLIB concerns nuclear reaction rate predictions calculated within a statistical Hauser-Feshbach approximation, which limits the reliability of the rates to reactions producing compound nuclei with a high enough level density. These calculations make use of global and coherent microscopic nuclear models for the quantities entering the rate calculations. The use of such models is utterly important, and makes the BRUSLIB rate library unique. A description of the Nuclear Network Generator NETGEN that complements the BRUSLIB package is also presented. NETGEN is a tool to generate nuclear reaction rates for temperature grids specified by the user. The information it provides can be used for a large variety of applications, including Big Bang nucleosynthesis, the energy generation and nucleosynthesis associated with the non-explosive and explosive hydrogen to silicon burning stages, or the synthesis of the heavy nuclides through the s-, alpha- and r-, rp- or p-processes.

astro-ph↗

arXiv2004

Muon-spin-relaxation and magnetic-susceptibility studies of the effects of nonmagnetic impurities on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ around x = 0.115

Zero-field muon-spin-relaxation (muSR) and magnetic-susceptibility measurements have been carried out in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x = 0.10, 0.115 and 0.13 changing the Zn-concentration y finely up to 0.10, with the aim to clarify effects of the nonmagnetic impurity Zn on the Cu-spin dynamics and superconductivity. The muSR measurements have revealed that, in each x, a magnetic order of Cu spins is observed at y ~ 0.0075, while it disappears and Cu spins turn into a fast fluctuating state for y > 0.03. From the magnetic-susceptibility measurements, on the other hand, it has been found that the volume fraction of the superconducting state rapidly decreases through the slight doping of Zn and that its y dependence corresponds to the y dependence of the volume fraction of the fast fluctuating region of Cu spins estimated from the muSR results. Both the rapid decrease of the volume fraction of the superconducting state and the formation of the magnetic order through the slight doping of Zn can be interpreted in terms of the development of the so-called "Swiss cheese" model. That is, it is concluded that Cu spins in a non-superconducting region around Zn exhibit slowing-down of the fluctuations or form an incoherent or coherent static magnetic order. Moreover, the formation of a non-superconducting region around Zn is considered to be due to the pinning of the dynamical spin correlation or dynamical stripe correlations.

cond-mat.supr-con↗

arXiv2004

Numerical Examination of the Stability of an Exact Two-dimensional Solution for Flux Pile-up Magnetic Reconnection

The Kelvin--Helmholtz (KH) and tearing instabilities are likely to be important for the process of fast magnetic reconnection that is believed to explain the observed explosive energy release in solar flares. Theoretical studies of the instabilities, however, typically invoke simplified initial magnetic and velocity fields that are not solutions of the governing magnetohydrodynamic (MHD) equations. In the present study, the stability of a reconnecting current sheet is examined using a class of exact global MHD solutions for steady state incompressible magnetic reconnection, discovered by Craig & Henton. Numerical simulation indicates that the outflow solutions where the current sheet is formed by strong shearing flows are subject to the KH instability. The inflow solutions where the current sheet is formed by a fast and weakly sheared inflow are shown to be tearing unstable. Although the observed instability of the solutions can be interpreted qualitatively by applying standard linear results for the KH and tearing instabilities, the magnetic field and plasma flow, specified by the Craig--Henton solution, lead to the stabilization of the current sheet in some cases. The sensitivity of the instability growth rate to the global geometry of magnetic reconnection may help in solving the trigger problem in solar flare research.

astro-ph↗

arXiv2004

Bosonization for disordered and chaotic systems

Using a supersymmetry formalism, we reduce exactly the problem of electron motion in an external potential to a new supermatrix model valid at all distances. All approximate nonlinear sigma models obtained previously for disordered systems can be derived from our exact model using a coarse-graining procedure. As an example, we consider a model for a smooth disorder and demonstrate that using our approach does not lead to a 'mode-locking' problem. As a new application, we consider scattering on strong impurities for which the Born approximation cannot be used. Our method provides a new calculational scheme for disordered and chaotic systems.

cond-mat.mes-hall↗

arXiv2003

Probing dark energy with gamma-ray bursts

We propose a new method to use gamma-ray bursts (GRBs) as an alternative probe of the dark energy. By calibrating luminosity-variability and luminosity-lag time relations at low redshift where distance-redshift relations have been already determined from type Ia supernova (SNIa), GRBs at high redshift can be used as a distance indicator. We investigate the potential impact of future GRB data on determining the current matter density $Ω_{m}$ and the dark energy equation of state $w$ which is assumed to be constant. We show that a combined analysis of a data set expected from the {\it Swift} and the current SNIa data results in excellent determination of both $Ω_{m}$ and $w$.

astro-ph↗

arXiv2003

Effects of neutrino oscillation on supernova neutrino: inverted mass hierarchy

We study the effects of neutrino oscillation on supernova neutrino in the case of the inverted mass hierarchy ($m_{3} \ll m_{1} < m_{2}$) as well as the normal mass hierarchy ($m_{1} < m_{2} \ll m_{3}$). Numerical analysis using a realistic supernova and presupernova model allows us to discuss quantitatively a possibility to probe neutrino oscillation parameters. We show that information about the mass hierarchy can be obtained if $θ_{13}$ is rather large ($\sin^{2}{2 θ_{13}} > 10^{-3}$) and that $θ_{13}$ can be well probed by SuperKamiokande if the neutrino mass hierarchy is the inverted case. Errors due to the uncertainty of the original neutrino spectra and the Earth effect are also discussed.

hep-ph↗

arXiv2003

Shock propagation and neutrino oscillation in supernova

The effect of the shock propagation on neutrino oscillation in supernova is studied paying attention to evolution of average energy of $ν_{e}$ and $\barν_{e}$. We show that the effect appears as a decrease in average $ν_{e}$ (in case of inverted mass hierarchy, $\barν_{e}$) energy at stellar surface as the shock propagates. It is found that the effect is significant 2 seconds after bounce if $3 \times 10^{-5} < \sin^{2}{θ_{13}} < 10^{-2}$.

astro-ph↗

arXiv2003

Semiclassical Study on Tunneling Processes via Complex-Domain Chaos

We investigate the semiclassical mechanism of tunneling process in non-integrable systems. The significant role of complex-phase-space chaos in the description of the tunneling process is elucidated by studying a simple scattering map model. Behaviors of tunneling orbits are encoded into symbolic sequences based on the structure of complex homoclinic tanglement. By means of the symbolic coding, the phase space itineraries of tunneling orbits are related with the amounts of imaginary parts of actions gained by the orbits, so that the systematic search of significant tunneling orbits becomes possible.

nlin.CD↗

arXiv2002

Earth effects on supernova neutrinos and their implications for neutrino parameters

We perform a detailed study of the Earth matter effects on supernova neutrinos with neutrino oscillation parameter LMA and small $θ_{13}$. The Earth effects show significant dependences on the neutrino path length inside the Earth and the value of $Δm^{2}_{12}$. We investigate rather optimistically a possibility that we can probe the value of $Δm^{2}_{12}$ by the Earth effects. We assume that $θ_{12}$ and the direction of the supernova are known with enough accuracy and that the resonance that occurs at higher density in supernova envelope is completely nonadiabatic. Further the neutrino spectra before neutrinos go through the Earth are assumed to be known. Then we show that making use of these dependences, we can obtain implication for the value of $Δm^{2}_{12}$ by comparing the observed energy spectrum to the predicted one. When SK detects neutrinos from supernova at 10kpc which traveled through the Earth (nadir angle $<$ 80 degree), $Δm^{2}_{12}$ can be determined with an accuracy of $\sim 10%$. In much of the neutrino-detection-time-$Δm^{2}_{12}$ plane, $Δm^{2}_{12}$ might be determined with an accuracy equal to or better than $\pm 0.5 \times 10^{-5} {\rm eV}^{2}$.

hep-ph↗

arXiv2002

Collisional Evolution of Galaxy Clusters and the Growth of Common Halos

We investigated the dynamical evolution of clusters of galaxies in virial equilibrium using Fokker-Planck models and self-consistent N-body models. In particular, we focused on the growth of a common halo, which is a cluster-wide halo formed by matter stripped from galaxies, and the development of a central density cusp. The Fokker-Planck models include the effects of two-body gravitational encounters both between galaxies and between galaxies and common halo particles. The effects of tidal mass stripping from the galaxies due to close galaxy-galaxy encounters and accompanying dissipation of the orbital kinetic energies of the galaxies were also taken into account in the Fokker-Planck models. We find that the results of the Fokker-Planck models are in excellent agreement with those of the N-body models regarding the growth of the common halo mass and the evolution of the cluster density profiles. In the central region of the cluster, a shallow density cusp, approximated by $ρ(r) \propto r^{-α}$ ($α\sim$ 1), develops. This shallow cusp results from the combined effects of two-body relaxation and tidal stripping. The cusp steepness, $α$, weakly depends on the relative importance of the tidal stripping. When the effect of stripping is important, the central velocity dispersion decreases as the central density increases and, consequently, a shallow ($α<2$) cusp is formed. In the limit of no stripping, usual gravothermal core collapse occurs, i.e. the central velocity dispersion increases as the central density increases with a steep ($α>2$) cusp left. We conclude from our consideration of the origin of the cusp demonstrated here that shallow cusps should develop in real galaxy clusters.

astro-ph↗