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T. Takahashi

Publications and source records attributed to T. Takahashi.

At least 253 records · Page 14Linked to original sources
arXiv1999

Quasiparticles in the superconducting state of Bi2Sr2CaCu2O8

Recent improvements in momentum resolution by a factor of 32 lead to qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. With improved resolution, we now see the intrinsic lineshape, which indicates the presence of true quasiparticles at the Fermi momentum in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.

cond-mat.supr-con↗
arXiv1999

Variability Pattern and the Spectral Evolution of the BL Lacertae Object PKS 2155-304

The TeV blazar PKS 2155-304 was monitored with the X-ray satellite ASCA in 1994 May, as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a large flare, where the 2-10 keV flux changed by a factor of 2 on a time scale of 3 x 10^4 sec. During the flare, the increase in the hard X-ray flux clearly preceded that observed in the soft X-rays, the spectral evolution tracking a `clockwise loop' in the flux versus photon index plane. Ascribing the energy-dependent variability to differential synchrotron cooling of relativistic electrons, we estimate the magnetic field B in the emission region. We tested two different methods of comparing the time series in various X-ray bands: (i) fitting the light curves to a Gaussian and searching for the time shift of the peak of the flare, and (ii) calculating the discrete correlation function. Both methods yielded a consistent solution of B ~ 0.1 G. We also found that the flare amplitude becomes larger as the photon energy increases, while the duration of the flare stays roughly constant through the ASCA energy band (0.7-7.5 keV). In the framework of the time dependent synchrotron self-Compton model in a homogeneous region, we consider a flare where the maximum Lorentz factor (Gmax) of the injected electrons increases uniformly throughout the emission volume. The temporal evolution of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtain B ~ 0.1-0.2 G and a region size R ~ 10^-2 pc, for relativistic beaming with a Doppler factor of D ~ 20-30. We discuss the significance of light travel time effects.

astro-ph↗
arXiv1999

Electronic Spectra and Their Relation to the (pi,pi) Collective Mode in High-Tc Superconductors

Photoemission spectra of Bi2Sr2CaCu2O8 reveal that the high energy feature near (pi,0), the "hump", scales with the superconducting gap and persists above Tc in the pseudogap phase. As the doping decreases, the dispersion of the hump increasingly reflects the wavevector (pi,pi) characteristic of the undoped insulator, despite the presence of a large Fermi surface. This can be understood from the interaction of the electrons with a collective mode, supported by our observation that the doping dependence of the resonance observed by neutron scattering is the same as that inferred from our data.

cond-mat.supr-con↗
arXiv1999

Log N - Log S Relations and Spectral Properties of Sources from the ASCA Large Sky Survey --- their Implications for the Origin of the Cosmic X-ray Background (CXB)

We carried out the first wide-area unbiased survey with the ASCA satellite in the 0.7-10 keV band around a north Galactic-pole region covering a continuous area of 7 square degrees (Large Sky Survey; LSS). To make the best use of ASCA capability, we developed a new source-detection method where the complicated detector responses are fully taken into account. Applying this method to the entire LSS data independently in the total (0.7-7 keV), hard (2-10 keV), and soft (0.7-2 keV) band, we detected 107 sources altogether with sensitivity limits of 6 x 10E-14 (0.7-7 keV), 1 x 10E-13 (2-10 keV), and 2 x 10E-14 erg sE-1 cmE-2 (0.7-2 keV), respectively. A complete list of the detected sources is presented. Based on detailed studies by Monte Carlo simulations, we evaluated effects of the source confusion and accurately derived Log N - Log S relation in each survey band. The Log N - Log S relation in the hard band is located on the extrapolation from the GINGA and HEAO1 results with the Euclidean slope of -3/2, while that in the soft band is consistent with the results by ROSAT. At these flux limits, 30 (+/- 3) percent of the CXB in the 0.7-7 keV band and 23 (+/- 3) percent in the 2-10 keV band have been resolved into discrete sources. The average spectrum of faint sources detected in the total band shows a photon index of 1.63 +/- 0.07 in the 0.7-10 keV range, consistent with the comparison of source counts between the hard and the soft energy band. Those detected in the hard band show a photon index of 1.49 +/- 0.10 in the 2-10 keV range. These spectral properties suggest that contribution of sources with hard energy spectra become significant at a flux of 10E-13 erg sE-1 cmE-2 (2-10 keV). The most plausible candidates are type-II AGNs, as indicated by on-going optical identifications.

astro-ph↗
arXiv1999

Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping Dependent ARPES Study

Comparing ARPES measurements on Bi2212 with penetration depth data, we show that a description of the nodal excitations of the d-wave superconducting state in terms of non-interacting quasiparticles is inadequate, and we estimate the magnitude and doping dependence of the Landau interaction parameter which renormalizes the linear T contribution to the superfluid density. Furthermore, although consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple coskx-cosky form, which suggests an increasing importance of long range interactions as the insulator is approached.

cond-mat.supr-con↗
arXiv1998

Temperature-induced spectral weight transfer in Bi_2Sr_2CaCu_2O_{8+delta}: a conventional view

In contrast with the recent photoemission observations by Shen et al. of anomalous temperature-induced momentum-dependent spectral weight transfer in Bi_2Sr_2CaCu_2O_{8+delta}, we find that in the same material, in spite of the unusual spectral lineshape change between superconducting and normal states, the integrated spectral weight displays minimum variation as a function of temperature, which is in agreement with the sum rule of angle-resolved photoemission: the integrated spectral weight is proportional to the momentum density, n(k).

cond-mat.supr-con↗
arXiv1998

Nature of Hard X-ray Source from Optical Identification of the ASCA Large Sky Survey

We present results of optical identification of the ASCA Large Sky Survey. X-ray sources which have hard X-ray spectra were identified with type-2 AGN at redshifts smaller than 0.5. It is supported that the absorbed X-ray spectrum of type-2 AGN makes the Cosmic X-ray Background harder in the hard X-ray band than type-1 AGN, which is main contributer in the soft X-ray band. Absence of type-2 AGN at redshift larger than 1 in the identified sample, which contrasts to the existence of 6 broad-line QSOs, suggests a deficiency of so-called ``type-2 QSO'' at high redshift.

astro-ph↗
arXiv1998

A Study of High Energy Emission from the TeV blazar Mrk 501 during Multiwavelength Observations in 1996

We present the results of a multiwavelength campaign for Mrk 501 performed in March 1996 with ASCA, EGRET, Whipple, and optical telescopes. In the X-ray band, a spectral break was observed around 2 keV. We report here for the first time the detection of high-energy gamma-ray flux from Mrk 501 with EGRET with 3.5 sigma significance (E>100 MeV). Higher flux was also observed in April/May 1996, with 4.0 sigma significance for E>100 MeV, and 5.2 sigma significance for E>500 MeV. The gamma-ray spectrum was measured to be flatter than most of the gamma-ray blazars. We find that the multiband spectrum in 1996 is consistent with that calculated from a one-zone SSC model in a homogeneous region. In the context of this model, we investigate the values of the magnetic field strength and the beaming factor allowed by the observational results. We compare the March 1996 multiwavelength spectrum with that in the flare state in April 1997. Between these two epochs, the TeV flux increase is well correlated with that observed in keV range. The keV and TeV amplitudes during the April 1997 flare are accurately reproduced, assuming that the population of synchrotron photons in 1996 are scattered by the newly injected relativistic electrons, having maximum energies of G_max = 6.0e6. However, the TeV spectrum observed during March 1996 campaign is flatter than predicted by our models. We find that this cannot be explained by either higher order Comptonization or the contribution of the `seed' IR photons from the host galaxy for the first-order external radiation Comptonization.

astro-ph↗
arXiv1998

ASCA Observations of Blazars and Multiband Analysis

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. The observations show a clear difference in the spectra between three subclasses of blazars, namely the High-energy peaked BL Lac objects (HBLs), Low-energy peaked BL Lac objects (LBLs), and quasar-hosted blazars (QHBs). We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission, with the Doppler factor consistent with the VLBI and/or gamma-ray variability data. For many QHBs, on the other hand, the gamma-rays cannot be solely due to the SSC mechanism unless the Doppler factor is significantly in excess of that inferred from VLBI data. We consider an alternative scenario consistent with the measured values of the Doppler factor, where the SSC component is still present in QHBs and it dominates in the X-ray band, but it is below the observed gamma-ray spectrum. With an assumption that the peak of the SSC emission is on the extrapolation of the X-ray spectrum, and adopting the Doppler factor of 10, we infer the magnetic field to be 0.1 - 1 Gauss, and Lorentz factors of electrons radiating at the peak of the nu F(nu) spectrum of 10^3 for QHBs; this is much lower than 10^5 for HBLs. This difference is most likely due to the large photon density expected in QHBs.

astro-ph↗
arXiv1998

Optical Identification of the Hardest X-ray Source in the ASCA Large Sky Survey

We report the optical identification of the hardest X-ray source (AX J131501+3141) detected in an unbiased wide-area survey in the 0.5--10 keV band, the ASCA Large Sky Survey. The X-ray spectrum of the source is very hard and is well reproduced by a power-law component (Gamma = 1.5^+0.7_-0.6) with N_H = 6^+4_-2 *10^22 cm^-2 (Sakano et al. 1998). We have found a galaxy with R=15.62 mag near the center of the error circle for the X-ray source. The optical spectrum of the galaxy shows only narrow emission lines whose ratios correspond to those of a type 2 Seyfert galaxy at z = 0.072, implying an absorption-corrected X-ray luminosity of 2*10^43 erg sec^-1 (2--10 keV) and M_B = -20.93 mag. A radio point source is also associated with the center of the galaxy. We thus identify the X-ray source with this galaxy as an obscured AGN. The hidden nature of the nucleus of the galaxy in the optical band is consistent with the X-ray spectrum. These results support the idea that the obscured AGNs/QSOs contribute significantly to the cosmic X-ray background in the hard band at the faint flux level.

astro-ph↗
arXiv1997

Destruction of the Fermi Surface in Underdoped High Tc Superconductors

The Fermi surface is a central concept in the theory of metals. Even though the optimally doped high temperature superconductors exhibit an anomalous normal state, angle resolved photoemission spectroscopy (ARPES) has revealed a large Fermi surface despite the absence of well-defined quasiparticles and the necessity of working at finite temperatures. However, the even more unusual behavior in the underdoped high temperature superconductors, which show a pseudogap above Tc, requires us to carefully re-examine this concept. Here, we present the first results on how the Fermi surface is destroyed as a function of temperature in underdoped Bi2212 using ARPES. We find the remarkable effect that different k points become gapped at different temperatures. This leads to a break up of the Fermi surface at a temperature T* into disconnected Fermi arcs which shrink with decreasing T, eventually collapsing to the point nodes of the dx2-y2 superconducting ground state below Tc. This novel behavior, where the Fermi surface does not form a continuous contour in momentum space, is unprecedented in that it occurs in the absence of long range order. Moreover, although the d-wave superconducting gap below Tc smoothly evolves into the pseudogap above Tc, the gaps at different k points are not related to one another above Tc the same way as they are below, implying an intimate, but non-trivial relation, between the two.

cond-mat.supr-con↗
arXiv1997

ARPES study of the superconducting gap and pseudogap in Bi2Sr2CaCu2O8+x

In this paper, we review some of our ARPES results on the superconducting and pseudo gaps in Bi2Sr2CaCu2O8+x. We find that optimally and overdoped samples exhibit a d-wave gap, which closes at the same temperature, Tc, for all k points. In underdoped samples, a leading edge gap is found up to a temperature T* > Tc. We find that T* scales with the maximum low temperature gap, increasing as the doping is reduced. The momentum dependence of the pseudogap is similar to that of the superconducting gap; however, the pseudogap closes at different temperatures for different k points.

cond-mat.supr-con↗
arXiv1997

Simple Photoelectric Absorption during Dipping in the ASCA Observation of XB1916-053

We report results of analysis of the ASCA observation of the Low Mass X-ray Binary dipping source XB1916-053 made on 1993, May 2nd. We have shown that the spectra can be well fitted by the same two-component model previously used to give good explanations of the very different dip sources X1755-338 and X1624-490, consisting of point-source blackbody emission from the neutron star plus extended Comptonised emission from the accretion disk corona. In XB1916-053, all levels of dipping can be fitted using kT_bb= 2.14+/-0.28 keV and power law photon index = 2.42 +/-0.21. Dipping is shown to be due to large increases of column density for the point-like blackbody, combined with the extended power law component being progressively covered by absorber until in the deepest parts of dips, the partial covering fraction approaches unity. This approach differs radically from the ``absorbed plus unabsorbed'' approach previously used in spectral modelling of XB1916-053 and similar sources, in which the normalisation of the unabsorbed component is allowed to decrease markedly in dipping, behavior generally attributed to the effects of electron scattering. Thus we have shown that spectral evolution in XB1916-053 can be explained simply in terms of photoelectric absorption without the need for substantial electron scattering. This explanation is supported by calculation of the relative importance of photoelectric absorption and electron scattering in the absorbing region which shows that little electron scattering is expected in the ASCA energy band.

astro-ph↗
arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. III. Probing the Inner Jet through Multiwavelength Correlations

In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10 d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with ground- based radio, IR, and optical telescopes. The light curves show a well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week there was at least one X-ray flare of comparable amplitude or perhaps ongoing stochastic X-ray variations. An extremely rapid change in UV flux was seen at the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous relativistic jet. We can rule out a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible if there was an instantaneous (t<<hours) injection of high energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions (e.g., behind a shock front and/or in an inhomogeneous jet). The present light curves are in sharp contrast to November 1991, when the amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hours. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes. (abbreviated abstract)

astro-ph↗
arXiv1997

Simulations of the Interaction Region in a Photon-Photon Collider

The status and initial performance of a simulation program CAIN for interaction region of linear colliders is described. The program is developed to be applicable for e+e-, e-e-, e-gamma and gamma-gamma linear colliders. As an example of an application, simulation of a gamma-gamma collider option of NLC is reported.

physics.acc-ph↗
arXiv1997

Measuring the two-photon decay width of Intermediate-mass Higgs at a photon-photon collider

Feasibility of a measurement of the partial decay width of the intermediate-mass Higgs boson into two photons at a photon-photon collider is studied by a simulation. The QCD radiative correction for quark pair background processes is taken into account for the realistic background estimation. It is found that the two-photon decay width can be measured with the statistical error of 7.6 % with about one year of experiment. The impact of the measurement of the two-photon decay width to look for the new physics beyond is demonstrated.

hep-ph↗
arXiv1997

The Cessation of Flickering during Dips in Cygnus X-1

We report the discovery of the cessation of flickering in dips in the black hole candidate Cygnus X-1, detected for the first time in the ASCA observation of May 9th., 1995. During this observation, particularly deep dipping took place resulting in strong changes in hardness ratio corresponding to absorption of the power law spectral component. The deadtime corrected light curve with high time resolution clearly shows a dramatic decrease in the extent of flickering in the band 0.7 - 4.0 keV during dipping, but in the band 4.0 - 10.0 keV, there is relatively little change. We show that the rms flickering amplitude in the band 0.7 - 4.0 keV is proportional to the X-ray intensity in this band which changes by a factor of almost three. This is direct evidence that the strong Low State flickering is intrinsic to the power law emission; ie takes place as part of the emission process. The rms amplitude is proportional to the intensity in the low energy band, except for a possible deviation from linearity at the lower intensities. If confirmed, this non-linearity could imply a process such as electron scattering of radiation which will tend to smear out the fluctuations, or a process of fluctuation generation which depends on radial position in the source. Thus timing observations during absorption dips can give information about the source region and may place constraints on its size.

astro-ph↗