10,016 research outputs found
Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources
Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and
Galactic superluminal jet sources share the common spectral characteristic that
they have unusually high disk temperatures which cannot be explained in the
framework of the standard optically thick accretion disk in the Schwarzschild
metric. On the other hand, the standard accretion disk around the Kerr black
hole might explain the observed high disk temperature, as the inner radius of
the Kerr disk gets smaller and the disk temperature can be consequently higher.
However, we point out that the observable Kerr disk spectra becomes
significantly harder than Schwarzschild disk spectra only when the disk is
highly inclined. This is because the emission from the innermost part of the
accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen
for a face-on disk. The Galactic superluminal jet sources are known to be
highly inclined systems, thus their energy spectra may be explained with the
standard Kerr disk with known black hole masses. For ULXs, on the other hand,
the standard Kerr disk model seems implausible, since it is highly unlikely
that their accretion disks are preferentially inclined, and, if edge-on Kerr
disk model is applied, the black hole mass becomes unreasonably large (> 300
M_solar). Instead, the slim disk (advection dominated optically thick disk)
model is likely to explain the observed super-Eddington luminosities, hard
energy spectra, and spectral variations of ULXs. We suggest that ULXs are
accreting black holes with a few tens of solar mass, which is not unexpected
from the standard stellar evolution scenario, and that their X-ray emission is
from the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte
Discovery of Spectral Transitions from Two Ultra-Luminous Compact X-Ray Sources in Ic342
Two {\it ASCA} observations were made of two ultra-luminous compact X-ray
sources (ULXs), Source 1 and Source 2, in the spiral galaxy IC 342. In the 1993
observation, Source 2 showed a 0.5--10 keV luminosity of
ergs s (assuming a distance of 4.0 Mpc), and a hard power-law spectrum
of photon index . As already reported, Source 1 was times
brighter on that occasion, and exhibited a soft spectrum represented by a
multi-color disk model of inner-disk temperature keV. The second
observation made in February 2000 revealed that Source 1 had made a transition
into a hard spectral state, while Source 2 into a soft spectral state. The ULXs
are therefore inferred to exhibit two distinct spectral states, and sometimes
make transitions between them. These results significantly reinforce the
scenario which describes ULXs as mass-accreting black holes.Comment: 11 pages, 3 figures; acceoted for ApJ
Gauged Nambu-Jona-Lasinio model with extra dimensions
We investigate phase structure of the D (> 4)-dimensional gauged
Nambu-Jona-Lasinio (NJL) model with extra dimensions
compactified on TeV scale, based on the improved ladder Schwinger-Dyson (SD)
equation in the bulk. We assume that the bulk running gauge coupling in the SD
equation for the SU(N_c) gauge theory with N_f massless flavors is given by the
truncated Kaluza-Klein effective theory and hence has a nontrivial ultraviolet
fixed point (UVFP). We find the critical line in the parameter space of two
couplings, the gauge coupling and the four-fermion coupling, which is similar
to that of the gauged NJL model with fixed (walking) gauge coupling in four
dimensions. It is shown that in the presence of such walking gauge interactions
the four-fermion interactions become ``nontrivial'' even in higher dimensions,
similarly to the four-dimensional gauged NJL model. Such a nontriviality holds
only in the restricted region of the critical line (``nontrivial window'') with
the gauge coupling larger than a non-vanishing value (``marginal triviality
(MT)'' point), in contrast to the four-dimensional case where such a
nontriviality holds for all regions of the critical line except for the pure
NJL point. In the nontrivial window the renormalized effective potential yields
a nontrivial interaction which is conformal invariant. The exisitence of the
nontrivial window implies ``cutoff insensitivity'' of the physics prediction in
spite of the ultraviolet dominance of the dynamics. In the formal limit D -> 4,
the nontrivial window coincides with the known condition of the nontriviality
of the four-dimensional gauged NJL model, .Comment: 34 pages, 6 figures, references added, to appear in Phys.Rev.D. The
title is changed in PR
The Three Spectral Regimes Found in the Stellar Black Hole XTE J1550--564 in its High/Soft State
The present paper describes the analysis of multiple RXTE/PCA data of the
black hole binary with superluminal jet, XTE J1550-564, acquired during its
1999--2000 outburst. The X-ray spectra show features typical of the high/soft
spectral state, and can approximately be described by an optically thick disk
spectrum plus a power-law tail. Three distinct spectral regimes, named standard
regime, anomalous regime, and apparently standard regime, have been found from
the entire set of the observed spectra. When the X-ray luminosity is well below
6E38 erg/s (assuming a distance of 5 kpc), XTE J1550-564 resides in the
standard regime, where the soft spectral component dominates the power-law
component and the observed disk inner radius is kept constant. When the
luminosity exceeds the critical luminosity, the apparently standard regime is
realized, where luminosity of the optically thick disk rises less steeply with
the temperature, and the spectral shape is moderately distorted from that of
the standard accretion disk. In this regime, radial temperature gradient of the
disk has been found to be flatter than that of the standard accretion disk. The
results of the apparently standard regime are suggestive of a slim disk (e.g.,
Abramowicz et al. 1988, Watarai et al. 2000) which is a solution predicted
under high mass accretion rate. In the intermediate anomalous regime, the
spectrum becomes much harder, and the disk inner radius derived using a simple
disk model spectrum apparently varies significantly with time. These properties
can be explained as a result of significant thermal inverse Comptonization of
the disk photons, as was found from GRO J1655-40 in its anomalous regime by
Kubota, Makishima and Ebisawa (2001).Comment: 29 pages, 10 figures; Resubmitted to ApJ by taking referee's comments
into consideratio
Investigation of heaterless hollow cathode breakdown
The development of long life high powered (>50A) hollow cathodes is of importance to meet the demand of increasingly powerful Gridded Ion engines and Hall Effect thrusters. High power cathodes typically operate at greater temperature ranges, which poses a significant challenge to maintain heater reliability. The heater component commonly used to raise the insert to emissive temperatures, has inherent reliability issues from thermal fatigue caused by thermal cycling with large temperature variations. A self-heating hollow cathode allows for potentially higher reliability through design simplicity of removing the heater component, and in addition there can be savings in mass, volume, ignition time and power. This study characterizes the initiation of the start-up process for a heaterless hollow cathode. As such the study analyses conditions of the initiation as a function of detailed geometrical and physical parameters. The Paschen curve can be seen to give a qualitative explanation for the breakdown voltage variance. The quantitative variations between the empirical results and Paschen curve are discussed in relation to non-uniform pressure simulations
Observational Evidence for Strong Disk Comptonization in GRO J1655-40
Analysis was made of the multiple XTE/PCA data on the promised black hole
candidate with superluminal jet, GRO J1655-40, acquired during its 1996--1997
outburst. The X-ray spectra can be adequately described by the sum of an
optically thick disk spectrum and a power-law. When the estimated 1--100 keV
power-law luminosity exceeds 1E37 erg/s (assuming a distance of 3.2 kpc), the
inner disk radius and the maximum color temperature derived from a simple
accretion disk model (a multi-color disk model) vary significantly with time.
These results reconfirm the previous report by Sobczak et al. (1999). In this
strong power-law state (once called ``very high state''), the disk luminosity
decreases with temperature, in contradiction to the prediction of the standard
Shakura-Sunyaev model. In the same state, the intensity of the power-law
component correlates negatively with that of the disk component, and positively
with the power-law photon index, suggesting that the strong power-law is simply
the missing optically thick disk emission. One possible explanation for this
behavior is inverse-Compton scattering in the disk. By re-fitting the same data
incorporating a disk Comptonization, the inner radius and temperature of the
underlying disk are found to become more constant. These results provide one of
the first observational confirmations of the scenario of disk Comptonization in
the strong power-law state. This strong power-law state seems to appear when
color temperature of the disk exceeds the certain threshold, 1.2 -- 1.3 keV.Comment: 10 pages 4 figures; submitted to ApJ
Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)
Comprehensive neutron-powder diffraction and Rietveld analyses were carried
out to clarify the relation between the crystal and magnetic structures of
La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6
octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the
equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic
easy-axis at low temperature is parallel to the c axis. As the JT distortion
becomes suppressed with increasing x, a planar ferromagnetic structure appears
at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure
at x =< 0.39. The canting angle between neighboring planes continuously
increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type
AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with
decrease of JT distortion can be ascribed to the change in the eg orbital state
from d3z^2-r^2 to dx^2-y^2
Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom
Recently, an intriguing doping dependence of the exchange energies in the
bilayer manganites has been observed in the neutron
scattering experiments. The intra-layer exchange only weakly changed with
doping while the inter-layer one drastically decreased. Here we propose a
theory which accounts for these experimental findings. We argue, that the
observed striking doping dependence of the exchange energies can be attributed
to the evaluation of the orbital level splitting with doping. The latter is
handled by the interplay between Jahn-Teller effect (supporting an axial
orbital) and the orbital anisotropy of the electronic band in the bilayer
structure (promoting an in-plane orbital), which is monitored by the Coulomb
repulsion. The presented theory, while being a mean-field type, describes well
the experimental data and also gives the estimates of the several interesting
energy scales involved in the problem.Comment: Added references, corrected typos. To appear in Phys. Rev.
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