14,284 research outputs found
Two-dimensional radiation-hydrodynamic model for limit-cycle oscillations of luminous accretion disks
We investigate the time evolution of luminous accretion disks around black
holes, conducting the two-dimensional radiation-hydrodynamic simulations. We
adopt the alpha prescription for the viscosity. The radial-azimuthal component
of viscous stress tensor is assumed to be proportional to the total pressure in
the optically thick region, while the gas pressure in the optically thin
regime. The viscosity parameter, alpha, is taken to be 0.1. We find the
limit-cycle variation in luminosity between high and low states. When we set
the mass input rate from the outer disk boundary to be 100 L_E/c^2, the
luminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddington
luminosity. It decays after retaining high value for about 40 s. Our numerical
results can explain the variation amplitude and duration of the recurrent
outbursts observed in microquasar, GRS 1915+105. We show that the
multi-dimensional effects play an important role in the high-luminosity state.
In this state, the outflow is driven by the strong radiation force, and some
part of radiation energy dissipated inside the disk is swallowed by the black
hole due to the photon-trapping effects. This trapped luminosity is comparable
to the disk luminosity. We also calculate two more cases: one with a much
larger accretion rate than the critical value for the instability and the other
with the viscous stress tensor being proportional to the gas pressure only even
when the radiation pressure is dominant. We find no quasi-periodic light
variations in these cases. This confirms that the limit-cycle behavior found in
the simulations is caused by the disk instability.Comment: 6 pages, 4 figures, accepted for publication in ApJ (ApJ 01 April
2006, v640, 2 issue
Unbounded-error One-way Classical and Quantum Communication Complexity
This paper studies the gap between quantum one-way communication complexity
and its classical counterpart , under the {\em unbounded-error}
setting, i.e., it is enough that the success probability is strictly greater
than 1/2. It is proved that for {\em any} (total or partial) Boolean function
, , i.e., the former is always exactly one half
as large as the latter. The result has an application to obtaining (again an
exact) bound for the existence of -QRAC which is the -qubit random
access coding that can recover any one of original bits with success
probability . We can prove that -QRAC exists if and only if
. Previously, only the construction of QRAC using one qubit,
the existence of -RAC, and the non-existence of
-QRAC were known.Comment: 9 pages. To appear in Proc. ICALP 200
Chandra X-ray Observatory Arcsecond Imaging of the Young, Oxygen-rich Supernova Remnant 1E0102.2-7219
We present observations of the young, Oxygen-rich supernova remnant
1E0102.2-7219 taken by the Chandra X-ray Observatory during Chandra's Orbital
Activation and Checkout phase. The boundary of the blast wave shock is clearly
seen for the first time, allowing the diameter of the remnant and the mean
blast wave velocity to be determined accurately. The prominent X-ray bright
ring of material may be the result of the reverse shock encountering ejecta;
the radial variation of O VII vs. O VIII emission indicates an ionizing shock
propagating inwards, possibly through a strong density gradient in the ejecta.
We compare the X-ray emission to Australia Telescope Compact Array 6 cm radio
observations (Amy and Ball) and to archival Hubble Space Telescope [O III]
observations. The ring of radio emission is predominantly inward of the outer
blast wave, consistent with an interpretation as synchrotron radiation
originating behind the blast wave, but outward of the bright X-ray ring of
emission. Many (but not all) of the prominent optical filaments are seen to
correspond to X-ray bright regions. We obtain an upper limit of ~9e33 erg/s (3
sigma) on any potential pulsar X-ray emission from the central region.Comment: Accepted for pulication in Ap. J. Letters. 4 pages, 6 figures (one
color figure). Formatted with emulateapj5. Revised to incorporate copyediting
changes. High-resolution postscript (3.02MB) and tiff versions of the color
figure are available from
http://chandra.harvard.edu/photo/cycle1/0015multi/index.htm
Entanglement measures and approximate quantum error correction
It is shown that, if the loss of entanglement along a quantum channel is
sufficiently small, then approximate quantum error correction is possible,
thereby generalizing what happens for coherent information. Explicit bounds are
obtained for the entanglement of formation and the distillable entanglement,
and their validity naturally extends to other bipartite entanglement measures
in between. Robustness of derived criteria is analyzed and their tightness
compared. Finally, as a byproduct, we prove a bound quantifying how large the
gap between entanglement of formation and distillable entanglement can be for
any given finite dimensional bipartite system, thus providing a sufficient
condition for distillability in terms of entanglement of formation.Comment: 7 pages, two-columned revtex4, no figures. v1: Deeply revised and
extended version: different entanglement measures are separately considered,
references are added, and some remarks are stressed. v2: Added a sufficient
condition for distillability in terms of entanglement of formation; published
versio
Ds0*(2317) and Ds1(2460) mesons in two-body B-meson decays
We analyze the branching ratios of B to D(*) + Ds0*(Ds1) decays using the
factorization hypothesis. The B to D(*) transition form factors are taken from
a model-independent analysis done by Caprini, Lellouch and Neubert based on
heavy quark spin symmetry and dispersive constraints, including short-distance
and power corrections. The leptonic decay constants fDs0* and fDs1 are
calculated assuming a molecular structure for the Ds0* and Ds1 mesons. The
calculated branching ratios of B-meson two-body decays are compared with
experimental data and other theoretical results.Comment: 12 pages, 2 figure
Transient and Highly Polarized Double-Peaked H-alpha Emission in the Seyfert 2 Nucleus of NGC 2110
We have discovered an extremely broad, double-peaked H-alpha emission line in
the polarized flux spectrum of NGC 2110, establishing that this well-studied
Seyfert 2 galaxy contains a disk-like hidden broad-line region (BLR). Several
properties of NGC 2110 suggest that it is an obscured twin of Arp 102B, the
prototypical double-peaked emission-line active galactic nucleus (AGN). A
comparison between our data and previous spectra of NGC 2110 indicates that the
double-peaked H-alpha feature is transient. The presence of a disk-like BLR in
NGC 2110 has important implications for AGNs: it expands the range of
properties exhibited by Seyfert 2 galaxies, and the fact that the BLR is
obscured by a torus-like structure provides the first evidence that
double-peaked emitters and classical Seyfert nuclei may have the same basic
parsec-scale geometry.Comment: 5 pages, including 3 postscript figures. Uses emulateapj. Accepted
for publication in ApJ Letter
Conformal Couplings in Induced Gravity
It is found that the induced gravity with conformal couplings requires the
conformal invariance in both classical and quantum levels for consistency. This
is also true for the induced gravity with an extended conformal coupling
interacting with torsion.Comment: 10 pages, Revtex3.0, to appear in General Relativity and Gravitatio
Broad Band X-Ray Observations of the Narrow Line X-Ray Galaxy NGC 5506
We present a detailed analysis of broad band X-ray data of the Seyfert 2
galaxy NGC5506. 2-10 keV band are detected during a 1-day ASCA observation,
while no significant change in the 2-10 keV continuum shape is found. The ASCA
spectrum consists of an absorbed power-law, a 'soft excess' below 2 keV, and an
Fe K emission line at 6.4 keV. The 'soft excess' can be well described
by either thermal emission from very low abundance material at a temperature
kT0.8 keV, or scattered/leaking flux from the primary power-law plus a
small amount of thermal emission. Analysis of ROSAT HRI data reveals that the
soft X-ray emission is extended on kpc scales in this object, and the extended
component may account for most of the soft X-ray excess observed by the ASCA.
The result suggests that in this type 2 AGN, the 'soft excess' at least partly
comes from an extended region, imposing serious problem for the model in which
the source is partially covered. Fe K profile is complex and can not be
satisfactorily modeled by a single gaussian. Models of either double gaussians,
or a narrow gaussian plus a line from a relativistic accretion disk viewed at
an inclination of about 40 provide good fits to the data. However,
the inclination of the disk can be substantially larger if there is a small
amount of excessive Fe K edge absorption. The intermediate inclinations for
NLXGs are consistent with the ideas that the inner accretion disk is aligned
with the outer obscuring torus.Comment: 8 pages, 5 postscript figures. to appear in Astrophy. J., 1999, April
2
Iridium-Catalyzed Enantioselective Hydrogenation of Vinylsilanes
We have screened a diverse array of iridium complexes derived from chiral N,P ligands as catalysts for the asymmetric hydrogenation of vinylsilanes, a transformation for which generally applicable catalysts were lacking. Several catalysts emerged from this study that enabled the highly enantioselective hydrogenation of a wide range of vinylsilanes with trisubstituted or disubstituted terminal C=C bonds bearing aryl, alkyl, ethoxycarbonyl, or hydroxymethyl substituents. In addition to trimethylsilyl and dimethyl(phenyl) silyl derivatives, trialkoxysilyl- and silacyclobutyl-substituted alkenes were used as substrates
GUT theories from Calabi-Yau 4-folds with SO(10) Singularities
We consider an SO(10) GUT model from F-theory compactified on an elliptically
fibered Calabi-Yau with a D5 singularity. To obtain the matter curves and the
Yukawa couplings, we use a global description to resolve the singularity. We
identify the vector and spinor matter representations and their Yukawa
couplings and we explicitly build the G-fluxes in the global model and check
the agreement with the semi-local results. As our bundle is of type SU(2k),
some extra conditions need to be applied to match the fluxes.Comment: 27 page
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