43,797 research outputs found
Toward a unified light curve model for multi-wavelength observations of V1974 Cygni (Nova Cygni 1992)
We present a unified model for optical, ultraviolet (UV), and X-ray light
curves of V1974 Cygni (Nova Cygni 1992). Based on an optically thick wind model
of nova outbursts, we have calculated light curves and searched for the best
fit model that is consistent with optical, UV, and X-ray observations. Our best
fit model is a white dwarf (WD) of mass 1.05 M_\sun with a chemical composition
of X=0.46, C+N+O=0.15, and Ne = 0.05 by mass weight. Both supersoft X-ray and
continuum UV 1455 \AA light curves are well reproduced. Supersoft X-rays
emerged on day ~ 250 after outburst, which is naturally explained by our model:
our optically thick winds cease on day 245 and supersoft X-rays emerge from
self-absorption by the winds. The X-ray flux keeps a constant peak value for ~
300 days followed by a quick decay on day ~ 600. The duration of X-ray flat
peak is well reproduced by a steady hydrogen shell burning on the WD. Optical
light curve is also explained by the same model if we introduce free-free
emission from optically thin ejecta. A t^{-1.5} slope of the observed optical
and infrared fluxes is very close to the slope of our modeled free-free light
curve during the optically thick wind phase. Once the wind stops, optical and
infrared fluxes should follow a t^{-3} slope, derived from a constant mass of
expanding ejecta. An abrupt transition from a t^{-1.5} slope to a t^{-3} slope
at day ~ 200 is naturally explained by the change from the wind phase to the
post-wind phase on day ~ 200. The development of hard X-ray flux is also
reasonably understood as shock-origin between the wind and the companion star.
The distance to V1974 Cyg is estimated to be ~ 1.7 kpc with E(B-V)= 0.32 from
the light curve fitting for the continuum UV 1455 \AA.Comment: 8 pages, 4 figures, to appear in the Astrophysical Journa
The X-ray Outburst of H1743-322: High-Frequency QPOs with a 3:2 Frequency Ratio
We observed the 2003 X-ray outburst of H1743-322 in a series of 130 pointed
observation with RXTE. We searched individual observations for high-frequency
QPOs (HFQPOs) and found only weak or marginal detections near 240 and 160 Hz.
We next grouped the observations in several different ways and computed the
average power-density spectra (PDS) in a search for further evidence of HFQPOs.
This effort yielded two significant results for those observations defined by
the presence of low-frequency QPOs (0.1-20 Hz) and an absence of
``band-limited'' power continua: (1) The 9 time intervals with the highest 7-35
keV count rates yielded an average PDS with a QPO at Hz. (; 3--35 keV); and (2) a second group with lower 7-35 keV count rates (26
intervals) produced an average PDS with a QPO at Hz (;
7--35 keV). The ratio of these two frequencies is . This finding
is consistent with results obtained for three other black hole systems that
exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of
H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles
the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms
of a resonance model that invokes frequencies set by general relativity for
orbital motions near a black-hole event horizon.Comment: 12 pages, 3 figures, submitted to Ap
Theoretical and Numerical Analysis of an Optimal Execution Problem with Uncertain Market Impact
This paper is a continuation of Ishitani and Kato (2015), in which we derived
a continuous-time value function corresponding to an optimal execution problem
with uncertain market impact as the limit of a discrete-time value function.
Here, we investigate some properties of the derived value function. In
particular, we show that the function is continuous and has the semigroup
property, which is strongly related to the Hamilton-Jacobi-Bellman
quasi-variational inequality. Moreover, we show that noise in market impact
causes risk-neutral assessment to underestimate the impact cost. We also study
typical examples under a log-linear/quadratic market impact function with
Gamma-distributed noise.Comment: 24 pages, 14 figures. Continuation of the paper arXiv:1301.648
Thermal Instability of Advection-Dominated Disks against Local Perturbations
Thermal instability is examined for advection-dominated one-temperature
accretion disks. We consider axisymmetric perturbations with short wavelength
in the radial direction. The viscosity is assumed to be sufficiently small for
the vertical hydrostatic balance to hold in perturbed states. The type of
viscosity is given either by the -viscosity or by a diffusion-type
stress tensor. Optically thick disks are found to be in general more unstable
than optically thin ones. When the thermal diffusion is present, the optically
thin disks become stable, but the optically thick disks are still unstable. The
instability of the advection-dominated disks is different from that of the
geometrically thin disks without advection. In the case of no advection, the
thermal mode behaves under no appreciable surface density change. In the case
of advection-dominated disks, however, the thermal mode occurs with no
appreciable pressure change (compared with the density change), when local
perturbations are considered. The variations of angular momentum and of surface
density associated with the perturbations lead to a thermal instability. The
astrophysical implications of this instability are briefly discussed.Comment: PASJ, in press 1996, Feb; uuencoded, compressed, PS file; also
available on WWW at http://fy.chalmers.se/~chen
From quantum circuits to adiabatic algorithms
This paper explores several aspects of the adiabatic quantum computation
model. We first show a way that directly maps any arbitrary circuit in the
standard quantum computing model to an adiabatic algorithm of the same depth.
Specifically, we look for a smooth time-dependent Hamiltonian whose unique
ground state slowly changes from the initial state of the circuit to its final
state. Since this construction requires in general an n-local Hamiltonian, we
will study whether approximation is possible using previous results on ground
state entanglement and perturbation theory. Finally we will point out how the
adiabatic model can be relaxed in various ways to allow for 2-local partially
adiabatic algorithms as well as 2-local holonomic quantum algorithms.Comment: Version accepted by and to appear in Phys. Rev.
Relativistic Diskoseismology. I. Analytical Results for 'Gravity Modes'
We generalize previous calculations to a fully relativistic treatment of
adiabatic oscillations which are trapped in the inner regions of accretion
disks by non-Newtonian gravitational effects of a black hole. We employ the
Kerr geometry within the scalar potential formalism of Ipser and Lindblom,
neglecting the gravitational field of the disk. This approach treats
perturbations of arbitrary stationary, axisymmetric, perfect fluid models. It
is applied here to thin accretion disks. Approximate analytic eigenfunctions
and eigenfrequencies are obtained for the most robust and observable class of
modes, which corresponds roughly to the gravity (internal) oscillations of
stars. The dependence of the oscillation frequencies on the mass and angular
momentum of the black hole is exhibited. These trapped modes do not exist in
Newtonian gravity, and thus provide a signature and probe of the strong-field
structure of black holes. Our predictions are relevant to observations which
could detect modulation of the X-ray luminosity from stellar mass black holes
in our galaxy and the UV and optical luminosity from supermassive black holes
in active galactic nuclei.Comment: 31 pages, 6 figures, uses style file aaspp4.sty, prepared with the
AAS LATEX macros v4.0, significant revision of earlier submission to include
modes with axial index m>
High efficiency dark-to-bright exciton conversion in carbon nanotubes
We report that dark excitons can have a large contribution to the emission
intensity in carbon nanotubes due to an efficient exciton conversion from a
dark state to a bright state. Time-resolved photoluminescence measurements are
used to investigate decay dynamics and diffusion properties of excitons, and we
obtain intrinsic lifetimes and diffusion lengths of bright excitons as well as
diffusion coefficients for both bright and dark excitons. We find that the
dark-to-bright transition rates can be considerably high, and that more than
half of the dark excitons can be transformed into the bright excitons. The
state transition rates have a large chirality dependence with a family pattern,
and the conversion efficiency is found to be significantly enhanced by adsorbed
air molecules on the surface of the nanotubes. Our findings show the nontrivial
significance of the dark excitons on the emission kinetics in low dimensional
materials, and demonstrate the potential for engineering the dark-to-bright
conversion process by using surface interactions.Comment: 7 pages, 4 figure
Single carbon nanotubes as ultrasmall all-optical memories
Performance improvements are expected from integration of photonic devices
into information processing systems, and in particular, all-optical memories
provide a key functionality. Scaling down the size of memory elements is
desirable for high-density integration, and the use of nanomaterials would
allow for devices that are significantly smaller than the operation
wavelengths. Here we report on all-optical memory based on individual carbon
nanotubes, where adsorbed molecules give rise to optical bistability. By
exciting at the high-energy tail of the excitonic absorption resonance,
nanotubes can be switched between the desorbed state and the adsorbed state. We
demonstrate reversible and reproducible operation of the nanotube optical
memory, and determine the rewriting speed by measuring the molecular adsorption
and desorption times. Our results underscore the impact of molecular-scale
effects on optical properties of nanomaterials, offering new design strategies
for photonic devices that are a few orders of magnitude smaller than the
optical diffraction limit.Comment: 8 pages, 6 figure
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