26,720 research outputs found
A Theoretical Light-Curve Model for the 1999 Outburst of U Scorpii
A theoretical light curve for the 1999 outburst of U Scorpii is presented in
order to obtain various physical parameters of the recurrent nova. Our U Sco
model consists of a very massive white dwarf (WD) with an accretion disk and a
lobe-filling, slightly evolved, main-sequence star (MS). The model includes a
reflection effect by the companion and the accretion disk together with a
shadowing effect on the companion by the accretion disk. The early visual light
curve (t ~ 1-15 days after maximum) is well reproduced by a thermonuclear
runaway model on a very massive WD close to the Chandrasekhar limit (M_{WD}=
1.37 \pm 0.01 M_\odot), in which optically thick winds blowing from the WD play
a key role in determining the nova duration. The duration of the strong wind
phase (t~0-17 days) is very consistent with the BeppoSAX supersoft X-ray
detection at t~19-20 days because supersoft X-rays are self-absorbed by the
massive wind. The envelope mass at the peak is estimated to be ~3x10^{-6}
M_\odot, which is indicating an average mass accretion rate ~2.5x10^{-7}
M_\odot yr^{-1} during the quiescent phase between 1987 and 1999. These
quantities are exactly the same as those predicted in a new progenitor model of
Type Ia supernovae.Comment: 7 pages, 3 figures, to appear in ApJL, vol. 52
Theoretical Sensitivity Analysis for Quantitative Operational Risk Management
We study the asymptotic behavior of the difference between the values at risk
VaR(L) and VaR(L+S) for heavy tailed random variables L and S for application
in sensitivity analysis of quantitative operational risk management within the
framework of the advanced measurement approach of Basel II (and III). Here L
describes the loss amount of the present risk profile and S describes the loss
amount caused by an additional loss factor. We obtain different types of
results according to the relative magnitudes of the thicknesses of the tails of
L and S. In particular, if the tail of S is sufficiently thinner than the tail
of L, then the difference between prior and posterior risk amounts VaR(L+S) -
VaR(L) is asymptotically equivalent to the expectation (expected loss) of S.Comment: 21 pages, 1 figure, 4 tables, forthcoming in International Journal of
Theoretical and Applied Finance (IJTAF
Ramification theory for varieties over a local field
We define generalizations of classical invariants of wild ramification for
coverings on a variety of arbitrary dimension over a local field. For an l-adic
sheaf, we define its Swan class as a 0-cycle class supported on the wild
ramification locus. We prove a formula of Riemann-Roch type for the Swan
conductor of cohomology together with its relative version, assuming that the
local field is of mixed characteristic.
We also prove the integrality of the Swan class for curves over a local field
as a generalization of the Hasse-Arf theorem. We derive a proof of a conjecture
of Serre on the Artin character for a group action with an isolated fixed point
on a regular local ring, assuming the dimension is 2.Comment: 159 pages, some corrections are mad
Epicyclic oscillations of non-slender fluid tori around Kerr black holes
Considering epicyclic oscillations of pressure-supported perfect fluid tori
orbiting Kerr black holes we examine non-geodesic (pressure) effects on the
epicyclic modes properties. Using a perturbation method we derive fully general
relativistic formulas for eigenfunctions and eigenfrequencies of the radial and
vertical epicyclic modes of a slightly non-slender, constant specific angular
momentum torus up to second-order accuracy with respect to the torus thickness.
The behaviour of the axisymmetric and lowest-order () non-axisymmetric
epicyclic modes is investigated. For an arbitrary black hole spin we find that,
in comparison with the (axisymmetric) epicyclic frequencies of free test
particles, non-slender tori receive negative pressure corrections and exhibit
thus lower frequencies. Our findings are in qualitative agreement with the
results of a recent pseudo-Newtonian study of analogous problem defined within
the Paczy{\'n}ski-Wiita potential. Implications of our results on the
high-frequency QPO models dealing with epicyclic oscillations are addressed.Comment: 24 pages, 8 figure
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
Astrophysical jets: observations, numerical simulations, and laboratory experiments
This paper provides summaries of ten talks on astrophysical jets given at the HEDP/HEDLA-08 International Conference in St. Louis. The talks are topically divided into the areas of observation, numerical modeling, and laboratory experiment. One essential feature of jets, namely, their filamentary (i.e., collimated) nature, can be reproduced in both numerical models and laboratory experiments. Another essential feature of jets, their scalability, is evident from the large number of astrophysical situations where jets occur. This scalability is the reason why laboratory experiments simulating jets are possible and why the same theoretical models can be used for both observed astrophysical jets and laboratory simulations
In-the-Gap SU UMa-Type Dwarf Nova, Var73 Dra with a Supercycle of about 60 Days
An intensive photometric-observation campaign of the recently discovered SU
UMa-type dwarf nova, Var73 Dra was conducted from 2002 August to 2003 February.
We caught three superoutbursts in 2002 October, December and 2003 February. The
recurrence cycle of the superoutburst (supercycle) is indicated to be 60
d, the shortest among the values known so far in SU UMa stars and close to
those of ER UMa stars. The superhump periods measured during the first two
superoutbursts were 0.104885(93) d, and 0.10623(16) d, respectively. A
0.10424(3)-d periodicity was detected in quiescence. The change rate of the
superhump period during the second superoutburst was , which
is an order of magnitude larger than the largest value ever known. Outburst
activity has changed from a phase of frequent normal outbursts and infrequent
superoutbursts in 2001 to a phase of infrequent normal outbursts and frequent
superoutbursts in 2002. Our observations are negative to an idea that this star
is an related object to ER UMa stars in terms of the duty cycle of the
superoutburst and the recurrence cycle of the normal outburst. However, to
trace the superhump evolution throughout a superoutburst, and from quiescence
more effectively, may give a fruitful result on this matter.Comment: 9 pages, 8 figures, submitted to A&
Superconductivity in Boron under pressure - why are the measured T's so low?
Using the full potential linear muffin-tin orbitals (FP-LMTO) method we
examine the pressure-dependence of superconductivity in the two metallic phases
of Boron: bct and fcc. Linear response calculations are carried out to examine
the phonon frequencies and electron-phonon coupling for various lattice
parameters, and superconducting transition temperatures are obtained from the
Eliashberg equation. In both bct and fcc phases the superconducting transition
temperature T is found to decrease with increasing pressure, due to
stiffening of phonons with an accompanying decrease in electron-phonon
coupling. This is in contrast to a recent report, where T is found to
increase with pressure. Even more drastic is the difference between the
measured T, in the range 4-11 K, and the calculated values for both bct and
fcc phases, in the range 60-100 K. The calculation reveals that the transition
from the fcc to bct phase, as a result of increasing volume or decreasing
pressure, is caused by the softening of the X-point transverse phonons. This
phonon softening also causes large electron-phonon coupling for high volumes in
the fcc phase, resulting in coupling constants in excess of 2.5 and T
nearing 100 K. We discuss possible causes as to why the experiment might have
revealed T's much lower than what is suggested by the present study. The
main assertion of this paper is that the possibility of high T, in excess
of 50 K, in high pressure pure metallic phases of boron cannot be ruled out,
thus substantiating the need for further experimental investigations of the
superconducting properties of high pressure pure phases of boron.Comment: 16 pages, 8 figures, 1 Tabl
Non-relativistic Lee Model on two Dimensional Riemannian Manifolds
This work is a continuation of our previous work (JMP, Vol. 48, 12, pp.
122103-1-122103-20, 2007), where we constructed the non-relativistic Lee model
in three dimensional Riemannian manifolds. Here we renormalize the two
dimensional version by using the same methods and the results are shortly given
since the calculations are basically the same as in the three dimensional
model. We also show that the ground state energy is bounded from below due to
the upper bound of the heat kernel for compact and Cartan-Hadamard manifolds.
In contrast to the construction of the model and the proof of the lower bound
of the ground state energy, the mean field approximation to the two dimensional
model is not similar to the one in three dimensions and it requires a deeper
analysis, which is the main result of this paper.Comment: 18 pages, no figure
Global Existence and Uniqueness of Solutions to the Maxwell-Schr{\"o}dinger Equations
The time local and global well-posedness for the Maxwell-Schr{\"o}dinger
equations is considered in Sobolev spaces in three spatial dimensions. The
Strichartz estimates of Koch and Tzvetkov type are used for obtaining the
solutions in the Sobolev spaces of low regularities. One of the main results is
that the solutions exist time globally for large data.Comment: 30 pages. In the revised version, the following modification was
made. (1) A line for dedication was added in the first page. (2) Some lines
were added at the bottom in page 4 and the top in page 5 in the first section
to make the description accurate. (3) Some typographical errors were
corrected throughout the pape
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