26,846 research outputs found
Time variability of accretion flows: effects of the adiabatic index and gas temperature
We report on next phase of our study of rotating accretion flows onto black
holes. We consider hydrodynamical (HD) accretion flows with a spherically
symmetric density distribution at the outer boundary but with spherical
symmetry broken by the introduction of a small, latitude-dependent angular
momentum. We study accretion flows by means of numerical two-dimensional,
axisymmetric, HD simulations for variety of the adiabatic index, and
the gas temperature at infinity, . Our work is an extension of work
done by Proga & Begelman who consider models for only . Our main
result is that the flow properties such as the topology of the sonic surface
and time behavior strongly depend on but little on . In
particular, for , the mass accretion rate shows large
amplitude, slow time-variability which is a result of mixing between slow and
fast rotating gas. This temporal behavior differs significantly from that in
models with \gamma\simless 5/3 where the accretion rate is relatively
constant and from that in models with \gamma\simgreat 1 where the accretion
exhibits small amplitude quasi-periodic oscillations. The key parameter
responsible for the differences is the sound speed of the accretion flow which
in turn determines whether the flow is dominated by gas pressure, radiation
pressure or rotation. Despite these differences the time-averaged mass
accretion rate in units of the corresponding Bondi rate is a weak function of
and .Comment: 31 pages, 14 figures, accepted for publication in ApJ, for full
resolution version goto http://users.camk.edu.pl/mmosc/ms.pd
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
Recurrent Nova IM Normae
We detected the second historical outburst of the 1920 nova IM Nor. Accurate
astrometry of the outbursting object revealed the true quiescent counterpart
having a magnitude of R=17.0 mag and B=18.0 mag. We show that the quiescent
counterpart shows a noticeable variation. From the comparison of light curves
and spectroscopic signatures, we propose that IM Nor and CI Aql comprise a new
class of recurrent novae bearing some characteristics similar to those of
classical novae. We interpret that the noticeable quiescent variation can be a
result of either high orbital inclination, which may be also responsible for
the low quiescent brightness, or the presence of high/low states. If the second
possibility is confirmed by future observations, IM Nor becomes the first
recurrent nova showing state changes in quiescence. Such state changes may
provide a missing link between recurrent novae and supersoft X-ray sources.Comment: 4 pages, 3 figures, submitted to Astronomy and Astrophysics Letter
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
Derivation of Green's Function of Spin Calogero-Sutherland Model by Uglov's Method
Hole propagator of spin 1/2 Calogero-Sutherland model is derived using
Uglov's method, which maps the exact eigenfunctions of the model, called
Yangian Gelfand-Zetlin basis, to a limit of Macdonald polynomials (gl_2-Jack
polynomials). To apply this mapping method to the calculation of 1-particle
Green's function, we confirm that the sum of the field annihilation operator on
Yangian Gelfand-Zetlin basis is transformed to the field annihilation operator
on gl_2-Jack polynomials by the mapping. The resultant expression for hole
propagator for finite-size system is written in terms of renormalized momenta
and spin of quasi-holes and the expression in the thermodynamic limit coincides
with the earlier result derived by another method. We also discuss the
singularity of the spectral function for a specific coupling parameter where
the hole propagator of spin Calogero-Sutherland model becomes equivalent to
dynamical colour correlation function of SU(3) Haldane-Shastry model.Comment: 36 pages, 8 figure
A generalized virial theorem and the balance of kinetic and potential energies in the semiclassical limit
We obtain two-sided bounds on kinetic and potential energies of a bound state
of a quantum particle in the semiclassical limit, as the Planck constant
\hbar\ri 0.
Proofs of these results rely on the generalized virial theorem obtained in
the paper as well as on a decay of eigenfunctions in the classically forbidden
region
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
Exceptional Points in Atomic Spectra
We report the existence of exceptional points for the hydrogen atom in
crossed magnetic and electric fields in numerical calculations. The resonances
of the system are investigated and it is shown how exceptional points can be
found by exploiting characteristic properties of the degeneracies, which are
branch point singularities. A possibility for the observation of exceptional
points in an experiment with atoms is proposed.Comment: 4 pages, 4 figures, 1 table, to be published in Physical Review
Letter
Existence of Density Functionals for Excited States and Resonances
We show how every bound state of a finite system of identical fermions,
whether a ground state or an excited one, defines a density functional.
Degeneracies created by a symmetry group can be trivially lifted by a
pseudo-Zeeman effect. When complex scaling can be used to regularize a
resonance into a square integrable state, a DF also exists.Comment: 4 pages, no figure
Extended Weak Coupling Limit for Friedrichs Hamiltonians
We study a class of self-adjoint operators defined on the direct sum of two
Hilbert spaces: a finite dimensional one called sometimes a ``small subsystem''
and an infinite dimensional one -- a ``reservoir''. The operator, which we call
a ``Friedrichs Hamiltonian'', has a small coupling constant in front of its
off-diagonal term. It is well known that under some conditions in the weak
coupling limit the appropriately rescaled evolution in the interaction picture
converges to a contractive semigroup when restricted to the subsystem. We show
that in this model, the properly renormalized and rescaled evolution converges
on the whole space to a new unitary evolution, which is a dilation of the above
mentioned semigroup. Similar results have been studied before \cite{AFL} in
more complicated models and they are usually referred to as "stochastic Limit".Comment: changes in notation and title, minor correction
- …