6,094 research outputs found
Where are all the gravastars? Limits upon the gravastar model from accreting black holes
The gravastar model, which postulates a strongly correlated thin shell of
anisotropic matter surrounding a region of anti-de Sitter space, has been
proposed as an alternative to black holes. We discuss constraints that
present-day observations of well-known black hole candidates place on this
model. We focus upon two black hole candidates known to have extraordinarily
low luminosities: the supermassive black hole in the Galactic Center,
Sagittarius A*, and the stellar-mass black hole, XTE J1118+480. We find that
the length scale for modifications of the type discussed in Chapline et al.
(2003) must be sub-Planckian.Comment: 11 pages, 4 figure
Turbulent Mixing in Clusters of Galaxies
We present a spherically-symmetric, steady-state model of galaxy clusters in
which radiative cooling from the hot gas is balanced by heat transport through
turbulent mixing. We assume that the gas is in hydrostatic equilibrium, and
describe the turbulent heat diffusion by means of a mixing length prescription
with a dimensionless parameter alpha_mix. Models with alpha_mix ~ 0.01-0.03
yield reasonably good fits to the observed density and temperature profiles of
cooling core clusters. Making the strong simplification that alpha_mix is
time-independent and that it is roughly the same in all clusters, the model
reproduces remarkably well the observed scalings of X-ray luminosity, gas mass
fraction and entropy with temperature. The break in the scaling relations at kT
\~ 1-2 keV is explained by the break in the cooling function at around this
temperature, and the entropy floor observed in galaxy groups is reproduced
naturally.Comment: Accepted for publication in ApJ
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
Dyson's Brownian Motion and Universal Dynamics of Quantum Systems
We establish a correspondence between the evolution of the distribution of
eigenvalues of a matrix subject to a random Gaussian perturbing
matrix, and a Fokker-Planck equation postulated by Dyson. Within this model, we
prove the equivalence conjectured by Altshuler et al between the space-time
correlations of the Sutherland-Calogero-Moser system in the thermodynamic limit
and a set of two-variable correlations for disordered quantum systems
calculated by them. Multiple variable correlation functions are, however, shown
to be inequivalent for the two cases.Comment: 10 pages, revte
The hard to soft spectral transition in LMXBs - affected by recondensation of gas into an inner disk
Soft and hard spectral states of X-ray transient sources reflect two modes of
accretion, accretion via a geometrically thin, optically thick disk or an
advection-dominated accretion flow (ADAF). The luminosity at transition between
these two states seems to vary from source to source, or even for the same
source during different outbursts, as observed for GX 339-4. We investigate how
the existence of an inner weak disk in the hard state affects the transition
luminosity. We evaluate the structure of the corona above an outer truncated
disk and the resulting disk evaporation rate for different irradiation. In some
cases, recent observations of X-ray transients indicate the presence of an
inner cool disk during the hard state. Such a disk can remain during quiescence
after the last outburst as long as the luminosity does not drop to very low
values (10^-4 to 10^-3 of the Eddington luminosity). Consequently, as part of
the matter accretes via the inner disk, the hard irradiation is reduced. The
hard irradiation is further reduced, occulted and partly reflected by the inner
disk. This leads to a hard-soft transition at a lower luminosity if an inner
disk exists below the ADAF. This seems to be supported by observations for GX
339-4.Comment: 9 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480
Multi-wavelength observations of the black hole X-ray binary XTE J1118+480
have offered abundant spectral and timing information about the source, and
have thus provided serious challenges to theoretical models. We propose a
coupled accretion-jet model to interpret the observations. We model the
accretion flow as an outer standard thin accretion disk truncated at a
transition radius by an inner hot accretion flow. The accretion flow accounts
for the observed UV and X-ray emission, but it substantially under-predicts the
radio and infrared fluxes, even after we allow for nonthermal electrons in the
hot flow. We attribute the latter components to a jet. We model the jet
emission by means of the internal shock scenario which is widely employed for
gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet
dominates the radio and infrared emission, the thin disk dominates the UV
emission, and the hot flow produces most of the X-ray emission. The optical
emission has contributions from all three components: jet, thin disk, and hot
flow. The model qualitatively accounts for timing features, such as the
intriguing positive and negative time lags between the optical and X-ray
emission, and the wavelength-dependent variability amplitude.Comment: 27 pages, 4 figures (one in color); to appear in ApJ in Feb. 200
Vector lattice model for stresses in granular materials
A vector lattice model for stresses in granular materials is proposed. A two
dimensional pile built by pouring from a point is constructed numerically
according to this model. Remarkably, the pile violates the Mohr Coulomb
stability criterion for granular matter, probably because of the inherent
anisotropy of such poured piles. The numerical results are also compared to the
earlier continuum FPA model and the (scalar) lattice -model
Fluctuation theorems for harmonic oscillators
We study experimentally the thermal fluctuations of energy input and
dissipation in a harmonic oscillator driven out of equilibrium, and search for
Fluctuation Relations. We study transient evolution from the equilibrium state,
together with non equilibrium steady states. Fluctuations Relations are
obtained experimentally for both the work and the heat, for the stationary and
transient evolutions. A Stationary State Fluctuation Theorem is verified for
the two time prescriptions of the torque. But a Transient Fluctuation Theorem
is satisfied for the work given to the system but not for the heat dissipated
by the system in the case of linear forcing. Experimental observations on the
statistical and dynamical properties of the fluctuation of the angle, we derive
analytical expressions for the probability density function of the work and the
heat. We obtain for the first time an analytic expression of the probability
density function of the heat. Agreement between experiments and our modeling is
excellent
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