877 research outputs found
Quark-nova remnants IV: Application to radio emitting AXP transients
(Abridged) XTE J1810-197 and 1E 1547.0-5408 are two transient AXPs exhibiting
radio emission with unusual properties. In addition, their spin down rates
during outburst show opposite trends, which so far has no explanation. Here, we
extend our quark-nova model for AXPs to include transient AXPs, in which the
outbursts are caused by transient accretion events from a Keplerian (iron-rich)
degenerate ring. For a ring with inner and outer radii of 23.5 km and 26.5 km,
respectively, our model gives a good fit to the observed X-ray outburst from
XTE J1810-197 and the behavior of temperature, luminosity, and area of the two
X-ray blackbodies with time. The two blackbodies in our model are related to a
heat front (i.e. Bohm diffusion front) propagating along the ring's surface and
an accretion hot spot on the quark star surface. Radio pulsations in our model
are caused by dissipation at the light cylinder of magnetic bubbles, produced
near the ring during the X-ray outburst. The delay between X-ray peak emission
and radio emission in our model is related to the propagation time of these
bubbles to the light cylinder. We predict a ~1 year and ~1 month delay for XTE
J1810-197 and 1E 1547.0-5408, respectively. The observed flat spectrum, erratic
pulse profile, and the pulse duration are all explained in our model as a
result of X-point reconnection events induced by the dissipation of the bubbles
at the light cylinder. The spin down rate of the central quark star can either
increase or decrease depending on how the radial drift velocity of the magnetic
islands changes with distance from the central star. We suggest an evolutionary
connection between transient AXPs and typical AXPs in our model.Comment: 16 journal pages, 4 figures and 1 table [Version accepted for
publication in A&A
Time as an operator/observable in nonrelativistic quantum mechanics
The nonrelativistic Schroedinger equation for motion of a structureless
particle in four-dimensional space-time entails a well-known expression for the
conserved four-vector field of local probability density and current that are
associated with a quantum state solution to the equation. Under the physical
assumption that each spatial, as well as the temporal, component of this
current is observable, the position in time becomes an operator and an
observable in that the weighted average value of the time of the particle's
crossing of a complete hyperplane can be simply defined: ... When the
space-time coordinates are (t,x,y,z), the paper analyzes in detail the case
that the hyperplane is of the type z=constant. Particles can cross such a
hyperplane in either direction, so it proves convenient to introduce an
indefinite metric, and correspondingly a sesquilinear inner product with
non-Hilbert space structure, for the space of quantum states on such a surface.
>... A detailed formalism for computing average crossing times on a z=constant
hyperplane, and average dwell times and delay times for a zone of interaction
between a pair of z=constant hyperplanes, is presented.Comment: 31 pages, no figures. Differs from published version by minor
corrections and additions, and two citation
Braking down an accreting protostar: disc-locking, disc winds, stellar winds, X-winds and Magnetospheric Ejecta
Classical T Tauri stars are low mass young forming stars that are surrounded
by a circumstellar accretion disc from which they gain mass. Despite this
accretion and their own contraction that should both lead to their spin up,
these stars seem to conserve instead an almost constant rotational period as
long as the disc is maintained. Several scenarios have been proposed in the
literature in order to explain this puzzling "disc-locking" situation: either
deposition in the disc of the stellar angular momentum by the stellar
magnetosphere or its ejection through winds, providing thereby an explanation
of jets from Young Stellar Objects.
In this lecture, these various mechanisms will be critically detailed, from
the physics of the star-disc interaction to the launching of self-confined jets
(disc winds, stellar winds, X-winds, conical winds). It will be shown that no
simple model can account alone for the whole bulk of observational data and
that "disc locking" requires a combination of some of them.Comment: 60 pages, 29 figures Lecture held in Evry Schatzman School 201
Axion Searches
The strong CP problem and its resolution through the existence of an axion
are briefly reviewed. The constraints on the axion from accelerator searches,
from the evolution of red giants and from supernova SN1987a combine to require
eV, where is the axion mass. On the other hand,
the constraint that axions do not overclose the universe implies m_a \gtwid
10^{-6} eV. If eV, axions contribute significantly to the
cosmological energy density in the form of cold dark matter. Dark matter axions
can be detected by resonant conversion to microwave photons in a cavity
permeated by a static magnetic field and tuned to the axion mass. Experiments
using this effect are described, as well as several other types of axion
searches.Comment: 12 pages, 2 figures, based on talks at the TAUP99 Conference (Sept.
6-10, 1999, Paris, France) and at Adrianfest (Sept. 24-25, 1999, Rochester,
NY
Accretion funnels onto weakly magnetized young stars
Aims : We re-examine the conditions required to steadily deviate an accretion
flow from a circumstellar disc into a magnetospheric funnel flow onto a slow
rotating young forming star. Methods : New analytical constraints on the
formation of accretion funnels flows due to the presence of a dipolar stellar
magnetic field disrupting the disc are derived. The Versatile Advection Code is
used to confirm these constraints numerically. Axisymmetric MHD simulations are
performed, where a stellar dipole field enters the resistive accretion disc,
whose structure is self-consistently computed. Results : The analytical
criterion derived allows to predict a priori the position of the truncation
radius from a non perturbative accretion disc model. Accretion funnels are
found to be robust features which occur below the co-rotation radius, where the
stellar poloidal magnetic pressure becomes both at equipartition with the disc
thermal pressure and is comparable to the disc poloidal ram pressure. We
confirm the results of Romanova et al. 2002 and find accretion funnels for
stellar dipole fields as low as 140 G in the low accretion rate limit of
. With our present numerical setup with no disc
magnetic field, we found no evidence of winds, neither disc driven nor X-winds,
and the star is only spun up by its interaction with the disc. Conclusions :
Weak dipole fields, similar in magnitude to those observed, lead to the
development of accretion funnel flows in weakly accreting T Tauri stars.
However, the higher accretion observed for most T Tauri stars () requires either larger stellar field strength and/or
different magnetic topologies to allow for magnetospheric accretion.Comment: 8 pages, 6 figures, accepted in A&
Nonlinear optical response of a gold surface in the visible range: A study by two-color sum-frequency generation spectroscopy. III. Simulations of the experimental SFG intensities
International audienceWe model the amplitude line shape and absolute phase of the infrared-visible sum-frequency signals produced by a thiolated polycrystalline gold surface as a function of the visible wavelength. We follow two hypotheses: in the interband scenario, the resonant features are attributed to interband transitions, whereas in the effective surface state scenario, they stem mostly from the excitation of surface transitions. We find that both scenarios lead to a satisfactory account of the experimental data and that only free electrons may spill out of the gold bulk, as expected. For the interband scenario, the balance between free and bound electron contributions to sum-frequency generation has to be adjusted to fit the data. The surface transitions are shown to take their origin inside gold and we investigate the surface states involved in such transitions, with a comparison to the silver surfaces. We finally provide a work program dedicated to discriminate between the two scenarios
Parametric Level Set Methods for Inverse Problems
In this paper, a parametric level set method for reconstruction of obstacles
in general inverse problems is considered. General evolution equations for the
reconstruction of unknown obstacles are derived in terms of the underlying
level set parameters. We show that using the appropriate form of parameterizing
the level set function results a significantly lower dimensional problem, which
bypasses many difficulties with traditional level set methods, such as
regularization, re-initialization and use of signed distance function.
Moreover, we show that from a computational point of view, low order
representation of the problem paves the path for easier use of Newton and
quasi-Newton methods. Specifically for the purposes of this paper, we
parameterize the level set function in terms of adaptive compactly supported
radial basis functions, which used in the proposed manner provides flexibility
in presenting a larger class of shapes with fewer terms. Also they provide a
"narrow-banding" advantage which can further reduce the number of active
unknowns at each step of the evolution. The performance of the proposed
approach is examined in three examples of inverse problems, i.e., electrical
resistance tomography, X-ray computed tomography and diffuse optical
tomography
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