19,550 research outputs found
Pulsar spin-down: the glitch-dominated rotation of PSR J0537-6910
The young, fast-spinning, X-ray pulsar J0537-6910 displays an extreme glitch
activity, with large spin-ups interrupting its decelerating rotation every ~100
days. We present nearly 13 years of timing data from this pulsar, obtained with
the {\it Rossi X-ray Timing Explorer}. We discovered 22 new glitches and
performed a consistent analysis of all 45 glitches detected in the complete
data span. Our results corroborate the previously reported strong correlation
between glitch spin-up size and the time to the next glitch, a relation that
has not been observed so far in any other pulsar. The spin evolution is
dominated by the glitches, which occur at a rate ~3.5 per year, and the
post-glitch recoveries, which prevail the entire inter-glitch intervals. This
distinctive behaviour provides invaluable insights into the physics of
glitches. The observations can be explained with a multi-component model which
accounts for the dynamics of the neutron superfluid present in the crust and
core of neutron stars. We place limits on the moment of inertia of the
component responsible for the spin-up and, ignoring differential rotation, the
velocity difference it can sustain with the crust. Contrary to its rapid
decrease between glitches, the spin-down rate increased over the 13 years, and
we find the long-term braking index , the only negative
braking index seen in a young pulsar. We briefly discuss the plausible
interpretations of this result, which is in stark contrast to the predictions
of standard models of pulsar spin-down.Comment: Minor changes to match the MNRAS accepted versio
Relativistic Two-stream Instability
We study the (local) propagation of plane waves in a relativistic,
non-dissipative, two-fluid system, allowing for a relative velocity in the
"background" configuration. The main aim is to analyze relativistic two-stream
instability. This instability requires a relative flow -- either across an
interface or when two or more fluids interpenetrate -- and can be triggered,
for example, when one-dimensional plane-waves appear to be left-moving with
respect to one fluid, but right-moving with respect to another. The dispersion
relation of the two-fluid system is studied for different two-fluid equations
of state: (i) the "free" (where there is no direct coupling between the fluid
densities), (ii) coupled, and (iii) entrained (where the fluid momenta are
linear combinations of the velocities) cases are considered in a
frame-independent fashion (eg. no restriction to the rest-frame of either
fluid). As a by-product of our analysis we determine the necessary conditions
for a two-fluid system to be causal and absolutely stable and establish a new
constraint on the entrainment.Comment: 15 pages, 2 eps-figure
R-mode oscillations and rocket effect in rotating superfluid neutron stars. I. Formalism
We derive the hydrodynamical equations of r-mode oscillations in neutron
stars in presence of a novel damping mechanism related to particle number
changing processes. The change in the number densities of the various species
leads to new dissipative terms in the equations which are responsible of the
{\it rocket effect}. We employ a two-fluid model, with one fluid consisting of
the charged components, while the second fluid consists of superfluid neutrons.
We consider two different kind of r-mode oscillations, one associated with
comoving displacements, and the second one associated with countermoving, out
of phase, displacements.Comment: 10 page
High-Order Contamination in the Tail of Gravitational Collapse
It is well known that the late-time behaviour of gravitational collapse is
{\it dominated} by an inverse power-law decaying tail. We calculate {\it
higher-order corrections} to this power-law behaviour in a spherically
symmetric gravitational collapse. The dominant ``contamination'' is shown to
die off at late times as . This decay rate is much {\it
slower} than has been considered so far. It implies, for instance, that an
`exact' (numerical) determination of the power index to within
requires extremely long integration times of order . We show that the
leading order fingerprint of the black-hole electric {\it charge} is of order
.Comment: 12 pages, 2 figure
Asymptotically Hyperbolic Non Constant Mean Curvature Solutions of the Einstein Constraint Equations
We describe how the iterative technique used by Isenberg and Moncrief to
verify the existence of large sets of non constant mean curvature solutions of
the Einstein constraints on closed manifolds can be adapted to verify the
existence of large sets of asymptotically hyperbolic non constant mean
curvature solutions of the Einstein constraints.Comment: 19 pages, TeX, no figure
Line Emission from Gas in Optically Thick Dust Disks around Young Stars
We present self-consistent models of gas in optically-thick dusty disks and
calculate its thermal, density and chemical structure. The models focus on an
accurate treatment of the upper layers where line emission originates, and at
radii AU. We present results of disks around stars where we have varied dust properties, X-ray luminosities and
UV luminosities. We separately treat gas and dust thermal balance, and
calculate line luminosities at infrared and sub-millimeter wavelengths from all
transitions originating in the predominantly neutral gas that lies below the
ionized surface of the disk. We find that the [ArII] 7m, [NeII]
12.8m, [FeI] 24m, [SI] 25m, [FeII] 26m, [SiII] 35 m,
[OI] 63m and pure rotational lines of H, HO and CO can be quite
strong and are good indicators of the presence and distribution of gas in
disks. We apply our models to the disk around the nearby young star, TW Hya,
and find good agreement between our model calculations and observations. We
also predict strong emission lines from the TW Hya disk that are likely to be
detected by future facilities. A comparison of CO observations with our models
suggests that the gas disk around TW Hya may be truncated to AU,
compared to its dust disk of 174 AU. We speculate that photoevaporation due to
the strong stellar FUV field from TW Hya is responsible for the gas disk
truncation.Comment: Accepted to Astrophysical Journa
Quantum measurements of atoms using cavity QED
Generalized quantum measurements are an important extension of projective or
von Neumann measurements, in that they can be used to describe any measurement
that can be implemented on a quantum system. We describe how to realize two
non-standard quantum measurements using cavity quantum electrodynamics (QED).
The first measurement optimally and unabmiguously distinguishes between two
non-orthogonal quantum states. The second example is a measurement that
demonstrates superadditive quantum coding gain. The experimental tools used are
single-atom unitary operations effected by Ramsey pulses and two-atom
Tavis-Cummings interactions. We show how the superadditive quantum coding gain
is affected by errors in the field-ionisation detection of atoms, and that even
with rather high levels of experimental imperfections, a reasonable amount of
superadditivity can still be seen. To date, these types of measurement have
only been realized on photons. It would be of great interest to have
realizations using other physical systems. This is for fundamental reasons, but
also since quantum coding gain in general increases with code word length, and
a realization using atoms could be more easily scaled than existing
realizations using photons.Comment: 10 pages, 5 figure
Allergic Rhinoconjunctivitis: the Role of Histamine
Allergic rhinoconjunctivitis is the most common atopic condition
encountered in clinical practice. Analysis of the pathogenesis of
this condition permits identification of optimal therapeutic
targets. The increased knowledge of the underlying pathophysiology
suggests that multiple inflammatory mediators are involved in the
pathogenesis of the allergic reaction in the ocular and nasal
mucosa. However, despite the presence of a wide range of different
mediators, it would appear that histamine plays a key role.
Experimental allergen challenge studies have demonstrated that
histamine is the only mediator which produces the full spectrum of
clinical manifestations of the acute allergic reaction when applied
to the mucosal surface. While both H1- and H2-receptors are present
in the nasal and ocular mucosa, only H1-receptor antagonists are
capable of inhibiting histamine-induced symptoms of allergic
rhinoconjunctivitis. Furthermore, although the exact role of
histamine in the immediate and prolonged allergic reaction has not
yet been fully elucidated, these findings do not exclude the
possibility that histamine is involved in these processes. The
available evidence therefore supports current clinical practice for
use of H1-receptor antagonist as a first-line therapy in
patients with this atopic condition
Gravitational-wave astronomy: the high-frequency window
This contribution is divided in two parts. The first part provides a
text-book level introduction to gravitational radiation. The key concepts
required for a discussion of gravitational-wave physics are introduced. In
particular, the quadrupole formula is applied to the anticipated
``bread-and-butter'' source for detectors like LIGO, GEO600, EGO and TAMA300:
inspiralling compact binaries. The second part provides a brief review of high
frequency gravitational waves. In the frequency range above (say) 100Hz,
gravitational collapse, rotational instabilities and oscillations of the
remnant compact objects are potentially important sources of gravitational
waves. Significant and unique information concerning the various stages of
collapse, the evolution of protoneutron stars and the details of the
supranuclear equation of state of such objects can be drawn from careful study
of the gravitational-wave signal. As the amount of exciting physics one may be
able to study via the detections of gravitational waves from these sources is
truly inspiring, there is strong motivation for the development of future
generations of ground based detectors sensitive in the range from hundreds of
Hz to several kHz.Comment: 21 pages, 5 figures, Lectures presented at the 2nd Aegean Summer
School on the Early Universe, Syros, Greece, September 200
- …