197,281 research outputs found
Timing analysis in microlensing
Timing analysis is a powerful tool used to determine periodic features of
physical phenomena. Here we review two applications of timing analysis to
gravitational microlensing events. The first one, in particular cases, allows
the estimation of the orbital period of binary lenses, which in turn enables
the breaking of degeneracies. The second one is a method to measure the
rotation period of the lensed star by observing signatures due to stellar spots
on its surface.Comment: 11 pages, 4 figures. To be published in International Journal of
Modern Physics D (IJMPD
Rayleigh - Taylor Gravity Waves and Quasiperiodic Oscillation Phenomenon in X-ray Binaries
Accretion onto compact objects in X-ray binaries [black hole, neutron star
(NS), white dwarf] is characterized by non-uniform flow density profiles. Such
an effect of heterogeneity in presence of gravitational forces and pressure
gradients exhibits Raylegh-Taylor gravity waves (RTGW). They should be seen as
quasioperiodic wave oscillations (QPO). In this paper I show that the main QPO
frequency, which is very close to the Keplerian frequency, is split into
separate frequencies (hybrid and low branch) under the influence of the
gravitational forces in the rotational frame of reference. The observed low and
high QPO frequencies are an intrinsic signature of the RTGW. I elaborate the
conditions for the density profile when the RTGW oscillations are stable. A
comparison of the inferred QPO frequencies with QPO observations is presented.
I find that hectohertz frequencies detected from NS binaries can be identified
as the RTGW low branch frequencies. I also predict that an observer can see the
double NS spin frequency during the NS long (super) burst events when the
pressure gradients and buoyant forces are suppressed. The Coriolis force is the
only force which acts in the rotational frame of reference and its presence
causes perfect coherent pulsations with a frequency twice of the NS spin.Comment: 14 pages and 2 figures, ApJ in pres
Superfluid instability of r-modes in "differentially rotating" neutron stars
Superfluid hydrodynamics affects the spin-evolution of mature neutron stars,
and may be key to explaining timing irregularities such as pulsar glitches.
However, most models for this phenomenon exclude the global instability
required to trigger the event. In this paper we discuss a mechanism that may
fill this gap. We establish that small scale inertial r-modes become unstable
in a superfluid neutron star that exhibits a rotational lag, expected to build
up due to vortex pinning as the star spins down. Somewhat counterintuitively,
this instability arises due to the (under normal circumstances dissipative)
vortex-mediated mutual friction. We explore the nature of the superfluid
instability for a simple incompressible model, allowing for entrainment
coupling between the two fluid components. Our results recover a previously
discussed dynamical instability in systems where the two components are
strongly coupled. In addition, we demonstrate for the first time that the
system is secularly unstable (with a growth time that scales with the mutual
friction) throughout much of parameter space. Interestingly, large scale
r-modes are also affected by this new aspect of the instability. We analyse the
damping effect of shear viscosity, which should be particularly efficient at
small scales, arguing that it will not be sufficient to completely suppress the
instability in astrophysical systems.Comment: RevTex, 11 figure
Gravimetric Soil Moisture Protocols
The purpose of this resource is to measure soil water content by mass. Students collect soil samples with a trowel or auger and weigh them, dry them, and then weigh them again. The soil water content is determined by calculating the difference between the wet sample mass and the dry sample mass. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school
Astrometric Image Centroid Displacements due to Gravitational Microlensing by the Ellis Wormhole
Continuing work initiated in an earlier publication (Abe, ApJ, 725 (2010)
787), we study the gravitational microlensing effects of the Ellis wormhole in
the weak-field limit. First, we find a suitable coordinate transformation, such
that the lens equation and analytic expressions of the lensed image positions
can become much simpler than the previous ones. Second, we prove that two
images always appear for the weak-field lens by the Ellis wormhole. By using
these analytic results, we discuss astrometric image centroid displacements due
to gravitational microlensing by the Ellis wormhole. The astrometric image
centroid trajectory by the Ellis wormhole is different from the standard one by
a spherical lensing object that is expressed by the Schwarzschild metric. The
anomalous shift of the image centroid by the Ellis wormhole lens is smaller
than that by the Schwarzschild lens, provided that the impact parameter and the
Einstein ring radius are the same. Therefore, the lensed image centroid by the
Ellis wormhole moves slower. Such a difference, though it is very small, will
be in principle applicable for detecting or constraining the Ellis wormhole by
using future high-precision astrometry observations. In particular, the image
centroid position gives us an additional information, so that the parameter
degeneracy existing in photometric microlensing can be partially broken. The
anomalous shift reaches the order of a few micro arcsec. if our galaxy hosts a
wormhole with throat radius larger than km. When the source moves
tangentially to the Einstein ring for instance, the maximum position shift of
the image centroid by the Ellis wormhole is 0.18 normalized by the Einstein
ring radius. For the same source trajectory, the maximum difference between the
centroid displacement by the Ellis wormhole lens and that by the Schwarzschild
one is -0.16 in the units of the Einstein radius.Comment: 29 pages, 6 figures, 2 tables, accepted by Ap
Damping of Neutron Star Shear Modes by Superfluid Friction
The forced motion of superfluid vortices in shear oscillations of rotating
solid neutron star matter produces damping of the mode. A simple model of the
unpinning and repinning processes is described, with numerical calculations of
the consequent energy decay times. These are of the order of 1 s or more for
typical anomalous X-ray pulsars but become very short for the general
population of radio pulsars. The superfluid friction processes considered here
may also be significant for the damping of r-modes in rapidly rotating neutron
stars.Comment: 7 LaTeX pages, 4 eps figures; accepted for publication in MNRA
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