2,857 research outputs found
EMRI corrections to the angular velocity and redshift factor of a mass in circular orbit about a Kerr black hole
This is the first of two papers on computing the self-force in a radiation
gauge for a particle moving in circular, equatorial orbit about a Kerr black
hole. In the EMRI (extreme-mass-ratio inspiral) framework, with mode-sum
renormalization, we compute the renormalized value of the quantity
, gauge-invariant under gauge transformations
generated by a helically symmetric gauge vector; and we find the related order
correction to the particle's angular velocity at fixed renormalized
redshift (and to its redshift at fixed angular velocity). The radiative part of
the perturbed metric is constructed from the Hertz potential which is extracted
from the Weyl scalar by an algebraic inversion\cite{sf2}. We then write the
spin-weighted spheroidal harmonics as a sum over spin-weighted spherical
harmonics and use mode-sum renormalization to find the renormalization
coefficients by matching a series in to the large- behavior of
the expression for . The
non-radiative parts of the perturbed metric associated with changes in mass and
angular momentum are calculated in the Kerr gauge
Models of helically symmetric binary systems
Results from helically symmetric scalar field models and first results from a
convergent helically symmetric binary neutron star code are reported here;
these are models stationary in the rotating frame of a source with constant
angular velocity omega. In the scalar field models and the neutron star code,
helical symmetry leads to a system of mixed elliptic-hyperbolic character. The
scalar field models involve nonlinear terms that mimic nonlinear terms of the
Einstein equation. Convergence is strikingly different for different signs of
each nonlinear term; it is typically insensitive to the iterative method used;
and it improves with an outer boundary in the near zone. In the neutron star
code, one has no control on the sign of the source, and convergence has been
achieved only for an outer boundary less than approximately 1 wavelength from
the source or for a code that imposes helical symmetry only inside a near zone
of that size. The inaccuracy of helically symmetric solutions with appropriate
boundary conditions should be comparable to the inaccuracy of a waveless
formalism that neglects gravitational waves; and the (near zone) solutions we
obtain for waveless and helically symmetric BNS codes with the same boundary
conditions nearly coincide.Comment: 19 pages, 7 figures. Expanded version of article to be published in
Class. Quantum Grav. special issue on Numerical Relativit
Hydrostatic Expansion and Spin Changes During Type I X-Ray Bursts
We present calculations of the spin-down of a neutron star atmosphere due to
hydrostatic expansion during a Type I X-ray burst. We show that (i) Cumming and
Bildsten overestimated the spin-down of rigidly-rotating atmospheres by a
factor of two, and (ii) general relativity has a small (5-10%) effect on the
angular momentum conservation law. We rescale our results to different neutron
star masses, rotation rates and equations of state, and present some detailed
rotational profiles. Comparing with recent observations of large frequency
shifts in MXB 1658-298 and 4U 1916-053, we find that the spin-down expected if
the atmosphere rotates rigidly is a factor of two to three less than the
observed values. If differential rotation is allowed to persist, we find that
the upper layers of the atmosphere spin down by an amount comparable to the
observed values; however, there is no compelling reason to expect the observed
spin frequency to be that of only the outermost layers. We conclude that
hydrostatic expansion and angular momentum conservation alone cannot account
for the largest frequency shifts observed during Type I bursts.Comment: Submitted to the Astrophysical Journal (13 pages, including 4
figures
Entangling flux qubits with a bipolar dynamic inductance
We propose a scheme to implement variable coupling between two flux qubits
using the screening current response of a dc Superconducting QUantum
Interference Device (SQUID). The coupling strength is adjusted by the current
bias applied to the SQUID and can be varied continuously from positive to
negative values, allowing cancellation of the direct mutual inductance between
the qubits. We show that this variable coupling scheme permits efficient
realization of universal quantum logic. The same SQUID can be used to determine
the flux states of the qubits.Comment: 4 pages, 4 figure
The rotational modes of relativistic stars: Numerical results
We study the inertial modes of slowly rotating, fully relativistic compact
stars. The equations that govern perturbations of both barotropic and
non-barotropic models are discussed, but we present numerical results only for
the barotropic case. For barotropic stars all inertial modes are a hybrid
mixture of axial and polar perturbations. We use a spectral method to solve for
such modes of various polytropic models. Our main attention is on modes that
can be driven unstable by the emission of gravitational waves. Hence, we
calculate the gravitational-wave growth timescale for these unstable modes and
compare the results to previous estimates obtained in Newtonian gravity (i.e.
using post-Newtonian radiation formulas). We find that the inertial modes are
slightly stabilized by relativistic effects, but that previous conclusions
concerning eg. the unstable r-modes remain essentially unaltered when the
problem is studied in full general relativity.Comment: RevTeX, 29 pages, 31 eps figure
Response to radiation in renal medullary carcinoma
Renal medullary carcinoma (RMC) is a rare and highly aggressive malignancy arising from the renal medulla and found mostly in patients with sickle cell trait. RMC usually presents with widely metastatic disease. We describe a young man diagnosed with metastatic RMC who sustained a complete response to systemic chemotherapy but developed brain metastases with leptomeningeal involvement and subsequently had a partial response to brain irradiation. The use of radiation in the management of RMC is reviewed. Due to the apparent propensity for RMC to spread to the central nervous system, prophylactic treatment such as craniospinal irradiation should be considered along with chemotherapy in patients with metastatic RMC to potentially improve the progression-free interval
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