405 research outputs found
Numerical evolution of secular bar-mode instability induced by the gravitational radiation reaction in rapidly rotating neutron stars
The evolution of a nonaxisymmetric bar-mode perturbation of rapidly rotating
stars due to a secular instability induced by gravitational wave emission is
studied in post-Newtonian simulations taking into account gravitational
radiation reaction. A polytropic equation of state with the polytropic index
is adopted. The ratio of the rotational kinetic energy to the
gravitational potential energy is chosen in the range between 0.2 and
0.26. Numerical simulations were performed until the perturbation grows to the
nonlinear regime, and illustrate that the outcome after the secular instability
sets in is an ellipsoidal star of a moderately large ellipticity \agt 0.7. A
rapidly rotating protoneutron star may form such an ellipsoid, which is a
candidate for strong emitter of gravitational waves for ground-based laser
interferometric detectors. A possibility that effects of magnetic fields
neglected in this work may modify the growth of the secular instability is also
mentioned.Comment: PRD, accepted for publicatio
Instability of non-supersymmetric smooth geometries
Recently certain non-supersymmetric solutions of type IIb supergravity were
constructed [hep-th/0504181], which are everywhere smooth, have no horizons and
are thought to describe certain non-BPS microstates of the D1-D5 system. We
demonstrate that these solutions are all classically unstable. The instability
is a generic feature of horizonless geometries with an ergoregion. We consider
the endpoint of this instability and argue that the solutions decay to
supersymmetric configurations. We also comment on the implications of the
ergoregion instability for Mathur's `fuzzball' proposal.Comment: v2: typos corrected, reference adde
Quasinormal modes and classical wave propagation in analogue black holes
Many properties of black holes can be studied using acoustic analogues in the
laboratory through the propagation of sound waves. We investigate in detail
sound wave propagation in a rotating acoustic (2+1)-dimensional black hole,
which corresponds to the ``draining bathtub'' fluid flow. We compute the
quasinormal mode frequencies of this system and discuss late-time power-law
tails. Due to the presence of an ergoregion, waves in a rotating acoustic black
hole can be superradiantly amplified. We compute superradiant reflection
coefficients and instability timescales for the acoustic black hole bomb, the
equivalent of the Press-Teukolsky black hole bomb. Finally we discuss
quasinormal modes and late-time tails in a non-rotating canonical acoustic
black hole, corresponding to an incompressible, spherically symmetric
(3+1)-dimensional fluid flow.Comment: 19 pages, 12 figures, ReVTeX4; v2: minor modifications and
correction
The X-Ray Properties of the Optically Brightest Mini-BAL Quasars from the Sloan Digital Sky Survey
We have compiled a sample of 14 of the optically brightest radio-quiet
quasars (~~17.5 and ~~1.9) in the Sloan Digital Sky Survey
Data Release 5 quasar catalog that have C IV mini-BALs present in their
spectra. X-ray data for 12 of the objects were obtained via a Chandra snapshot
survey using ACIS-S, while data for the other two quasars were obtained from
archival XMM-Newton observations. Joint X-ray spectral analysis shows the
mini-BAL quasars have a similar average power-law photon index
() and level of intrinsic absorption () as non-BMB (neither BAL nor mini-BAL) quasars.
Mini-BAL quasars are more similar to non-BMB quasars than to BAL quasars in
their distribution of relative X-ray brightness (assessed with
). Relative colors indicate mild dust reddening in the
optical spectra of mini-BAL quasars. Significant correlations between
and UV absorption properties are confirmed for a sample
of 56 sources combining mini-BAL and BAL quasars with high signal-to-noise
ratio rest-frame UV spectra, which generally supports models in which X-ray
absorption is important in enabling driving of the UV absorption-line wind. We
also propose alternative parametrizations of the UV absorption properties of
mini-BAL and BAL quasars, which may better describe the broad absorption
troughs in some respects.Comment: ApJ accepted; 21 pages, 11 figures, and 9 table
Evolution of predator dispersal in relation to spatio-temporal prey dynamics : how not to get stuck in the wrong place!
Peer reviewedPublisher PD
Reverberation Mapping Measurements of Black Hole Masses in Six Local Seyfert Galaxies
We present the final results from a high sampling rate, multi-month,
spectrophotometric reverberation mapping campaign undertaken to obtain either
new or improved Hbeta reverberation lag measurements for several relatively
low-luminosity AGNs. We have reliably measured thetime delay between variations
in the continuum and Hbeta emission line in six local Seyfert 1 galaxies. These
measurements are used to calculate the mass of the supermassive black hole at
the center of each of these AGNs. We place our results in context to the most
current calibration of the broad-line region (BLR) R-L relationship, where our
results remove outliers and reduce the scatter at the low-luminosity end of
this relationship. We also present velocity-resolved Hbeta time delay
measurements for our complete sample, though the clearest velocity-resolved
kinematic signatures have already been published.Comment: 52 pages (AASTeX: 29 pages of text, 8 tables, 7 figures), accepted
for publication in the Astrophysical Journa
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