300 research outputs found
Perturbative calculation of quasi-normal modes of Schwarzschild black holes
We discuss a systematic method of analytically calculating the asymptotic
form of quasi-normal frequencies of a four-dimensional Schwarzschild black hole
by expanding around the zeroth-order approximation to the wave equation
proposed by Motl and Neitzke. We obtain an explicit expression for the
first-order correction and arbitrary spin. Our results are in agreement with
the results from WKB and numerical analyses in the case of gravitational waves.Comment: 11 pages; references added and a sign error corrected; to appear in
CQ
Symmetry of massive Rarita-Schwinger fields
We derive the general lagrangian and propagator for a vector-spinor field in
-dimensions and show that the physical observables are invariant under the
so-called point transformation symmetry. Until now the symmetry has not been
exploited in any non-trival way, presumably because it is not an invariance of
the classical action nor is it a gauge symmetry. Nevertheless, we develop a
technique for exploring the consequences of the symmetry leading to a conserved
vector current and charge. The current and charge are identically zero in the
free field case and only contribute in a background such as a electromagnetic
or gravitational field. The current can couple spin-3/2 fields to vector and
scalar fields and may have important consequences in intermediate energy hadron
physics as well as linearized supergravity. The consistency problem which
plagues higher spin field theories is then discussed and and some ideas
regarding the possiblity of solutions are presented.Comment: 26 pages, 1 figure; revised using referee comments, Journal ref.
adde
Approximating RR Lyrae light curves using cubic polynomials
In this paper, we use cubic polynomials to approximate RR Lyrae light curves
and apply the method to HST data of RR Lyraes in the halo of M31. We compare
our method to the standard method of Fourier decomposition and find that the
method of cubic polynomials eliminates virtually all ringing effects and does
so with significantly fewer parameters than the Fourier technique. Further, for
RRc stars the parameters in the fit are all physical. Our study also reveals a
number of additional periodicites in this data not found previously: we find 23
RRc stars, 29 RRab stars and 3 multiperiodic stars.Comment: 6 pages, MNRAS accepte
CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids
We present a new derivation of the CORS Baade-Wesselink method in the
Walraven photometric system. We solved the complete Baade-Wesselink equation by
calibrating the surface brightness function with a recent grid of atmosphere
models. The new approach was adopted to estimate the mean radii of a sample of
Galactic Cepheids for which are available precise light curves in the Walraven
bands. Current radii agree, within the errors, quite well with Cepheid radii
based on recent optical and near-infrared interferometric measurements. We also
tested the impact of the projection factor on the Period-Radius relation using
two different values (p=1.36, p=1.27) that bracket the estimates available in
the literature. We found that the agreement of our Period-Radius relation with
similar empirical and theoretical Period-Radius relations in the recent
literature, improves by changing the projection factor from p=1.36 to p=1.27.
Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with
a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of
the selected Cepheids, we also derived the Period-Luminosity relation in the V
band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag,
for p=1.27. It agrees quite well with recent results in the literature, while
the relation for p=1.36 deviates by more than 2sigma. We conclude that, even
taking into account the intrinsic dispersion of the obtained Period-Luminosity
relations, that is roughly of the same order of magnitude as the effect of the
projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA
Hysteresis of atmospheric parameters of 12 RR Lyrae stars based on multichannel simultaneous Stroemgren photometry
RR Lyrae stars have been observed to improve the insight into the processes
at work in their atmospheres. Simultaneous Stroemgren-photometry allows to
obtain a rapid sequence of measurements in which photometric indices are
unaffected by non-optimum observing conditions. The indices y, b-y, and c_1 are
used with an established calibration to derive T_eff and to derive the gravity,
log g_BJ from the Balmer jump, throughout the pulsation cycle. By employing the
equations for stellar structure, additional parameters can be derived.
Stroemgren photometry and its calibration in terms of T_eff and log g can be
used to determine the run of R and the atmosphere pulsation velocity. We find
that the Balmer-line strengths are correlated with T_eff and that the strength
of the Ca_ii K line correlates well with the radius of the star and thus the
pulsation-dependent density of the atmosphere. The density in the stellar
atmosphere fluctuates as indicated by the changes in the gravity log g_BJ,
derived from c_1, between 2.3 and 4.5 dex. Also the Stroemgren metal index,
m_1, fluctuates. We find a disagreement between log g(T,L,M), the gravity
calculated from T_eff, L, and the mass M,and the gravity log g_BJ. This can be
used to reassess the mass and the absolute magnitude of an individual star.The
curves derived for the pulsational velocity V_pul differ from curves obtained
from spectra needed to apply the Baade-Wesselink method; we think these
differences are due to phase dependent differences in the optical depth levels
sampled in continuum photometry and in spectroscopy. We find an atmospheric
oscillation in these fundamental mode RR Lyrae stars of periodicity P/7.Comment: 12 pages, 11 figure
Variability of Active Galactic Nuclei from the Optical to X-ray Regions
Some progress in understanding AGN variability is reviewed. Reprocessing of
X-ray radiation to produce significant amounts of longer-wavelength continua
seems to be ruled out. In some objects where there has been correlated X-ray
and optical variability, the amplitude of the optical variability has exceeded
the amplitude of X-ray variability. We suggest that accelerated particles
striking material could be linking X-ray and optical variability (as in
activity in the solar chromosphere). Beaming effects could be significant in
all types of AGN. The diversity in optical/X-ray relationships at different
times in the same object, and between different objects, might be explained by
changes in geometry and directions of motion relative to our line of sight.
Linear shot-noise models of the variability are ruled out; instead there must
be large-scale organization of variability. Variability occurs on
light-crossing timescales rather than viscous timescales and this probably
rules out the standard Shakura-Sunyaev accretion disk. Radio-loud and
radio-quiet AGNs have similar continuum shapes and similar variability
properties. This suggests similar continuum origins and variability mechanisms.
Despite their extreme X-ray variability, narrow-line Seyfert 1s (NLS1s) do not
show extreme optical variability.Comment: Invited talk given at Euro Asian Astronomical Society meeting in
Moscow, June 2002. 20 pages, 4 figures. References update
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