16,067 research outputs found
Periodic Gravitational Waves From Small Cosmic String Loops
We consider a population of small, high-velocity cosmic string loops. We
assume the typical length of these loops is determined by the gravitational
radiation scale and use the results of \cite{Polchinski:2007rg} which pointed
out their highly relativistic nature. A study of the gravitational wave
emission from such a population is carried out. The large Lorentz boost
involved causes the lowest harmonics of the loops to fall within the frequency
band of the LIGO detector. Due to this feature the gravitational waves emitted
by such loops can be detected in a periodic search rather than in burst or
stochastic analysis.
It is shown that, for interesting values of the string tension
(10^{-10}\lsim G\mu\lsim 10^{-8}) the detector can observe loops at
reasonably high redshifts and that detection is, in principle, possible. We
compute the number of expected observations produced by such a process. For a
10 hour search we find that this number is of order . This is a
consequence of the low effective number density of the loops traveling along
the line of sight. However, small probabilities of reconnection and longer
observation times can improve the result.Comment: 1+15 pages, 7 figure
Numerical sunspot models: Robustness of photospheric velocity and magnetic field structure
MHD simulations of sunspots have successfully reproduced many aspects of
sunspot fine structure as consequence of magneto convection in inclined
magnetic field. We study how global sunspot properties and penumbral fine
structure depend on the magnetic top boundary condition as well as on grid
spacing. The overall radial extent of the penumbra is subject to the magnetic
top boundary condition. All other aspects of sunspot structure and penumbral
fine structure are resolved at an acceptable level starting from a grid
resolution of 48 [24] km (horizontal [vertical]). We find that the amount of
inverse polarity flux and the overall amount of overturning convective motions
in the penumbra are robust with regard to both, resolution and boundary
conditions. At photospheric levels Evershed flow channels are strongly
magnetized. We discuss in detail the relation between velocity and magnetic
field structure in the photosphere and point out observational consequences.Comment: 23 pages, 22 figures, 2 movies, accepted for publication in Ap
First Kepler results on compact pulsators II: KIC 010139564, a new pulsating subdwarf B (V361 Hya) star with an additional low-frequency mode
We present the discovery of nonradial pulsations in a hot subdwarf B star
based on 30.5 days of nearly continuous time-series photometry using the
\emph{Kepler} spacecraft. KIC 010139564 is found to be a short-period pulsator
of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes
whose periods range from 130 to 190 seconds. It also shows one periodicity at a
period of 3165 seconds. If this periodicity is a high order g-mode, then this
star may be the hottest member of the hybrid DW Lyn stars. In addition to the
resolved pulsation frequencies, additional periodic variations in the light
curve suggest that a significant number of additional pulsation frequencies may
be present. The long duration of the run, the extremely high duty cycle, and
the well-behaved noise properties allow us to explore the stability of the
periodic variations, and to place strong constraints on how many of them are
independent stellar oscillation modes. We find that most of the identified
periodicities are indeed stable in phase and amplitude, suggesting a rotation
period of 2-3 weeks for this star, but further observations are needed to
confirm this suspicion.Comment: 10 pages, accepted for publication in MNRA
CU Comae: a new field double-mode RR Lyrae, the most metal poor discovered to date
We report the discovery of a new double-mode RR Lyrae variable (RRd) in the
field of our Galaxy: CU Comae. CU Comae is the sixth such RRd identified to
date and is the most metal-poor RRd ever detected. Based on BVI CCD photometry
spanning eleven years of observations, we find that CU Comae has periods
P0=0.5441641 +/-0.0000049d and P1=0.4057605 +/-0.0000018d. The amplitude of the
primary (first-overtone) period of CU Comae is about twice the amplitude of the
secondary (fundamental) period. The combination of the fundamental period of
pulsation P0 and the period ratio of P1/P0=0.7457 places the variable on the
metal-poor side of the Petersen diagram, in the region occupied by M68 and M15
RRd's. A mass of 0.83 solar masses is estimated for CU Comae using an updated
theoretical calibration of the Petersen diagram. High resolution spectroscopy
(R=30,000) covering the full pulsation cycle of CU Comae was obtained with the
2.7 m telescope of the Mc Donald Observatory, and has been used to build up the
radial velocity curve of the variable. Abundance analysis done on the four
spectra taken near minimum light (phase: 0.54 -- 0.71) confirms the metal poor
nature of CU Comae, for which we derive [Fe/H]=-2.38 +/-0.20. This value places
this new RRd at the extreme metal-poor edge of the metallicity distribution of
the RR Lyrae variables in our Galaxy.Comment: 21 pages including 8 Tables, Latex, 11 Figures. Accepted for
publication in The Astronomical Journal, October 2000 issu
The model constraints from the observed trends for the quasi-periodic oscillation in RE J1034+396
We analyze the time variability of the X-ray emission of RE J1034+396 -- an
active galactic nucleus with the first firm detection of a quasi-periodic
oscillations (QPO). Based on the results of a wavelet analysis, we find a drift
in the QPO central frequency. The change in the QPO frequency correlates with
the change in the X-ray flux with a short time delay. The data specifically
suggest a linear dependence between the QPO period and the flux, and this gives
important constraints on the QPO models. In particular, it excludes explanation
in terms of the orbiting hot spot model close to a black hole. Linear
structures such as shocks, spiral waves, or very distant flares are favored.Comment: Astronomy & Astrophysics, in pres
Evidence for Compact Dark Matter in Galactic Halos
Clumped dark matter arises naturally within the framwork of generic
cosmological dark matter models. Invoking the existence of dark matter clumps
can also solve may unexplained mysteries in astrophysics and geology or
geophysics, eg. the galactic gamma-ray halo and the periodic terrestrial flood
basalt volcanic episodes. Clumped dark matter is dynamically stable to friction
and will not heat the disk. Such clumps may have already been discovered in the
form of dwarf spheroidals, and further searches are encouraged by the results
of this paper.Comment: Revised Version, includes new relevant references, Latex File, 16
pages, no figure
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