9,887 research outputs found
The instrumental polarization of the Nasmyth focus polarimetric differential imager NAOS/CONICA (NACO) at the VLT - Implications for time-resolved polarimetric measurements of Sgr A*
We report on the results of calibrating and simulating the instrumental
polarization properties of the ESO VLT adaptive optics camera system
NAOS/CONICA (NACO) in the Ks-band. We use the Stokes/Mueller formalism for
metallic reflections to describe the instrumental polarization. The model is
compared to standard-star observations and time-resolved observations of bright
sources in the Galactic center. We find the instrumental polarization to be
highly dependent on the pointing position of the telescope and about 4% at
maximum. We report a polarization angle offset of 13.28{\deg} due to a position
angle offset of the half-wave plate that affects the calibration of NACO data
taken before autumn 2009. With the new model of the instrumental polarization
of NACO it is possible to measure the polarization with an accuracy of 1% in
polarization degree. The uncertainty of the polarization angle is < 5{\deg} for
polarization degrees > 4%. For highly sampled polarimetric time series we find
that the improved understanding of the polarization properties gives results
that are fully consistent with the previously used method to derive the
polarization. The small difference between the derived and the previously
employed polarization calibration is well within the statistical uncertainties
of the measurements, and for Sgr A* they do not affect the results from our
relativistic modeling of the accretion process.Comment: 16 pages, 15 figures, 5 tables, accepted by A&A on 2010 October 1
The Galactic Center stellar cluster: The central arcsecond
With 10 years of high-resolution imaging data now available on the stellar
cluster in the Galactic Center, we analyze the dynamics of the stars at
projected distances from the central black hole candidate
Sagittarius A* (Sgr A*). We find evidence for radial anisotropy of the cluster
of stars surrounding Sgr A*. We confirm/find accelerated motion for 6 stars,
with 4 stars having passed the pericenter of their orbits during the observed
time span. We calculated/constrained the orbital parameters of these stars. All
orbits have moderate to high eccentricities. The center of acceleration
coincides with the radio position of Sgr A*. From the orbit of the star S2, the
currently most tightly constrained one, we determine the mass of Sgr A* to
M and its position to mas East and
mas South of the nominal radio position. The data provide
compelling evidence that Sgr A* is a single supermassive black hole.Comment: 7 pages, 3 Figures; to be published in Astron. Nachr., Vol. 324, No.
S1 (2003), Special Supplement "The central 300 parsecs of the Milky Way",
Eds. A. Cotera, H. Falcke, T. R. Geballe, S. Markof
Compressive high-frequency waves riding on an Alfv\'en/ion-cyclotron wave in a multi-fluid plasma
In this paper, we study the weakly-compressive high-frequency plasma waves
which are superposed on a large-amplitude Alfv\'en wave in a multi-fluid plasma
consisting of protons, electrons, and alpha particles. For these waves, the
plasma environment is inhomogenous due to the presence of the low-frequency
Alfv\'en wave with a large amplitude, a situation that may apply to space
plasmas such as the solar corona and solar wind. The dispersion relation of the
plasma waves is determined from a linear stability analysis using a new
eigenvalue method that is employed to solve the set of differential wave
equations which describe the propagation of plasma waves along the direction of
the constant component of the Alfv\'en wave magnetic field. This approach also
allows one to consider weak compressive effects. In the presence of the
background Alfv\'en wave, the dispersion branches obtained differ significantly
from the situation of a uniform plasma. Due to compressibility, acoustic waves
are excited and couplings between various modes occur, and even an instability
of the compressive mode. In a kinetic treatment, these plasma waves would be
natural candidates for Landau-resonant wave-particle interactions, and may thus
via their damping lead to particle heating.Comment: 15 pages, 5 figure
Kobayashi geodesics in A_g
We consider Kobayashi geodesics in the moduli space of abelian varieties A_g
that is, algebraic curves that are totally geodesic submanifolds for the
Kobayashi metric. We show that Kobayashi geodesics can be characterized as
those curves whose logarithmic tangent bundle splits as a subbundle of the
logarithmic tangent bundle of A_g.
Both Shimura curves and Teichmueller curves are examples of Kobayashi
geodesics, but there are other examples. We show moreover that non-compact
Kobayashi geodesics always map to the locus of real multiplication and that the
Q-irreducibility of the induced variation of Hodge structures implies that they
are defined over a number field.Comment: 2nd version, AMSLATeX, 20 pages. We fixed a gap in the proof of Lemma
3.1. The main theorems of our preprint are unchange
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