1,980 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
Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster
The aims of this work are to provide accurate photometry in multiple
near-infrared broadband filters, to determine the power-law index of the
extinction-law toward the central parsec of the Galaxy, to provide measurements
of the absolute extinction toward the Galactic center, and finally to measure
the spatial variability of extinction on arcsecond scales.We use adaptive
optics observations of the central parsec of the Milky Way. Absolute values for
the extinction in the H, Ks, and L'-bands as well as of the power-law indices
of the H to Ks and Ks to L' extinction-laws are measured based on the
well-known properties of red clump stars. Extinction maps are derived based on
H-Ks and Ks-L' colors. We present Ks-band photometry for ~7700 stars (H and L'
photometry for a subset). From a number of recently published values we compute
a mean distance of the Galactic center of R_0=8.03+-0.15 kpc, which has an
uncertainty of just 2%. Based on this R_0 and on the RC method, we derive
absolute mean extinction values toward the central parsec of the Galaxy of
A_H=4.48+-0.13 mag, A_Ks=2.54+-0.12$ mag, and A_L'=1.27+-0.18 mag. We estimate
values of the power-law indices of the extinction-law of
alpha_{H-Ks}=2.21+-0.24 and alpha_{Ks-L'}=1.34+-0.29. A Ks-band extinction map
for the Galactic center is computed based on this extinction law and on stellar
H-Ks colors. Mean extinction values in a circular region with 0.5" radius
centered on Sagittarius A* are A_{H, SgrA*}=4.35+-0.12, A_{Ks,
SgrA*}=2.46+-0.03, and A_{L', SgrA*}=1.23+-0.08.Comment: accepted for publication by Astronomy & Astrophysics; please contact
RS for higher quality figure
Effect of zooplankton-mediated trophic cascades on marine microbial food web components (bacteria, nanoflagellates, ciliates)
To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions
Intensive monitoring of the strongly variable BL Lac S5 0716+714
The BL Lac object S5 0716+714 was monitored during a multifrequency campaign
in 1996. Preliminary analysis of the optical, ROSAT and RXTE data are
presented. Strong variability on short time scales was observed. The data
suggest an interpretation within a multi-component model.Comment: To appear in The Active X-ray Sky: Results from BeppoSAX and
Rossi-XTE, Rome, Italy, 21-24 October, 1997. Eds.: L. Scarsi, Bradt, P.
Giommi and F. Fiore. PS-file avialable at
http://www.lsw.uni-heidelberg.de/projects/extragalactic/bl_lac.htm
First proper motions of thin dust filaments at the Galactic Center
Context: L'-band (3.8 micron) images of the Galactic Center show a large
number of thin filaments in the mini-spiral, located west of the mini-cavity
and along the inner edge of the Northern Arm. One possible mechanism that could
produce such structures is the interaction of a central wind with the
mini-spiral. Additionally, we identify similar features that appear to be
associated with stars. Aims: We present the first proper motion measurements of
the thin dust filaments observed in the central parsec around SgrA* and
investigate possible mechanisms that could be responsible for the observed
motions. Methods: The observations have been carried out using the NACO
adaptive optics system at the ESO VLT. The images have been transformed to a
common coordinate system and features of interest were extracted. Then a
cross-correlation technique could be performed in order to determine the
offsets between the features with respect to their position in the reference
epoch. Results: We derive the proper motions of a number of filaments and 2
cometary shaped dusty sources close (in projection) to SgrA*. We show that the
shape and the motion of the filaments does not agree with a purely Keplerian
motion of the gas in the potential of the supermassive black hole at the
position of SgrA*. Therefore, additional mechanisms must be responsible for
their formation and motion. We argue that the properties of the filaments are
probably related to an outflow from the disk of young mass-losing stars around
SgrA*. In part, the outflow may originate from the black hole itself. We also
present some evidence and theoretical considerations that the outflow may be
collimated.Comment: accepted for publication by A&
Near infrared flares of Sagittarius A*: Importance of near infrared polarimetry
We report on the results of new simulations of near-infrared (NIR)
observations of the Sagittarius A* (Sgr A*) counterpart associated with the
super-massive black hole at the Galactic Center. The observations have been
carried out using the NACO adaptive optics (AO) instrument at the European
Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru
telescope (13 June 2004, 30 July 2005, 1 June 2006, 15 May 2007, 17 May 2007
and 28 May 2008). We used a model of synchrotron emission from relativistic
electrons in the inner parts of an accretion disk. The relativistic simulations
have been carried out using the Karas-Yaqoob (KY) ray-tracing code. We probe
the existence of a correlation between the modulations of the observed flux
density light curves and changes in polarimetric data. Furthermore, we confirm
that the same correlation is also predicted by the hot spot model. Correlations
between intensity and polarimetric parameters of the observed light curves as
well as a comparison of predicted and observed light curve features through a
pattern recognition algorithm result in the detection of a signature of
orbiting matter under the influence of strong gravity. This pattern is detected
statistically significant against randomly polarized red noise. Expected
results from future observations of VLT interferometry like GRAVITY experiment
are also discussed.Comment: 26 pages, 38 figures, accepted for publication by A&
Simultaneous NIR/sub-mm observation of flare emission from SgrA*
We report on a successful, simultaneous observation and modeling of the
sub-millimeter to near-infrared flare emission of the Sgr A* counterpart
associated with the super-massive black hole at the Galactic center. Our
modeling is based on simultaneous observations that have been carried out on 03
June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and
the LABOCA bolometer at the APEX telescope. Inspection and modeling of the
light curves show that the sub-mm follows the NIR emission with a delay of
1.5+/-0.5 hours. We explain the flare emission delay by an adiabatic expansion
of the source components.Comment: 12 pages, 9 figures, 3 tables, in press with A&
Coordinated NIR/mm observations of flare emission from Sagittarius A*
We report on a successful, simultaneous observation and modelling of the
millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart
associated with the supermassive black hole at the Galactic centre (GC). We
present a mm/sub-mm light curve of Sgr A* with one of the highest quality
continuous time coverages and study and model the physical processes giving
rise to the variable emission of Sgr A*.Comment: 14 pages, 16 figure
The extreme luminosity states of Sagittarius A*
We discuss mm-wavelength radio, 2.2-11.8um NIR and 2-10 keV X-ray light
curves of the super massive black hole (SMBH) counterpart of Sagittarius A*
(SgrA*) near its lowest and highest observed luminosity states. The luminosity
during the low state can be interpreted as synchrotron emission from a
continuous or even spotted accretion disk. For the high luminosity state SSC
emission from THz peaked source components can fully account for the flux
density variations observed in the NIR and X-ray domain. We conclude that at
near-infrared wavelengths the SSC mechanism is responsible for all emission
from the lowest to the brightest flare from SgrA*. For the bright flare event
of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC
model combined with adiabatic expansion can explain the related peak
luminosities and different widths of the flare profiles obtained in the NIR and
X-ray regime as well as the non detection in the radio domain.Comment: 18 pages, 13 figures, accepted by A&
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