353 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
On the orientation of the Sagittarius A* system
The near-infrared emission from the black hole at the Galactic center (Sgr
A*) has unique properties. The most striking feature is a suggestive periodic
sub-structure that has been observed in a couple of flares so far. Using
near-infrared polarimetric observations and modelling the quasi-periodicity in
terms of an orbiting blob, we try to constrain the three dimensional
orientation of the Sgr A* system. We report on so far unpublished polarimetric
data from 2003. They support the observations of a roughly constant mean
polarization angle of 60 degr \pm 20 degr from 2004-2006. Prior investigations
of the 2006 data are deepened. In particular, the blob model fits are evaluated
such that constraints on the position angle of Sgr A* can be derived.
Confidence contours in the position-inclination angle plane are derived. On a
3sigma level the position angle of the equatorial plane normal is in the range
60 degr - 108 degr (east of north) in combination with a large inclination
angle. This agrees well with recent independent work in which radio
spectral/morphological properties of Sgr A* and X-ray observations,
respectively, have been used. However, the quality of the presently available
data and the uncertainties in our model bring some ambiguity to our
conclusions.Comment: 10 pages, 4 figures; Research Note accepted by A&A for publicatio
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&
The mean infrared emission of SagittariusA*
(abridged) The massive black hole at the center of the Milky Way,
SagittariusA* is, in relative terms, the weakest accreting black hole
accessible to observations. At the moment, the mean SED of SgrA* is only known
reliably in the radio to mm regimes. The goal of this paper is to provide
constraints on the mean emission from SgrA* in the near-to-mid infrared.
Excellent imaging quality was reached in the MIR by using speckle imaging
combined with holographic image reconstruction, a novel technique for this kind
of data. No counterpart of SgrA* is detected at 8.6 microns. At this
wavelength, SgrA* is located atop a dust ridge, which considerably complicates
the search for a potential point source. An observed 3 sigma upper limit of ~10
mJy is estimated for the emission of SgrA* at 8.6 microns, a tighter limit at
this wavelength than in previous work. The de-reddened 3 sigma upper limit,
including the uncertainty of the extinction correction, is ~84 mJy . Based on
the available data, it is argued that, with currently available instruments,
SgrA* cannot be detected in the MIR, not even during flares. At 4.8 and 3.8
microns, on the other hand, SgrA* is detected at all times, at least when
considering timescales of a few up to 13 min. We derive well-defined
time-averaged, de-reddened flux densities of 3.8+-1.3 mJy at 4.8 microns and
5.0+-0.6 mJy at 3.8 microns. Observations with NIRC2/Keck and NaCo/VLT from the
literature provide good evidence that SgrA* also has a fairly well-defined
de-reddened mean flux of 0.5-2.5 mJy at wavelengths of 2.1-2.2 microns. We
present well-constrained anchor points for the SED of SgrA* on the
high-frequency side of the Terahertz peak. The new data are in general
agreement with published theoretical SEDs of the mean emission from SgrA*, but
we expect them to have an appreciable impact on the model parameters in future
theoretical work.Comment: accepted for publication by Astronomy & Astrophysics on 20 June 201
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&
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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