1,234 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 Near-Infrared Photometric Properties of Bright Giants in the Central Regions of the Galactic Bulge
Images recorded through broad (J, H, K), and narrow (CO, and 2.2micron
continuum) band filters are used to investigate the photometric properties of
bright (K < 13.5) stars in a 6 x 6 arcmin field centered on the SgrA complex.
The giant branch ridgelines in the (K, J-K) and (K, H-K) color-magnitude
diagrams are well matched by the Baade's Window (BW) M giant sequence if the
mean extinction is A_K ~ 2.8 mag. Extinction measurements for individual stars
are estimated using the M_K versus infrared color relations defined by M giants
in BW, and the majority of stars have A_K between 2.0 and 3.5 mag. The
extinction is locally high in the SgrA complex, where A_K ~ 3.1 mag.
Reddening-corrected CO indices, CO_o, are derived for over 1300 stars with J,
H, and K brightnesses, and over 5300 stars with H and K brightnesses. The
distribution of CO_o values for stars with K_o between 11.25 and 7.25 can be
reproduced using the M_K versus CO_o relation defined by M giants in BW. The
data thus suggest that the most metal-rich giants in the central regions of the
bulge and in BW have similar photometric properties and 2.3micron CO strengths.
Hence, it appears that the central region of the bulge does not contain a
population of stars that are significantly more metal-rich than what is seen in
BW.Comment: 29 pages, including 14 figure
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&
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
K-band polarimetry of an Sgr A* flare with a clear sub-flare structure
Context: The supermassive black hole at the Galactic center, SgrA*, shows
frequent radiation outbursts, often called 'flares'. In the near-infrared some
of these flares were reported as showing intrinsic quasi-periodicities. The
flux peaks associated with the quasi-periodic behavior were found to be highly
polarized. Aims: The aim of this work is to present new evidence to support
previous findings of the properties of the polarized radiation from SgrA* and
to again provide strong support for the quasi-periodicity of ~18+-3 min
reported earlier. Methods: Observations were carried out at the European
Southern Observatory's Very Large Telescope on Paranal, Chile. We used the
NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the
polarimetric lightcurves with a hot-spot model, we addressed the question of
whether the data are consistent with this model. To fit the observed data we
used a general relativistic ray-tracing code in combination with a simple
hot-spot/ring model. Results: We report on new polarization measurements of a
K-band flare from the supermassive black hole at the Galactic center. The data
provide very strong support for a quasi-periodicity of 15.5+-2 min. The mean
polarization of the flare is consistent with the direction of the electric
field vector that was reported in previous observations. The data can be
modeled successfully with a combined blob/ring model. The inclination i of the
blob orbit must be i > 20 deg on a 3sigma level, and the dimensionless spin
parameter of the black hole is derived to be a > 0.5.Comment: accepted by A&A Letters for publication; 5 pages, 9 figure
X-ray Images of Hot Accretion Flows
We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor
change
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
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