2,150 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
Radio Variability of Sagittarius A* - A 106 Day Cycle
We report the presence of a 106-day cycle in the radio variability of Sgr A*
based on an analysis of data observed with the Very Large Array (VLA) over the
past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of
cycle frequency to frequency width f/Delta_f= 2.2+/-0.3. The periodic signal is
also clearly observed at 2 cm. At 3.6 cm the detection of a periodic signal is
marginal. No significant periodicity is detected at both 6 and 20 cm. Since the
sampling function is irregular we performed a number of tests to insure that
the observed periodicity is not the result of noise. Similar results were found
for a maximum entropy method and periodogram with CLEAN method. The probability
of false detection for several different noise distributions is less than 5%
based on Monte Carlo tests. The radio properties of the pulsed component at 1.3
cm are spectral index alpha ~ 1.0+/- 0.1 (for S nu^alpha), amplitude Delta
S=0.42 +/- 0.04 Jy and characteristic time scale Delta t_FWHM ~ 25 +/- 5 days.
The lack of VLBI detection of a secondary component suggests that the
variability occurs within Sgr A* on a scale of ~5 AU, suggesting an instability
of the accretion disk.Comment: 14 Pages, 3 figures. ApJ Lett 2000 accepte
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&
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
Multiwavelength VLBI observations of Sagittarius A*
The compact radio source Sgr\,A*, associated with the super massive black
hole at the center of the Galaxy, has been studied with VLBA observations at 3
frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The
total VLBI flux density of Sgr\,A* varies from day to day. The variability is
correlated at the 3 observing frequencies with higher variability amplitudes
appearing at the higher frequencies. For the modulation indices, we find 8.4\,%
at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is
inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron
self-absorption with a turnover frequency at or above 86\,GHz. The radio
spectral index correlates with the flux density, which is harder (more inverted
spectrum) when the source is brighter. The average source size does not appear
to be variable over the 10-day observing interval. However, we see a tendency
for the sizes of the minor axis to increase with increasing total flux, whereas
the major axis remains constant. Towards higher frequencies, the position angle
of the elliptical Gaussian increases, indicative of intrinsic structure, which
begins to dominate the scatter broadening. At cm-wavelength, the source size
varies with wavelength as , which is interpreted as the
result of interstellar scatter broadening. After removal of this scatter
broadening, the intrinsic source size varies as . The
VLBI closure phases at 22, 43, and 86\,GHz are zero within a few degrees,
indicating a symmetric or point-like source structure. In the context of an
expanding plasmon model, we obtain an upper limit of the expansion velocity of
about 0.1\,c from the non-variable VLBI structure. This agrees with the
velocity range derived from the radiation transport modeling of the flares from
the radio to NIR wavelengths.}Comment: 14pages, 14 Figures, Accepted for publication in 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&
Resolving the Host Galaxy of the Nearby QSO I Zw 1 with Sub-Arcsecond Multi-Transition Molecular Line Observations
We present the first sub-kpc 0.7" (~ 850 pc) resolution 12CO(1-0) molecular
line observations of the ISM in the host galaxy of the QSO I Zw 1. The
observations were obtained with the BIMA mm-interferometer in its compact A
configuration. The BIMA data are complemented by new observations of the
12CO(2-1) and 13CO(1-0) line with IRAM Plateau de Bure mm-interferometer (PdBI)
at 0.9" and 1.9" resolution, respectively. These measurements, which are part
of a multi-wavelength study of the host galaxy of I Zw 1, are aimed at
comparing the ISM properties of a QSO host with those of nearby galaxies as
well as to obtain constraints on galaxy formation/evolution models. Our images
of the 12CO(1-0) line emission show a ring-like structure in the circumnuclear
molecular gas distribution with an inner radius of about 1.2 kpc. The presence
of such a molecular gas ring was predicted from earlier lower angular
resolution PdBI 12CO(1-0) observations. A comparison of the BIMA data with IRAM
PdBI 12CO(2-1) observations shows variations in the excitation conditions of
the molecular gas in the innermost 1.5" comprising the nuclear region of I Zw
1. The observed properties of the molecular cloud complexes in the disk of the
host galaxy suggest that they can be the sites of massive circumnuclear star
formation, and show no indications of excitation by the nuclear AGN. This all
indicates that the molecular gas in a QSO host galaxy has similar properties to
the gas observed in nearby low luminosity AGNs.Comment: to be published in ApJ 1 July 2004 issu
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&
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