59 research outputs found
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 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
Source-intrinsic near-infrared properties of Sgr A*: Total intensity measurements
We present a comprehensive data description for Ks-band measurements of Sgr
A*. We characterize the statistical properties of the variability of Sgr A* in
the near-infrared, which we find to be consistent with a single-state process
forming a power-law distribution of the flux density. We discover a linear
rms-flux relation for the flux-density range up to 12 mJy on a timescale of 24
minutes. This and the power-law flux density distribution implies a
phenomenological, formally non-linear statistical variability model with which
we can simulate the observed variability and extrapolate its behavior to higher
flux levels and longer timescales. We present reasons why data with our cadence
cannot be used to decide on the question whether the power spectral density of
the underlying random process shows more structure at timescales between 25 min
and 100 min compared to what is expected from a red noise random process.Comment: Accepted to ApJS, August 27, 201
Evidence for a large-scale helical magnetic field in the quasar 3C 454.3
Most current theoretical models link the launching of relativistic jets from active galactic nuclei to the presence of twisted magnetic fields close to the supermassive black hole. While these models predict a large-scale, ordered, helical magnetic field near the central engine, it is not clear if, and to what extent, this order is preserved further downstream in the jet. Here, we present compelling evidence that suggests that the radio emission from the jet of the quasar 3C 454.3 exhibits multiple signatures of a large-scale, ordered helical magnetic field component at a distance of hundreds of parsecs from the launching point. Our results provide observational support for magnetic jet launching models and indicate that the ordered helical field component may remain stable over a large distance down the jet
On the location of the supermassive black hole in CTA102
Context. Relativistic jets in active galactic nuclei are one of the most powerful phenomena in the Universe. They form in the surroundings of the supermassive black holes as a by-product of accretion onto the central black hole in active galaxies. The flow in the jets propagates at velocities close to the speed of light. The distance between the first part of the jet that is visible in radio images (core) and the black hole is still a matter of debate. Aims. Only very-long-baseline interferometry observations resolve the innermost compact regions of the radio jet. These observations can access the jet base, and by combining data at different wavelenghts, address the physical parameters of the outflow from its emission. Methods. We have performed an accurate analysis of the frequency-dependent shift of the VLBI core location for a multi-wavelength set of images of the blazar CTA 102 including data from 6 cm down to 3 mm. Results. The measure of the position of the central black hole, with mass ~10^8.93M_⊙, in the blazar CTA 102 reveals a distance of ~8 × 10^4 gravitational radii to the 86 GHz core, in agreement with similar measures obtained for other blazars and distant radio galaxies, and in contrast with recent results for the case of nearby radio galaxies, which show distances between the black hole and the radio core that can be two orders of magnitude smaller
An evolving hot spot orbiting around Sgr A*
Here we report on recent near-infrared observations of the Sgr A* counterpart
associated with the super-massive ~ 4x10^6 M_sun black hole at the Galactic
Center. We find that the May 2007 flare shows the highest sub-flare contrast
observed until now, as well as evidence for variations in the profile of
consecutive sub-flares. We modeled the flare profile variations according to
the elongation and change of the shape of a spot due to differential rotation
within the accretion disk.Comment: 7 pages, 5 figures, contribution for the conference "The Universe
under the Microscope" (AHAR 2008), to be published in Journal of Physics:
Conference Series by Institute of Physics Publishin
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&
Tidal effects on small bodies by massive black holes
The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy
harbours a supermassive black hole, whose mass has been measured from stellar
orbital motions. Sgr A is therefore the nearest laboratory where super-massive
black hole astrophysics can be tested, and the environment of black holes can
be investigated. Since it is not an active galactic nucleus, it also offers the
possibility of observing the capture of small objects that may orbit the
central black hole. We study the effects of the strong gravitational field of
the black hole on small objects, such as a comet or an asteroid. We also
explore the idea that the flares detected in Sgr A might be produced by the
final accretion of single, dense objects with mass of the order of 10^20 g, and
that their timing is not a characteristic of the sources, but rather of the
space-time of the central galactic black hole in which they are moving. We find
that tidal effects are strong enough to melt the solid object, and present
calculations of the temporal evolution of the light curve of infalling objects
as a function of various parameters. Our modelling of tidal disruption suggests
that during tidal squeezing, the conditions for synchrotron radiation can be
met. We show that the light curve of a flare can be deduced from dynamical
properties of geodesic orbits around black holes and that it depends only
weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte
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