774 research outputs found
Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*
The recent detection of Sagittarius A* at lambda = 1.3 mm on a baseline from
Hawaii to Arizona demonstrates that millimeter wavelength very long baseline
interferometry (VLBI) can now spatially resolve emission from the innermost
accretion flow of the Galactic center region. Here, we investigate the ability
of future millimeter VLBI arrays to constrain the spin and inclination of the
putative black hole and the orientation of the accretion disk major axis within
the context of radiatively inefficient accretion flow (RIAF) models. We examine
the range of baseline visibility and closure amplitudes predicted by RIAF
models to identify critical telescopes for determining the spin, inclination,
and disk orientation of the Sgr A* black hole and accretion disk system. We
find that baseline lengths near 3 gigalambda have the greatest power to
distinguish amongst RIAF model parameters, and that it will be important to
include new telescopes that will form north-south baselines with a range of
lengths. If a RIAF model describes the emission from Sgr A*, it is likely that
the orientation of the accretion disk can be determined with the addition of a
Chilean telescope to the array. Some likely disk orientations predict
detectable fluxes on baselines between the continental United States and even a
single 10-12 m dish in Chile. The extra information provided from closure
amplitudes by a four-antenna array enhances the ability of VLBI to discriminate
amongst model parameters.Comment: Accepted for publication in ApJ
Approaching the event horizon: 1.3mm VLBI of SgrA*
Advances in VLBI instrumentation now allow wideband recording that
significantly increases the sensitivity of short wavelength VLBI observations.
Observations of the super-massive black hole candidate at the center of the
Milky Way, SgrA*, with short wavelength VLBI reduces the scattering effects of
the intervening interstellar medium, allowing observations with angular
resolution comparable to the apparent size of the event horizon of the putative
black hole. Observations in April 2007 at a wavelength of 1.3mm on a three
station VLBI array have now confirmed structure in SgrA* on scales of just a
few Schwarzschild radii. When modeled as a circular Gaussian, the fitted
diameter of SgrA* is 37 micro arcsec (+16,-10; 3-sigma), which is smaller than
the expected apparent size of the event horizon of the Galactic Center black
hole. These observations demonstrate that mm/sub-mm VLBI is poised to open a
new window onto the study of black hole physics via high angular resolution
observations of the Galactic Center.Comment: 6 pages, 4 figures, Proceedings for "The Universe under the
Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be
published in Journal of Physics: Conference Series by Institute of Physics
Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
Perturbing open cavities: Anomalous resonance frequency shifts in a hybrid cavity-nanoantenna system
The influence of a small perturbation on a cavity mode plays an important
role in fields like optical sensing, cavity quantum electrodynamics and cavity
optomechanics. Typically, the resulting cavity frequency shift directly relates
to the polarizability of the perturbation. Here we demonstrate that particles
perturbing a radiating cavity can induce strong frequency shifts that are
opposite to, and even exceed, the effects based on the particles'
polarizability. A full electrodynamic theory reveals that these anomalous
results rely on a non-trivial phase relation between cavity and nanoparticle
radiation, allowing back-action via the radiation continuum. In addition, an
intuitive model based on coupled mode theory is presented that relates the
phenomenon to retardation. Because of the ubiquity of dissipation, we expect
these findings to benefit the understanding and engineering of a wide class of
systems.Comment: 15 pages, 12 figure
Masses of Nearby Supermassive Black Holes with Very-Long Baseline Interferometry
Dynamical mass measurements to date have allowed determinations of the mass M
and the distance D of a number of nearby supermassive black holes. In the case
of Sgr A*, these measurements are limited by a strong correlation between the
mass and distance scaling roughly as M ~ D^2. Future very-long baseline
interferometric (VLBI) observations will image a bright and narrow ring
surrounding the shadow of a supermassive black hole, if its accretion flow is
optically thin. In this paper, we explore the prospects of reducing the
correlation between mass and distance with the combination of dynamical
measurements and VLBI imaging of the ring of Sgr A*. We estimate the signal to
noise ratio of near-future VLBI arrays that consist of five to six stations,
and we simulate measurements of the mass and distance of Sgr A* using the
expected size of the ring image and existing stellar ephemerides. We
demonstrate that, in this best-case scenario, VLBI observations at 1 mm can
improve the error on the mass by a factor of about two compared to the results
from the monitoring of stellar orbits alone. We identify the additional sources
of uncertainty that such imaging observations have to take into account. In
addition, we calculate the angular diameters of the bright rings of other
nearby supermassive black holes and identify the optimal targets besides Sgr A*
that could be imaged by a ground-based VLBI array or future space-VLBI missions
allowing for refined mass measurements.Comment: 8 pages, 4 figures, 2 tables, refereed version, accepted for
publication in Ap
Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance
We show that interferometry can be applied to study irregular, rapidly
rotating structures, as are expected in the turbulent accretion flow near a
black hole. Specifically, we analyze the lagged covariance between
interferometric baselines of similar lengths but slightly different
orientations. For a flow viewed close to face-on, we demonstrate that the peak
in the lagged covariance indicates the direction and angular velocity of the
emission pattern from the flow. Even for moderately inclined flows, the
covariance robustly estimates the flow direction, although the estimated
angular velocity can be significantly biased. Importantly, measuring the
direction of the flow as clockwise or counterclockwise on the sky breaks a
degeneracy in accretion disk inclinations when analyzing time-averaged images
alone. We explore the potential efficacy of our technique using
three-dimensional, general relativistic magnetohydrodynamic (GRMHD)
simulations, and we highlight several baseline pairs for the Event Horizon
Telescope (EHT) that are well-suited to this application. These results
indicate that the EHT may be capable of estimating the direction and angular
velocity of the emitting material near Sagittarius A*, and they suggest that a
rotating flow may even be utilized to improve imaging capabilities.Comment: 8 Pages, 4 Figures, accepted for publication in Ap
Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*
Millimeter very-long baseline interferometry (mm-VLBI) provides the novel
capacity to probe the emission region of a handful of supermassive black holes
on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole
at the center of the Milky Way, this provides access to the region in the
immediate vicinity of the horizon. Broderick et al. (2009) have already shown
that by leveraging spectral and polarization information as well as accretion
theory, it is possible to extract accretion-model parameters (including black
hole spin) from mm-VLBI experiments containing only a handful of telescopes.
Here we repeat this analysis with the most recent mm-VLBI data, considering a
class of aligned, radiatively inefficient accretion flow (RIAF) models. We find
that the combined data set rules out symmetric models for Sgr A*'s flux
distribution at the 3.9-sigma level, strongly favoring length-to-width ratios
of roughly 2.4:1. More importantly, we find that physically motivated accretion
flow models provide a significantly better fit to the mm-VLBI observations than
phenomenological models, at the 2.9-sigma level. This implies that not only is
mm-VLBI presently capable of distinguishing between potential physical models
for Sgr A*'s emission, but further that it is sensitive to the strong
gravitational lensing associated with the propagation of photons near the black
hole. Based upon this analysis we find that the most probable magnitude,
viewing angle, and position angle for the black hole spin are
a=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24)
east of north, where the errors quoted are the 1-sigma and 2-sigma
uncertainties.Comment: 15 pages, 10 figures, submitted to Ap
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