500 research outputs found
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.
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
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
The Event Horizon of M87
The 6 billion solar mass supermassive black hole at the center of the giant
elliptical galaxy M87 powers a relativistic jet. Observations at millimeter
wavelengths with the Event Horizon Telescope have localized the emission from
the base of this jet to angular scales comparable to the putative black hole
horizon. The jet might be powered directly by an accretion disk or by
electromagnetic extraction of the rotational energy of the black hole. However,
even the latter mechanism requires a confining thick accretion disk to maintain
the required magnetic flux near the black hole. Therefore, regardless of the
jet mechanism, the observed jet power in M87 implies a certain minimum mass
accretion rate. If the central compact object in M87 were not a black hole but
had a surface, this accretion would result in considerable thermal
near-infrared and optical emission from the surface. Current flux limits on the
nucleus of M87 strongly constrain any such surface emission. This rules out the
presence of a surface and thereby provides indirect evidence for an event
horizon.Comment: 9 pages, 2 figures, submitted to Ap
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
Event-Horizon-Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk
Observations of the black hole in the center of the Milky Way with the Event
Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is
smaller than the size of the black-hole shadow. This can be reconciled with the
spectral properties of the source, if the accretion flow is seen at a
relatively high inclination (50-60 degrees). Such an inclination makes the
angular momentum of the flow, and perhaps of the black hole, nearly aligned
with the angular momenta of the orbits of stars that lie within 3 arcsec from
the black hole. We discuss the implications of such an alignment for the
properties of the black hole and of its accretion flow. We argue that future
Event-Horizon-Telescope observations will not only refine the inclination of
Sgr A* but also measure precisely its orientation on the plane of the sky.Comment: To appear in the Astrophysical Journa
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