133 research outputs found
Imaging Optically-Thin Hot Spots Near the Black Hole Horizon of Sgr A* at Radio and Near-Infrared Wavelengths
Sub milli-arcsecond astrometry and imaging of the black hole Sgr A* at the
Galactic centre may become possible in the near future at infrared and
sub-millimetre wavelengths. Motivated by observations of short-term infrared
and X-ray variability of Sgr A*, in a previous paper we computed the expected
images and light curves, including polarization, associated with an compact
emission region orbiting the central black hole. We extend this work, using a
more realistic hot-spot model and including the effects of opacity in the
underlying accretion flow. We find that at infrared wavelengths the qualitative
features identified by our earlier work are present, namely it is possible to
extract the black hole mass and spin from spot images and light curves of the
observed flux and polarization. At radio wavelengths, disk opacity produces
significant departures from the infrared behaviour, but there are still generic
signatures of the black hole properties. Detailed comparison of these results
with future data can be used to test general relativity and to improve existing
models for the accretion flow in the immediate vicinity of the black hole.Comment: 13 pages, 26 figures, submitted to MNRA
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
Limits on the Position Wander of Sgr A*
We present measurements with the VLBA of the variability in the centroid
position of Sgr A* relative to a background quasar at 7-mm wavelength. We find
an average centroid wander of 71 +/- 45 micro-arcsec for time scales between 50
and 100 min and 113 +/- 50 micro-arcsec for timescales between 100 and 200 min,
with no secular trend. These are sufficient to begin constraining the viability
of the hot-spot model for the radio variability of Sgr A*. It is possible to
rule out hot spots with orbital radii above 15GM_SgrA*/c^2 that contribute more
than 30% of the total 7-mm flux. However, closer or less luminous hot spots
remain unconstrained. Since the fractional variability of Sgr A* during our
observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s
radio variability remains consistent with these limits. Improved monitoring of
Sgr A*'s centroid position has the potential to place significant constraints
upon the existence and morphology of inhomogeneities in a supermassive black
hole accretion flow.Comment: 14 pages, 3 figures submitted to Ap
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
Estimating the Parameters of Sgr A*'s Accretion Flow Via Millimeter VLBI
Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, for the
first time, directly probed distances comparable to the horizon scale of a
black hole. This provides unprecedented access to the environment immediately
around the horizon of an accreting black hole. We leverage both existing
spectral and polarization measurements and our present understanding of
accretion theory to produce a suite of generic radiatively inefficient
accretion flow (RIAF) models of Sgr A*, which we then fit to these recent
millimeter-VLBI observations. We find that if the accretion flow onto Sgr A* is
well described by a RIAF model, the orientation and magnitude of the black
hole's spin is constrained to a two-dimensional surface in the spin,
inclination, position angle parameter space. For each of these we find the
likeliest values and their 1-sigma & 2-sigma errors to be a=0(+0.4+0.7),
inclination=50(+10+30)(-10-10) degrees, and position angle=-20(+31+107)(-16-29)
degrees, when the resulting probability distribution is marginalized over the
others. The most probable combination is a=0(+0.2+0.4), inclination=90(-40-50)
degrees and position angle=-14(+7+11)(-7-11) degrees, though the uncertainties
on these are very strongly correlated, and high probability configurations
exist for a variety of inclination angles above 30 degrees and spins below
0.99. Nevertheless, this demonstrates the ability millimeter-VLBI observations,
even with only a few stations, to significantly constrain the properties of Sgr
A*.Comment: 10 pages, 7 figures, accepted by Ap
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