2,224 research outputs found
Accretion Disk Evolution With Wind Infall I. General Solution and Application to Sgr A*
The evolution of an accretion disk can be influenced significantly by the
deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such
a mass influx impacts the long-term behavior of the disk by providing
additional sources of viscosity and heating. In this paper, we derive and solve
the disk equations when these effects are taken into account. We present a
survey of models with various wind configurations and demonstrate that the disk
spectrum may then differ substantially from that of a standard alpha-disk. In
particular, it is likely that a wind-fed disk has a significant infrared bump
due to the deposition of energy in its outer region. We apply some of the
results of our calculations to the Galactic Center black hole candidate Sgr A*
and show that if a fossil disk is present in this source, it must have a very
low viscosity parameter (alpha<10^-4) and the Bondi-Hoyle wind must be
accreting with a very high specific angular momentum to prevent it from
circularizing in the inner disk region where its impact would be most
noticeable.Comment: accepted for The Astrophysical Journal, AAS LaTex, 20 pages, also
available at http://www.astro.umd.edu/~hfalcke/publications.html#wintercep
Outflow-Dominated Emission from the Quiescent Massive Black Holes in NGC 4621 and NGC 4697
The nearby elliptical galaxies NGC 4621 and NGC 4697 each host a supermassive
black hole with a mass more than 1e8 Solar masses. Analysis of archival Chandra
data and new NRAO Very Large Array data shows that each galaxy contains a
low-luminosity active galactic nucleus (LLAGN), identified as a faint, hard
X-ray source that is astrometrically coincident with a faint 8.5-GHz source.
The latter has a diameter less than 0.3 arcsec (26 pc for NGC 4621, 17 pc for
NGC 4697). The black holes energizing these LLAGNs have Eddington ratios L(2-10
keV) / L(Edd) ~ 1e-9, placing them in the so-called quiescent regime. The
emission from these quiescent black holes is radio-loud, with log Rx = log
nuLnu(8.5 GHz) / L(2-10 keV) ~ -2, suggesting the presence of a radio outflow.
Also, application of the radio-X-ray-mass relation from Yuan & Cui for
quiescent black holes predicts the observed radio luminosities nuLnu(8.5 GHz)
to within a factor of a few. Significantly, that relation invokes X-ray
emission from the outflow rather than from an accretion flow. The faint, but
detectable, emission from these two massive black holes is therefore consistent
with being outflow-dominated. Observational tests of this finding are
suggested.Comment: 11 pages; 4 figures: emulateapj; to appear in Ap
Viewing the Shadow of the Black Hole at the Galactic Center
In recent years, the evidence for the existence of an ultra-compact
concentration of dark mass associated with the radio source Sgr A* in the
Galactic Center has become very strong. However, an unambiguous proof that this
object is indeed a black hole is still lacking. A defining characteristic of a
black hole is the event horizon. To a distant observer, the event horizon casts
a relatively large ``shadow'' with an apparent diameter of ~10 gravitational
radii due to bending of light by the black hole, nearly independent of the
black hole spin or orientation. The predicted size (~30 micro-arcseconds) of
this shadow for Sgr A* approaches the resolution of current
radio-interferometers. If the black hole is maximally spinning and viewed
edge-on, then the shadow will be offset by ~8 micro-arcseconds from the center
of mass, and will be slightly flattened on one side. Taking into account
scatter-broadening of the image in the interstellar medium and the finite
achievable telescope resolution, we show that the shadow of Sgr A* may be
observable with very long-baseline interferometry at sub-millimeter
wavelengths, assuming that the accretion flow is optically thin in this region
of the spectrum. Hence, there exists a realistic expectation of imaging the
event horizon of a black hole within the next few years.Comment: 5 pages, 1 figure (color), (AAS)Tex, to appear in The Astrophysical
Journal Letters, Vol. 528, L13 (Jan 1, 2000 issue); also available at
http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#bhimag
The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm
We present new high-resolution observations of Sagittarius A* at wavelengths
of 17.4 to 23.8 cm with the Very Large Array in A configuration with the Pie
Town Very Long Baseline Array antenna. We use the measured sizes to calibrate
the interstellar scattering law and find that the major axis size of the
scattering law is smaller by ~6% than previous estimates. Using the new
scattering law, we are able to determine the intrinsic size of Sgr A* at
wavelengths from 0.35 cm to 6 cm using existing results from the VLBA. The new
law increases the intrinsic size at 0.7 cm by ~20% and <5% at 0.35 cm. The
intrinsic size is 13^{+7}_{-3} Schwarzschild radii at 0.35 cm and is
proportional to lambda^gamma, where gamma is in the range 1.3 to 1.7.Comment: ApJL, in pres
Hot Accretion With Conduction: Spontaneous Thermal Outflows
Motivated by the low-collisionality of gas accreted onto black holes in Sgr
A* and other nearby galactic nuclei, we study a family of 2D advective
accretion solutions with thermal conduction. While we only impose global
inflow, the accretion flow spontaneously develops bipolar outflows. The role of
conduction is key in providing the extra degree of freedom (latitudinal energy
transport) necessary to launch these rotating thermal outflows. The sign of the
Bernoulli constant does not discriminate between inflowing and outflowing
regions. Our parameter survey covers mass outflow rates from ~ 0 to 13% of the
net inflow rate, outflow velocities from ~0 to 11% of the local Keplerian
velocity and outflow opening angles from ~ 0 to 60 degs. As the magnitude of
conduction is increased, outflows can adopt a conical geometry, pure inflow
solutions emerge, and the limit of 2D non-rotating Bondi-like solutions is
eventually reached. These results confirm that radiatively-inefficient, hot
accretion flows have a hydrodynamical propensity to generate bipolar thermal
outflows.Comment: 38 pages, 10 figures, accepted for publication in Ap
Actin-based propulsion of spatially extended objects
We propose a mathematical model of the actin-based propulsion of spatially extended obstacles. It starts from the properties of individual actin filaments and includes transient attachment to the obstacle, polymerization as well as cross-linking. Two particular geometries are discussed, which apply to the motion of protein-coated beads in a cell-like medium and the leading edge of a cell protrusion, respectively. The model gives rise to both steady and saltatory movement of beads and can explain the experimentally observed transitions of the dynamic regime with changing bead radius and protein surface density. Several spatiotemporal patterns are obtained with a soft obstacle under tension, including the experimentally observed spontaneous emergence of lateral traveling waves in crawling cells. Thus, we suggest a unifying mechanism for systems that are currently described by differential concepts
Modeling the X-ray Contribution of X-ray Binary Jets
Astrophysical jets exist in both XRBs and AGN, and seem to share common
features, particularly in the radio. While AGN jets are known to emit X-rays,
the situation for XRB jets is not so clear. Radio jets have been resolved in
several XRBs in the low/hard state, establishing that some form of outflow is
routinely present in this state. Interestingly, the flat-to-inverted radio
synchrotron emission associated with these outflows strongly correlates with
the X-ray emission in several sources, suggesting that the jet plasma plays a
role at higher frequencies. In this same state, there is increasing evidence
for a turnover in the IR/optical where the flat-to-inverted spectrum seems to
connect to an optically thin component extending into the X-rays. We discuss
how jet synchrotron emission is likely to contribute to the X-rays, in addition
to inverse Compton up-scattering, providing a natural explanation for these
correlations and the turnover in the IR/optical band. We present model
parameters for fits to several sources, and address some common misconceptions
about the jet model.Comment: 4 pages, 1 Table, conference proceedings for "The Physics of
Relativistic Jets in the Chandra and XMM Era, Bologna, 2002", Eds. G.
Brunetti, D. E. Harris, R. M. Sambruna & G. Sett
Towards Proper Motions in the Local Group
Key and still largely missing parameters for measuring the mass content and
distribution of the Local Group are the proper motion vectors of its member
galaxies. The problem when trying to derive the gravitational potential of the
Local Group is that usually only radial velocities are known, and hence
statistical approaches have to be used. The expected proper motions for
galaxies within the Local Group, ranging from 20 to 100 as/yr, are
detectable with VLBI using the phase-referencing technique. We present
phase-referencing observations of bright masers in IC~10 and M33 with respect
to background quasars. We observed the HO masers in IC10 three times over a
period of two months to check the accuracy of the relative positions. The
relative positions were obtained by modeling the interferometer phase data for
the maser sources referenced to the background quasars. The model allowed for a
relative position shift for the source and a single vertical atmospheric delay
error in the correlator model for each antenna. The rms of the relative
positions for the three observations is only 0.01 mas, which is approximately
the expected position error due to thermal noise. Also, we present a method to
measure the geometric distance to M33. This will allow re-calibration of the
extragalactic distance scale based on Cepheids. The method is to measure the
relative proper motions of two HO maser sources on opposite sides of M33.
The measured angular rotation rate, coupled with other measurements of the
inclination and rotation speed of the galaxy, yields a direct distance
measurement.Comment: 4 pages, Proceedings of the 6th European VLBI Network Symposium, Ros,
E., Porcas, R.W., Zensus, J.A. (eds.), MPIfR, Bonn, Germany (2002); Also
availabe http://www.mpifr-bonn.mpg.de/staff/abrunthaler/brunthal01.p
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