67 research outputs found
VLBI Observations of Water Masers in the Circumstellar Envelope of IRC+60169
Water masers around an AGB star, IRC+60169, were observed at four epochs
using the Japanese VLBI networks. The distribution of the maser features is
limited in a thick-shell region, which has inner and outer expansion velocities
of 7 km/s and 14 km/s at radii of 25 mas and 120 mas, respectively. The
distribution of the red-shifted features exhibits a ring-like structure, the
diameter of which is 30 mas, and corresponds to the inner radius of the maser
shell. This implies that dense gas around the star obscures red-shifted
emission. Although a position--radial velocity diagram for the maser features
is consistent with a spherical shell model, the relative proper motions do not
indicate an expansion motion of the shell. A remarkable property has been found
that is a possible periodic change of the alignment pattern of water maser
spots.Comment: 9 pages including 7 figures, to appear in PASJ, Vol. 54, No.
Magnetic Driving of Relativistic Outflows in Active Galactic Nuclei. I. Interpretation of Parsec-Scale Accelerations
There is growing evidence that relativistic jets in active galactic nuclei
undergo extended (parsec-scale) acceleration. We argue that, contrary to some
suggestions in the literature, this acceleration cannot be purely hydrodynamic.
Using exact semianalytic solutions of the relativistic MHD equations, we
demonstrate that the parsec-scale acceleration to relativistic speeds inferred
in sources like the radio galaxy NGC 6251 and the quasar 3C 345 can be
attributed to magnetic driving. Additional observational implications of this
model will be explored in future papers in this series.Comment: 6 pages, 2 figures, submitted to Ap
Multi-epoch VLBA observations of 3C 66A
We present the results of six-epoch Very Long Baseline Array (VLBA)
observations of 3C~66A. The high-resolution Very Long Baseline Interferometer
(VLBI) maps obtained at multi-frequency (2.3, 8.4, and 22.2 GHz) simultaneously
enabled us to identify the brightest compact component with the core. We find
that the spectrum of the core can be reasonably fitted by the synchrotron
self-absorption model. Our VLBA maps show that the jet of 3C~66A has two
bendings at about 1.2 and 4 mas from the core. We also give possible
identifications of our jet components with the components in previous VLBA
observations by analysing their proper motions. We find consistent differences
of the position from the core in one component between different frequencies at
six epochs.Comment: 10 pages, 5 figures, received 30 January 2007, accepted 22 March 200
Orbital Motion in the Radio Galaxy 3C 66B: Evidence for a Supermassive Black Hole Binary
Supermassive black hole binaries may exist in the centers of active galactic
nuclei like quasars and radio galaxies and mergers between galaxies may result
in the formation of supermassive binaries during the course of galactic
evolution. Using the very-long-baseline interferometer, we imaged the radio
galaxy 3C 66B at radio frequencies and found that the unresolved radio core of
3C 66B shows well-defined elliptical motions with a period of 1.05 +/- 0.03
years, which provides a direct detection of a supermassive black hole binary.Comment: 8 pages, 3 figures, to appear in the Scienc
Large-Scale Regular Morphological Patterns in the Radio Jet of NGC 6251
We report on large-scale, regular morphological patterns found in the radio
jet of the nearby radio galaxy NGC 6251. Investigating morphological properties
of this radio jet from the nucleus to a radial distance of 300 arcsec
( 140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, &
Willis, we find three chains, each of which consists of five radio knots. We
also find that eight radio knots in the first two chains consist of three small
sub-knots (the triple-knotty substructures). We discuss the observational
properties of these regular morphological patterns.Comment: 8 figures, 15 pages, accepted for publication in A
Is Thermal Expansion Driving the Initial Gas Ejection in NGC 6251?
In this paper, we explore the possibility that the radiative properties of
the most compact region in NGC 6251* may be understood in the same sense as Sgr
A*, though with some telling differences that may hint at the nature of jet
formation. We show that observations of this object with ASCA, ROSAT, HST and
VLBI together may be hinting at a picture in which Bondi-Hoyle accretion from
an ambient ionized medium feeds a standard disk accreting at ~ 4.0*10^{22} g
s^{-1}. Somewhere near the event horizon, this plasma is heated to >10^{11} K,
where it radiates via thermal synchrotron (producing a radio component) and
self-Comptonization (accounting for a nonthermal X-ray flux). This temperature
is much greater than its virial value and the hot cloud expands at roughly the
sound speed (~0.1c), after which it begins to accelerate on a parsec scale to
relativistic velocities. In earlier work, the emission from the extended jet
has been modeled successfully using nonthermal synchrotron self-Compton
processes, with a self-absorbed inner core. In the picture we are developing
here, the initial ejection of matter is associated with a self-absorbed thermal
radio component that dominates the core emission on the smallest scales. The
nonthermal particle distributions responsible for the emission in the extended
jet are then presumably energized, e.g., via shock acceleration, within the
expanding, hot gas. The power associated with this plasma represents an
accretion efficiency of about 0.54, requiring dissipation in a prograde disk
around a rapidly spinning black hole (with spin parameter a~1).Comment: 17 pages, 1 figures, to appear in Ap
Sub-parsec-scale Accleration of the Radio Jet in the Powerful Radio Galaxy NGC 6251
In order to investigate the genesis of powerful radio jet, we have mapped the
central 10 pc region of the nearby radio galaxy NGC 6251 with a 0.2 pc
resolution using Very Long Baseline Interferometer (VLBI) at two radio
frequencies, 5 GHz and 15 GHz, we have found the sub-parsec-scale counterjet
for the first time in this radio galaxy. This discovery allows us to
investigate the jet acceleration based on the relativistic beaming model.Comment: 7 pages with 7 figures. To appear in PASJ, 52, No. 5, Oct. 25, 200
What Happened to the NGC 6251 Counterjet?
We have used the VLBA to produce a high dynamic range image of the nucleus of
NGC 6251 at 1.6 GHz and snapshot images at 5.0, 8.4, and 15.3 GHz to search for
emission from a parsec-scale counterjet. Previous VLBI images at 1.6 GHz have
set a lower limit for the jet/counterjet brightness ratio near the core at
about 80:1, which is larger than expected given the evidence that the radio
axis is fairly close to the plane of the sky. A possible explanation is that
the inner few pc of the counterjet is hidden by free-free absorption by ionized
gas associated with an accretion disk or torus. This would be consistent with
the nearly edge-on appearance of the arcsecond-scale dust disk seen in the
center of NGC 6251 by HST. We detect counterjet emission close to the core at
1.6 GHz, but not at the higher frequencies. Given that the optical depth of
free-free absorption falls off more rapidly with increasing frequency than the
optically thin synchrotron emission from a typical radio jet, this result
implies that the absence of a detectable parsec-scale counterjet at high
frequencies is not due to free-free absorption unless the density of ionized
gas is extremely high and we have misidentified the core at 1.6 GHz. The most
likely alternative is a large jet/counterjet brightness ratio caused by
relativistic beaming, which in turn requires the inner radio axis to be closer
to our line of sight than the orientation of the HST dust disk would suggest.Comment: Accepted for publication in ApJ. Includes 10 figure
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