94 research outputs found
Positional Coincidence of H2O Maser and a Plasma Obscuring Torus in Radio Galaxy NGC 1052
We present multi-frequency simultaneous VLBA observations at 15, 22 and 43
GHz towards the nucleus of the nearby radio galaxy NGC 1052. These three
continuum images reveal a double-sided jet structure, whose relative intensity
ratios imply that the jet axis is oriented close to the sky plane. The steeply
rising spectra at 15-43 GHz at the inner edges of the jets strongly suggest
that synchrotron emission is absorbed by foreground thermal plasma. We detected
H2O maser emission in the velocity range of 1550-1850 km/s, which is redshifted
by 50-350 km/s with respect to the systemic velocity of NGC 1052. The
redshifted maser gas appears projected against both sides of the jet, in the
same manner as the HI seen in absorption. The H2O maser gas are located where
the free-free absorption opacity is large. This probably imply that the masers
in NGC 1052 are associated with a circumnuclear torus or disk as in the nucleus
of NGC 4258. Such circumnuclear structure can be the sence of accreting onto
the central engine.Comment: 18 pages, 7 figures, accepted for publication in Ap
The Dense Plasma Torus Around the Nucleus of an Active Galaxy NGC 1052
A subparsec-scale dense plasma torus around an active galactic nucleus (AGN)
is unveiled. We report on very-long-baseline interferometry (VLBI) observations
at 2.3, 8.4, and 15.4 GHz towards the active galaxy NGC 1052. The convex
spectra of the double-sided jets and the nucleus imply that synchrotron
emission is obscured through free--free absorption (FFA) by the foreground cold
dense plasma. A trichromatic image was produced to illustrate the distribution
of the FFA opacity. We found a central condensation of the plasma which covers
about 0.1 pc and 0.7 pc of the approaching and receding jets, respectively. A
simple explanation for the asymmetric distribution is the existence of a thick
plasma torus perpendicular to the jets. We also found an ambient FFA absorber,
whose density profile can be ascribed to a spherical distribution of the
isothermal King model. The coexistence of torus-like and spherical
distributions of the plasma suggests a transition from radial accretion to
rotational accretion around the nucleus.Comment: 10 pages, to appear in Publ. Astron. Soc. Japan, vol.53, No.2 (2001
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
High Resolution VSOP Imaging of a Southern Blazar PKS 1921-293 at 1.6 GHz
We present a high resolution 1.6 GHz VSOP image of the southern blazar PKS
1921-293. The image shows a typical core-jet morphology, consistent with
ground-based VLBI images. However, the addition of data from the space antenna
has greatly improved the angular resolution (especially along the north-south
direction for this source), and thus allowed us to clearly identify the core.
Model fitting reveals an inner jet component ~1.5 mas north of the core. This
jet feature may be moving on a common curved path connecting the jet within a
few parsecs to the 10-parsec-scale jet. The compact core has a brightness
temperature of 2.6*10**12 K (in the rest frame of the quasar), an indication of
relativistic beaming. We analyzed the source in terms of three models,
involving the inverse Compton catastrophe, an inhomogeneous relativistic jet,
and the equipartition of energy between the radiating particles and the
magnetic field. Our analysis of this gamma-ray-quiet blazar shows no preference
to any particular one of these models.Comment: 7 pages including 2 figures and 1 table, PASJLaTeX, accepted for
publication in PAS
Multifrequency Polarimetry of the Nrao 140 Jet: Possible Detection of a Helical Magnetic Field and Constraints on its Pitch Angle
We present results from multifrequency polarimetry of NRAO 140 using the Very
Long Baseline Array. These observations allow us to reveal the distributions of
both the polarization position angle and the Faraday rotation measure (RM).
These distributions are powerful tools to discern the projected and
line-of-sight components of the magnetic field, respectively. We find a
systematic gradient in the RM distribution, with its sign being opposite at
either side of the jet with respect to the jet axis. The sign of the RM changes
only with the direction of the magnetic field component along the line of
sight, so this can be explained by the existence of helical magnetic components
associated with the jet itself. We derive two constraints for the pitch angle
of the helical magnetic field from the distributions of the RM and the
projected magnetic field; the RM distribution indicates that the helical fields
are tightly wound, while that of the projected magnetic field suggests they are
loosely wound around the jet axis. This inconsistency may be explained if the
Faraday rotator is not cospatial with theemitting region. Our results may point
toward a physical picture in which an ultra-relativistic jet (spine) with a
loosely wound helical magnetic field is surrounded by a sub-relativistic wind
layer (sheath) with a tightly wound helical magnetic field.Comment: 12 pages, 4 figures, ApJ, in pres
ALMA polarimetry measures magnetically aligned dust grains in the torus of NGC 1068
The obscuring structure surrounding active galactic nuclei (AGN) can be
explained as a dust and gas flow cycle that fundamentally connects the AGN with
their host galaxies. This structure is believed to be associated with dusty
winds driven by radiation pressure. However, the role of magnetic fields, which
are invoked in almost all models for accretion onto a supermassive black hole
and outflows, is not thoroughly studied. Here we report the first detection of
polarized thermal emission by means of magnetically aligned dust grains in the
dusty torus of NGC 1068 using ALMA Cycle 4 polarimetric dust continuum
observations (, pc; 348.5 GHz, m). The polarized torus
has an asymmetric variation across the equatorial axis with a peak polarization
of \% and position angle of (B-vector) at
pc east from the core. We compute synthetic polarimetric observations of
magnetically aligned dust grains assuming a toroidal magnetic field and
homogeneous grain alignment. We conclude that the measured 860 m continuum
polarization arises from magnetically aligned dust grains in an optically thin
region of the torus. The asymmetric polarization across the equatorial axis of
the torus arises from 1) an inhomogeneous optical depth, and 2) a variation of
the velocity dispersion, i.e. variation of the magnetic field turbulence at
sub-pc scales, from the eastern to the western region of the torus. These
observations and modeling constrain the torus properties beyond spectral energy
distribution results. This study strongly supports that magnetic fields up to a
few pc contribute to the accretion flow onto the active nuclei.Comment: 19 pages, 11 figures (Accepted for Publication to ApJ
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