170 research outputs found
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
The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales
We present the detection of compact radio structures of fourteen radio-loud
narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array
observations at 5 GHz, which were performed in 2013. While 50\% of the sources
of our sample show a compact core only, the remaining 50\% exhibit a core-jet
structure. The measured brightness temperatures of the cores range from
to K with a median value of K, indicating
that the radio emission is from non-thermal jets, and that, likely, most
sources are not strongly beamed, then implying a lower jet speed in these
radio-loud NLS1 galaxies. In combination with archival data taken at multiple
frequencies, we find that seven sources show flat or even inverted radio
spectra, while steep spectra are revealed in the remaining seven objects.
Although all these sources are very radio-loud with , their jet
properties are diverse, in terms of their milli-arcsecond (mas) scale (pc
scale) morphology and their overall radio spectral shape. The evidence for slow
jet speeds (i.e., less relativistic jets), in combination with the low
kinetic/radio power, may offer an explanation for the compact VLBA radio
structure in most sources. The mildly relativistic jets in these high accretion
rate systems are consistent with a scenario, where jets are accelerated from
the hot corona above the disk by the magnetic field and the radiation force of
the accretion disk. Alternatively, a low jet bulk velocity can be explained by
low spin in the Blandford-Znajek mechanism.Comment: 39 pages, 17 figures, ApJS accepte
Rotating and infalling motion around the high-mass young stellar object Cepheus A-HW2 observed with the methanol maser at 6.7 GHz
We have measured the internal proper motions of the 6.7 GHz methanol masers
associated with Cepheus A (Cep A) HW2 using Very Long Baseline Interferometery
(VLBI) observations. We conducted three epochs of VLBI monitoring observations
of the 6.7 GHz methanol masers in Cep A-HW2 with the Japanese VLBI Network
(JVN) over the period between 2006-2008. In 2006, we were able to use
phase-referencing to measure the absolute coordinates of the maser emission
with an accuracy of a few milliarcseconds. We compared the maser distribution
with other molecular line observations that trace the rotating disk. We
measured the internal proper motions for 29 methanol maser spots, of which 19
were identified at all three epochs and the remaining ten at only two epochs.
The magnitude of proper motions ranged from 0.2 to 7.4 km/s, with an average of
3.1 km/s. Although there are large uncertainties in the observed internal
proper motions of the methanol maser spots in Cep A, they are well fitted by a
disk that includes both rotation and infall velocity components. The derived
rotation and infall velocities at the disk radius of 680 au are 0.5 +- 0.7 and
1.8 +- 0.7 km/s, respectively. Assuming that the modeled disk motion accurately
represents the accretion disk around the Cep A-HW2 high-mass YSO, we estimated
the mass infall rate to be 3 x 10^{-4} n_8 Msun/yr (n_8 is the gas volume
density in units of 10^{8} cm^{-3}). The combination of the estimated mass
infall rate and the magnitude of the fitted infall velocity suggests that Cep
A-HW2 is at an evolutionary phase of active gas accretion from the disk onto
the central high-mass YSO. The infall momentum rate is estimated to be 5 x
10^{-4} n_8 Msun/yr km/s, which is larger than the estimated stellar radiation
pressure of the HW2 object, supporting the hypothesis that this object is in an
active gas accretion phase.Comment: 16 pages, 6 figures, 5 tables, accepted for publication in Astronomy
& Astrophysic
Pair Plasma Dominance in the Parsec-Scale Relativistic Jet of 3C345
We investigate whether a pc-scale jet of 3C345 is dominated by a normal
plasma or an electron-positron plasma. We present a general condition that a
jet component becomes optically thick for synchrotron self-absorption, by
extending the method originally developed by Reynolds et al. The general
condition gives a lower limit of the electron number density, with the aid of
the surface brightness condition, which enables us to compute the magnetic
field density. Comparing the lower limit with another independent constraint
for the electron density that is deduced from the kinetic luminosity, we can
distinguish the matter content. We apply the procedure to the five components
of 3C345 (C2, C3, C4, C5, and C7) of which angular diameters and radio fluxes
at the peak frequencies were obtainable from literature. Evaluating the
representative values of Doppler beaming factors by their equipartition values,
we find that all the five components are likely dominated by an
electron-positron plasma. The conclusion does not depend on the lower cutoff
energy of the power-law distribution of radiating particles.Comment: 17 page
Simulation of I-V Hysteresis Branches in An Intrinsic Stack of Josephson Junctions in High Superconductors
I-V characteristics of the high T superconductor
BiSrCaCO shows a strong hysteresis, producing many
branches. The origin of hysteresis jumps is studied by use of the model of
multi-layered Josephson junctions proposed by one of the authors (T. K.). The
charging effect at superconducting layers produces a coupling between the next
nearest neighbor phase-differences, which determines the structure of
hysteresis branches. It will be shown that a solution of phase motions is
understood as a combination of rotating and oscillating phase-differences, and
that, at points of hysteresis jumps, there occurs a change in the number of
rotating phase-differences. Effects of dissipation are analyzed. The
dissipation in insulating layers works to damp the phase motion itself, while
the dissipation in superconducting layers works to damp relative motions of
phase-differences. Their effects to hysteresis jumps are discussed.Comment: 18 pages, Latex, 8 figures. To be appear in Phys.Rev.B Vol.60(1999
New candidate GHz Peaked Spectrum and Compact Steep Spectrum Sources
Data from a recent Australia Telescope Compact Array (ATCA) program of
multi-frequency, multi-epoch monitoring of 202 active galactic nuclei with
declinations < +10 degrees have been searched for GHz Peaked Spectrum (GPS) and
Compact Steep Spectrum (CSS) radio sources. Supplementary data at higher and
lower frequencies, where available, have been used to further examine the
spectral properties of previously reported and new candidate GPS and CSS
sources. The ATCA monitoring program has allowed the variability and
polarization properties of sources previously reported as GPS and CSS sources,
and the majority of new GPS and CSS candidates, to be investigated, confirming
that these are useful diagnostics in discriminating genuine GPS and CSS sources
from variable sources that display similar spectra only temporarily. GPS
sources are confirmed to be generally more compact, and less polarized, than
CSS sources, although CSS sources show evidence for being somewhat less
variable than GPS sources at 1.4 and 2.5GHz. In addition, the widths of GPS
spectra are examined, and a significant difference is found in the GPS sample
of Snellen et al. (2000) between sources with compact double (CD) or compact
symmetric object (CSO) morphologies and sources with other morphologies, in
that CD and CSO sources have generally narrower spectra. Possible reasons for
this difference are considered.Comment: 19 pages, 9 figures, accepted for publication in A&
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