69 research outputs found
What Kinds of Accretion Disks Are There in the Nuclei of Radio Galaxies?
It seems to be a widely accepted opinion that the types of accretion disks
(or flows) generally realized in the nuclei of radio galaxies and in further
lower mass-accretion rate nuclei are inner, hot, optically thin, radiatively
inefficient accretion flows (RIAFs) surrounded by outer, cool, optically thick,
standard type accretion disks. However, observational evidence for the
existence of such outer cool disks in these nuclei is rather poor. Instead,
recent observations sometimes suggest the existence of inner cool disks of
non-standard type, which develop in the region very close to their central
black holes. Taking NGC 4261 as a typical example of such light eating nuclei,
for which both flux data ranging from radio to X-ray and data for the
counterjet occultation are available, we examine the plausibility of such a
picture for the accretion states as mentioned above, based on model
predictions. It is shown that the explanation of the gap seen in the counterjet
emission in terms of the free-free absorption by an outer standard disk is
unrealistic, and moreover, the existence itself of such an outer standard disk
seems very implausible. Instead, the model of RIAF in an ordered magnetic field
(so called resistive RIAF model) can well serve to explain the emission gap in
terms of the synchrotron absorption, as well as to reproduce the observed
features of the overall spectral energy distribution (SED). This model also
predicts that the RIAF state starts directly from an interstellar hot gas phase
at around the Bondi radius and terminates at the inner edge whose radius is
about 100 times the Schwartzschild radii. Therefore, there is a good
possibility for a cool disk to develop within this innermost region.Comment: 8 pages, 3 figures, to appear in PASJ, Vol.62, No.
Internal Motion of 6.7-GHz Methanol Masers in H II Region S269
We present the first internal motion measurement of the 6.7-GHz methanol
maser within S269, a small HII region in the outer Galaxy, which was carried
out in 2006 and 2011 using the Japanese VLBI Network (JVN). Several maser
groups and weak isolated spots were detected in an area spanning by ~200 mas
(1000 AU). Three remarkable maser groups are aligned at a position angle of 80
degree. Two of three maser groups were also detected by a previous observation
in 1998, which allowed us to study a long-term position variation of maser
spots from 1998 to 2011. The angular separation between the two groups
increased ~10 mas, which corresponds to an expansion velocity of ~10 km s^{-1}.
Some velocity gradient (~10^{-2} km s^{-1} mas^{-1}) in the overall
distribution was found. The internal motion between the maser groups support
the hypothesis that the methanol masers in S269 could trace a bipolar outflow.Comment: 8 pages, 6 figures, accepted for publication in PAS
VLBI observations of the most radio-loud, narrow-line quasar SDSS J094857.3+002225
We observed the narrow-line quasar SDSS J094857.3+002225, which has the
highest known radio loudness for a narrow-line Seyfert~1 galaxy (NLS1), at
1.7--15.4 GHz with the Very Long Baseline Array (VLBA). This is the first
very-long-baseline interferometry (VLBI) investigation for a radio-loud NLS1.
We independently found very high brightness temperatures from (1) its
compactness in a VLBA image and (2) flux variation among the VLBA observation,
our other observations with the VLBA, and the Very Large Array (VLA). A Doppler
factor larger than 2.7--5.5 was required to meet an intrinsic limit of
brightness temperature in the rest frame. This is evidence for highly
relativistic nonthermal jets in an NLS1. We suggest that the Doppler factor is
one of the most crucial parameters determining the radio loudness of NLS1s. The
accretion disk of SDSS J094857.3+002225 is probably in the very high state,
rather than the high/soft state, by analogy with X-ray binaries with strong
radio outbursts and superluminal jets such as GRS 1915+105.Comment: 6 pages, 2 figures, accepted for publication in PAS
Very Long Baseline Array Imaging of Parsec-scale Radio Emissions in Nearby Radio-quiet Narrow-line Seyfert 1 Galaxies
We conducted Very Long Baseline Array (VLBA) observations of seven nearby
narrow-line Seyfert 1 (NLS1) galaxies at 1.7 GHz (18cm) with milli-arcsecond
resolution. This is the first systematic very long baseline interferometry
(VLBI) study focusing on the central parsec-scale regions of radio-quiet NLS1s.
Five of the seven were detected at a brightness temperature of >~5x10^6 K and
contain radio cores with high brightness temperatures of >6x10^7 K, indicating
a nonthermal process driven by jet-producing central engines as is observed in
radio-loud NLS1s and other active galactic nucleus (AGN) classes. VLBA images
of MRK 1239, MRK 705, and MRK 766 exhibit parsec-scale jets with clear linear
structures. A large portion of the radio power comes from diffuse emission
components that are distributed within the nuclear regions (<~300 pc), which is
a common characteristic throughout the observed NLS1s. Jet kinetic powers
limited by the Eddington limit may be insufficient to allow the jets escape to
kiloparsec scales for these radio-quiet NLS1s with low-mass black holes of
<~10^7 M_sun.Comment: 12 pages, 3 figures, 3 tables, accepted for publication in Ap
Dense Plasma Torus in the GPS Galaxy NGC 1052
We report results from nearly simultaneous pentachromatic VLBI observations
towa rds a nearby GPS galaxy NGC 1052. The observations at 1.6 and 4.8 GHz with
VSOP, and at 2.3, 8.4, and 15.4 GHz wit h VLBA, provide linear resolutions of
pc. Convex spectra of a double-sided jet imply that synchrotron
emission is obscured through foreground cold dense plasma, in terms of
free--free absorption (FFA). We found a central condensation of the plasma
which covers about 0.1 pc and 1 pc of the approaching and receding jets,
respectively. A simple model with a geometrically thick plasma torus
perpendicular to the jets is established to explain the asymmetric distribution
of FFA opacities.Comment: 9 pages, 4 figures, other comment
Short-Term Variability of PKS1510-089
We searched a short-term radio variability in an active galactic nucleus PKS
1510-089. A daily flux monitoring for 143 days at 8.4 GHz was performed, and
VLBI observations at 8.4, 22, and 43 GHz were carried out 4 times during the
flux monitoring period. As a result, variability with time scale of 20 to 30
days was detected. The variation patterns were well alike on three frequencies,
moreover those at 22 and 43 GHz were synchronized. These properties support
that this short-term variability is an intrinsic one. The Doppler factor
estimated from the variability time scale is 47. Since the Doppler factor is
not extraordinary large for AGN, such intrinsic variability with time scale
less than 30 days would exist in other AGNs.Comment: 14 pages, 4 figure
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
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