88 research outputs found
A New System Noise Measurement Method Using a 2-bit Analog-To-Digital Converter
We propose a new method to measure the system noise temperature, , using a 2-bit analog-to-digital converter (ADC). The statistics of the
digitized signal in a four-level quantization brings us information about the
bias voltage and the variance, which reflects the power of the input signal.
Comparison of the variances in {\it hot} and {\it sky} circumstances yields
without a power meter. We performed test experiments using the
Kagoshima 6-m radio telescope and a 2-bit ADC to verify this method. Linearity
in the power-variance relation was better than 99% within the dynamic range of
10 dB. Digitally measured coincided with that of conventional
measurement with a power meter in 1.8-% difference or less for elevations of
. No significant impact was found by the bias voltages
within the range between -3.7 and +12.8% with respect to the threshold voltage.
The proposed method is available for existing interferometers that have a
multi-level ADC, and release us from troubles caused by power meters.Comment: to appear in the Publications of the Astronomical Society of Japan,
Vol.62, No.5; 12 pages, 6 figure
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
Free-Free Absorption and the Unified Scheme
We report Very-Long-Baseline Array (VLBA) observations at 2.3, 8.4, and 15.4
GHz towards nine GHz-Peaked Spectrum (GPS) sources. One Seyfert 1 galaxy, one
Seyfert 2 galaxy, three radio galaxies, and four quasars were included in our
survey. We obtained spatial distributions of the Free-Free Absorption (FFA)
opacity with milliarcsec resolutions for all sources. It is found that type-1
(Seyfert 1 and quasars) and type-2 (Seyfert 2 and radio galaxies) sources
showed different distributions of the FFA opacities. The type-1 sources tend to
show more asymmetric opacity distributions towards a double lobe, while those
of the type-2 sources are rather symmetric. Our results imply that the
different viewing angle of the jet causes the difference of FFA opacity along
the external absorber. This idea supports the unified scheme between quasars
and radio galaxies, proposed by Barthel (1989).Comment: 17 pages, including 8 figures. Proceedings of the 3rd Compact Steep
Spectrum and GHz Peaked Spectrum Sources, 2002, Greece. Submitted to the
Publications of Astronomical Society of Australi
Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy
We have developed the Smoothed Bandpass Calibration (SBC) method and the best
suitable scan pattern to optimize radio spectroscopic observations. Adequate
spectral smoothing is applied to the spectrum toward OFF-source blank sky
adjacent to a target source direction for the purpose of bandpass correction.
Because the smoothing process reduces noise, the integration time for
OFF-source scans can be reduced keeping the signal-to-noise ratio. Since the
smoothing is not applied to ON-source scans, the spectral resolution for line
features is kept. An optimal smoothing window is determined by bandpass
flatness evaluated by Spectral Allan Variance (SAV). An efficient scan pattern
is designed to the OFF-source scans within the bandpass stability timescale
estimated by Time-based Allan Variance (TAV). We have tested the SBC using the
digital spectrometer, VESPA, on the VERA Iriki station. For the targeted noise
level of 5e-4 as a ratio to the system noise, the optimal smoothing window was
32 - 60 ch in the whole bandwidth of 1024 ch, and the optimal scan pattern was
designed as a sequence of 70-s ON + 10-s OFF scan pairs. The noise level with
the SBC was reduced by a factor of 1.74 compared with the conventional method.
The total telescope time to achieve the goal with the SBC was 400 s, which was
1/3 of 1200 s required by the conventional way. Improvement in telescope time
efficiency with the SBC was calculated as 3x, 2x and 1.3x for single-beam,
dual-beam, and on-the-fly (OTF) scans, respectively. The SBC works to optimize
scan patterns for observations from now, and also works to improve
signal-to-noise ratios of archival data if ON- and OFF-source spectra are
individually recorded, though the efficiency depends on the spectral stability
of the receiving system.Comment: 12 pages, 11 figures, to appear in the Publications of Astronomical
Society of Japan, Vol.64, No.
VLBI Imagings of Kilo-parsec Knot in 3C 380
We investigate observational properties of a kilo-parsec scale knot in
radio-loud quasar 3C 380 by using two epoch archival data obtained by Very Long
Baseline Interferometry (VLBI) at 5 GHz on 1998 July and 2001 April. We succeed
in obtaining the highest spatial resolution image of the bright knot K1 located
at 732 milliarcseconds, or more than 20 kpc de-projected, downstream from the
nucleus three times better than previously obtained highest resolution image by
Papageorgiou et al. (2006). Our images reveal, with new clarity, "inverted
bow-shock" structure in K1 facing the nucleus and its morphology resembles a
conical shock wave. By comparing the two epoch images directly, we explore the
kinematics of K1 and obtain the upper limit of apparent velocity, 0.25 mas/yr
or 9.8 c of K1 for the first time. The upper limit of apparent velocity is
marginally smaller than superluminal motions seen in the core region. Further
new epoch VLBI observations are necessary to measure the proper motion at K1.Comment: 15 pages, 4 figures, accepted for publication in PAS
A New Test for the Absorption Mechanism of GPS Radio Sources Using Polarization Properties
We consider the use of polarization properties as a means to discriminate
between Synchrotron Self-Absorption (SSA) and Free--Free Absorption (FFA) in
GHz-Peaked Spectrum (GPS) sources. The polarization position angle (PA) of
synchrotron radiation at high frequencies for the optically thin regime is
perpendicular to the magnetic field, whereas it is parallel to the magnetic
field at low frequencies for the optically thick regime. Therefore, SSA
produces a change in PA of across the spectral peak, while FFA
does not result in such a change. We analyzed polarization data from VLA
observations for six GPS sources to see if such a change in PA was present. Our
results indicate that there is no significant evidence for change
in PA across the spectral peak, suggesting that FFA is more likely than SSA for
low-frequency cutoffs in these sources
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
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
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