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, $T_{\rm sys}$, 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 $T_{\rm sys}$ 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 $T_{\rm sys}$ coincided with that of conventional measurement with a power meter in 1.8-% difference or less for elevations of $10^{\circ} - 88^{\circ}$. 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 $\sim 0.1$ 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 $90^{\circ}$ 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 $90^{\circ}$ 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