Astrometry of H2_{2}O Masers in Nearby Star-Forming Regions with VERA --- IV. L1448C

We have carried out multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22~GHz H$_{2}$O masers associated with a Class 0 protostar L1448C in the Perseus molecular cloud. The maser features trace the base of collimated bipolar jet driven by one of the infrared counter parts of L1448C named as L1448C(N) or L1448-mm A. We detected possible evidences for apparent acceleration and precession of the jet according to the three-dimensional velocity structure. Based on the phase-referencing VLBI astrometry, we have successfully detected an annual parallax of the H$_{2}$O maser in L1448C to be 4.31$\pm$0.33~milliarcseconds (mas) which corresponds to a distance of 232$\pm$18~pc from the Sun. The present result is in good agreement with that of another H$_{2}$O maser source NGC~1333 SVS13A in the Perseus molecular cloud, 235~pc. It is also consistent with the photometric distance, 220~pc. Thus, the distance to the western part of the Perseus molecular cloud complex would be constrained to be about 235~pc rather than the larger value, 300~pc, previously reported.Comment: 15 pages, 5 figures, accepted for publication in PAS

Bigradient Phase Referencing

We propose bigradient phase referencing (BPR), a new radio-observation technique, and report its performance using the Japanese very-long-baseline-interferometry network (JVN). In this method, a weak source is detected by phase-referencing using a primary calibrator, in order to play a role as a secondary calibrator for phase-referencing to a weak target. We will be given the opportunity to select a calibrator from lots of milli-Jansky sources, one of which may be located at the position closer to the target. With such a smaller separation, high-quality phase-referencing can be achieved. Furthermore, a subsequent more-sophisticated calibration can relocate array's focus to a hypothetical point much closer to the target; a higher quality of phase referencing is available. Our demonstrative observations with strong radio sources have proved the capabilities of BPR in terms of image dynamic ranges and astrometric reproducibility. The image dynamic range on a target has been improved with a factor of about six compared to that of normal phase-referencing; the resultant position difference of target's emission between two epochs was only 62+-50 micro-arcsecond, even with less than 2300-km baselines at 8.4 GHz and fast-switching of a target-calibrator pair of a 2.1-degree separation.Comment: 10 pages, 4 figures, accepted for publication in PAS

Millimeter-VLBI Observations of Low-luminosity Active Galactic Nuclei with Source-frequency Phase Referencing

We report millimeter-VLBI results of low-luminosity active galactic nuclei (M84 and M87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M84 at 88 GHz. The derived brightness temperature of the M84 core was about 7.2 × 109 K, which could serve as a lower limit because the core down to 30 Schwarzschild radii was still unresolved in our 88 GHz observations. We successfully determined the core shifts of M87 at 22-44 GHz and 44-88 GHz through the source-frequency phase-referencing technique. The jet apex of M87 could be deduced at ~46 μas upstream of the 43 GHz core from core-shift measurements. The estimated magnetic field strength of the 88 GHz core of M87 is 4.8 ± 2.4 G, which is at the same magnitude of 1-30 G near the event horizon probed by the Event Horizon Telescope

VLBI imaging of M81* at 3.4 mm with source-frequency phase-referencing

We report on the first VLBI image of the M81 nucleus (M81*) at a wavelength of 3.4 mm, obtained with the source-frequency phase-referencing (SFPR) technique. Thanks to the SFPR calibration, the coherent integration time could be eventually increased by more than an order of magnitude, which enabled the detection of fringes at the level of 45 mJy beam-1 with a dynamic range higher than 130:1. This paves the way toward future mm/sub-mm VLBI observations of weaker sources. From the analysis of the M81* visibilities, a core size of ~50 uas at 3.4 mm was estimated. This follows the power-law relationship with wavelength lambda^0.88, reported previously at lower frequencies. These results constrain the core size (at 3.4 mm) to a minimum of ~80 Schwarzschild radii of M81*.Comment: 10 pages, 5 figures,accepted by ApJ Lette

First Phase Development of Korea-Japan Joint VLBI Correlator and Its Current Progress

The first phase of the Korea-Japan Joint VLBI Correlator (KJJVC) development has been completed and installed to correlate the observed data from KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry) in October 2009. KJJVC is able to process 16 stations, a maximum of 8 Gbps/station, and 8,192 output channels for VLBI data. The system configuration, the experimental results, and future plans are introduced in this paper

Astrometry of Star Forming Region IRAS 05137+3919 in the far outer Galaxy

We present the results of astrometric observations with VERA toward the H2O maser sources in IRAS 05137+3919, which is thought to be located in the far outer Galaxy. We have derived the parallax of \pi = 0.086 +/- 0.027 mas, which corresponds to the source distance of D=11.6+5.3-2.8 kpc. Although the parallax measurement is only 3-sigma level and thus the distance uncertainty is considerably large, we can strongly constrain the minimum distance to this source, locating the source at the distance from the Sun greater than 8.3 kpc (or 16.7 kpc from the Galaxy's center) at 90% confidence level. Our results provide an astrometric confirmation that this source is located in the far outer Galaxy beyond 15 kpc from the Galaxy center, indicating that IRAS 05137+3919 is one of the most distant star-forming regions from the Galaxy center.Comment: 7 pages, 2 figures, to appear in PASJ vol 63. No1 (VERA special issue

SNR periodical variation of Chang’E-3 spacecraft orbiting the Moon

AbstractChang’E-3 spacecraft was orbiting the Moon from December 6–14, 2013, and very long baseline interferometry (VLBI) observations were performed to improve the accuracy of its orbit determination. In the process of recording VLBI raw data, 2 bits quantization was implemented. Interesting phenomenon was that signal-to-noise ratio (SNR) of each VLBI station experienced periodical change and had large variation on amplitude while in the Moon’s orbit, whereas SNR kept in a stable level after Chang’E-3 landed on the Moon. Influence of varying elevation angle on SNR was analyzed and compensation of 2 bits quantization harmonics to SNR calculation was investigated. Most importantly, telescope system noise temperature increase caused by the Moon was computed along the time of Chang’E-3 orbiting the Moon, and well matched SNR changing trend in terms of correlation coefficients