scispace - formally typeset
Search or ask a question
Author

Hideki Ujihara

Bio: Hideki Ujihara is an academic researcher from National Institute of Information and Communications Technology. The author has contributed to research in topics: Very-long-baseline interferometry & Wideband. The author has an hindex of 6, co-authored 22 publications receiving 204 citations.

Papers
More filters
Journal ArticleDOI
TL;DR: In this paper, the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry) were presented, and the results demonstrate the high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.
Abstract: We present the results of the first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The observations of a pair of H2O maser sources, W 49N and OH 43.8−0.1, were carried out on 2002 May 29 and July 23, and fringes of the H2O maser lines at 22GHz were successfully detected. While the residual fringe phases of both sources showed rapid variations over 360 ◦ due to the atmospheric fluctuation, the differential phase between the two sources remained constant for 1 hour with an r.m.s. of 8 ◦ , demonstrating that the atmospheric phase fluctuation was effectively removed by dual-beam phase referencing. An analysis based on the Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for an arbitrarily long time. These results demonstrate VERA’s high capability of phase referencing, indicating that it is a promising tool for phase-referencing VLBI astrometry at 10 µas-level accuracy.

67 citations

Journal ArticleDOI
TL;DR: The first dual-beam observations of a pair of H2O maser sources W49N and OH43.1 have been carried out on 2002 May 29 and July 23 as discussed by the authors.
Abstract: We present the results of first dual-beam observations with VERA (VLBI Exploration of Radio Astrometry). The first dual-beam observations of a pair of H2O maser sources W49N and OH43.8-0.1 have been carried out on 2002 May 29 and July 23, and fringes of H2O maser lines at 22 GHz have been successfully detected. While the residual fringe phases of both sources show rapid variations over 360 degree due to the atmospheric fluctuation, the differential phase between the two sources remains constant for 1 hour with r.m.s. of 8 degree, demonstrating that the atmospheric phase fluctuation is removed effectively by the dual-beam phase referencing. The analysis based on Allan standard deviation reveals that the differential phase is mostly dominated by white phase noise, and the coherence function calculated from the differential phase shows that after phase referencing the fringe visibility can be integrated for arbitrarily long time. These results demonstrate VERA's high capability of phase referencing, indicating that VERA is a promising tool for phase referencing VLBI astrometry at 10 microarcsec level accuracy.

58 citations

Journal ArticleDOI
TL;DR: In this paper, very long baseline interferometry (VLBI) is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources.
Abstract: The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons. Very long baseline interferometry is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources. This approach overcomes limitations of the performance of satellite transfer techniques.

35 citations

Journal ArticleDOI
TL;DR: In this paper, the results of multiepoch VLBI observations for a 6.7 GHz methanol maser toward the UC H II region of W 3(OH) with Japanese VLBi Network (JVN) were derived, and the trigonometric annual parallax to be 0.598 ˙ 0.067 mas.
Abstract: We present the results of multiepoch VLBI observations for a 6.7 GHz methanol maser toward the UC H II region of W 3(OH) with Japanese VLBI Network (JVN). Based on phase-referencing VLBI astrometry, we derived the trigonometric annual parallax to be 0.598 ˙ 0.067 mas, corresponding to a distance of 1.67 +0:21 � 0:17 kpc. This is the first detection of parallax for a 6.7 GHz methanol maser with JVN, and demonstrates that JVN/VERA is capable of conducting VLBI astrometry for 6.7 GHz methanol maser sources within few kpc from the Sun. Based on in-beam mapping of the W 3(OH) methanol maser, we also measured the internal proper motions of its 6.7 GHz methanol maser for the first time. The internal proper motions basically show north–south expansion with a velocity of few km s � 1 , being similar to OH masers. The spatial distribution and the internal proper motions of 6.7 GHz

17 citations

01 Jan 2003
TL;DR: The VERA system as mentioned in this paper is the first VLBI array to be designed to be free from the atmosphere phase fluctuations, and it has four antennas with 2,300 km maximum baseline length in Japan.
Abstract: VERA is the first VLBI array to be designed to be free from the atmosphere phase fluctuations. It has four VLBI station with 2,300 km maximum baseline length in Japan. To compensate phase fluctuations of interferometer visibilities, which are mainly caused by the atmosphere, VERA antenna has two receivers and observes two objects simultaneously. By the comparison the visibility phase between two beams , simultaneous phase referencing VLBI will be achieved. The goal accuracy of astrometry observations is 10 micro arcseconds. . Currently the construction of four stations was complete. And test observations are undergoing. We show the scientific goal and instrumental accuracy of VERA. 1.INTRODUCTION The VLBI technique is one of radio interferometry, which combines signals from celestial objects by tape recorders not cables. Then it is possible to extend antenna separations over the earth. At 1997, it became around 30,000 km by using a spacecraft.(Hirabayashi, et al., 1999) For astrometry observations, VLBI technique supplies the most accurate data. By the mid-1990s, the VLBI technique has achieved the positional accuracy up to a few sub milli acrsecond(Ma, et al., 1998). The group delay and delay rate at dual-frequency 2.3 and 8.4 GHz are determined for each celestial objects with each pairs of observing antennas. These data are compiled and positions of each celestial objects and antenna stations are solved. Recently phase referencing technique is used and has improved the accuracy of the astrometry observations. But current phase referencing technique is mainly used an antenna nodding method, which is that each antenna is switched between a calibrator and a target object with more than 30 second interval. This way is effective for lower than 8GHz observations. For higher frequency observations, nodding interval should be less than 30 seconds. It makes large loss for actual observing time because of large antenna slewing time. Then VERA; VLBI Exploration of Radio Astrometry, has adopted a two-beam system, which observe separated celestial objects simultaneously (Kobayashi, et al., 2001). It makes complete phase referencing VLBI observations possible. With 2,300 km maximu m baseline length, around 10-micro arc second accuracy will be achieved at astrometry observations. Main scientific targets for VERA are to determine the structure and kinematics of our galaxy (Honma, et al., 2000). By the measurements of the distance for the Galactic maser objects, three-dimensional distribution and proper motion will show the Galactic structure. VERA aims the 10-micro arc second accuracy to measure positions of maser objects, which is equivalent to the 10 % error at the Galactic center distance. 2.VERA system outlook VERA has four VLBI stations in Japan with the 2,300 km maximum baseline length and the 1,200 km minimum baseline length. The station distribution map is shown at Figure 1. Two stations are in small islands, one is in Honshu island; main island of Japan, and the other is in Kyusyu island. The antenna of VERA has twenty-meter diameter shown at Figure 2. The receiver cabin is below the main surface to get more than 2 degrees width of the field of view. In this cabin, two-beam receiving system is installed, which is shown at Figure 3. Two-beam system is required that the maximum separation is 2.2 degree and the minimum separation is 0.4 degree. To realize it, movable receivers are used with the Stewart platform, which is consisted with six jacks. Six degree of freedom are restricted by Fig. 1 Array configuration of VERA in Japan Fig.2 VERA 20m diameter antenna Fig. 3 Two beam receiver platform Fig. 4 Coaxial spiral feed array for S and X bands the six jack length. Highly strong jacks are used for accurate positioning. The receiver platform moves on the focus plane according to the separation angle. The tolerance of positioning is 0.3 mm for the phase calibration between two beams. Radiators on the antenna surface are used to calibrate the difference of effective ray path between two beams . Aperture efficiencies and system noise temperatures are shown at Table 1. The loss of signals by the dome on the receiver cabin is a penalty for the two-beam system. The 5% loss for 22 GHz and 8% for 43 GHz is expected from the laboratory test. And ray injection angle dependence is expected to be small. Observing frequencies are Q and K bands for celestial maser object observations and S and X bands for geodesy observations. 43 GHz and 22 GHz band receivers are used cooled HEMT amplifier. The expected system temperatures at good weather conditions are around 150 K and 250 K at K and Q band, respectively. And aperture efficiency of 22 GHz is around 50 % and degradation is small at 2-degree separation. But at 43 GHz, the aperture efficiency drops 10% at the 2-degree separation, because the aperture illumination loss become higher. For S and X band, a spiral array feed is used, which is shown at Figure 4. The coaxial feed is newly developed. Data acquisition system was newly developed. Observed signal is sampled with 2 channels , 1 Giga sample per second and 2-bit sampling mode. A digital filter processor filters these data. And the data recorder has a capability of 1 Giga bit per second recording and it is compatible with previous VSOP 256 Mbps recording system. A tape cart system is used and automatic 32 hours operation is possible. And Input and output interfaces are adapted to VSI; VLBI Standard Interface. The correlator used for the VSOP project is reused for VERA correlator. It is a 10-station VLBI correlator and has a capability of 256 Mega sample per second and 2-bit sampling for each station. It is used as a 5 station, 512Mega sample per second, 2-bit sampling correlator. Also the correlator input interface is modified for the VSI. The total fringe sensitivity is shown in Table 2. Normally the reference object is selected a background compact radio galaxies and quasers, which are more than 100 times further than galactic objects. If the reference object has enough intensity, it is possible to integrate the observing maser object signal for long time. Then the sensitivity is lower than usual single beam VLBI arrays. Table 1. Aperture efficiency and system noise temperature 2 deg. 0 deg. 2 deg. 0 deg. separation

14 citations


Cited by
More filters
01 Jan 2016
TL;DR: The interferometry and synthesis in radio astronomy is universally compatible with any devices to read and is available in the book collection an online access to it is set as public so you can download it instantly.
Abstract: Thank you for reading interferometry and synthesis in radio astronomy. As you may know, people have look numerous times for their favorite novels like this interferometry and synthesis in radio astronomy, but end up in malicious downloads. Rather than enjoying a good book with a cup of tea in the afternoon, instead they are facing with some malicious virus inside their computer. interferometry and synthesis in radio astronomy is available in our book collection an online access to it is set as public so you can download it instantly. Our book servers spans in multiple locations, allowing you to get the most less latency time to download any of our books like this one. Merely said, the interferometry and synthesis in radio astronomy is universally compatible with any devices to read.

630 citations

Journal ArticleDOI
TL;DR: In this article, the authors measured the annual parallax of the maser features in SVS 13 of 4.25 ˙ 0.32 mas, corresponding to a distance of 235 ˘ 18 pc from the Sun.
Abstract: We report on the results of multiepoch very long baseline interferometry (VLBI) observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H2O masers associated with the young stellar object SVS 13 in the NGC 1333 region. We carried out phase-referencing VLBI astrometry, and measured the annual parallax of the maser features in SVS 13 of 4.25 ˙ 0.32 mas, corresponding to a distance of 235 ˙ 18 pc from the Sun. Our result is consistent with a photometric distance of 220 pc, previously reported. Even though the maser features were detectable only for 6 months, the present result provides the distance to NGC 1333 with much higher accuracy than photometric methods. The absolute positions and proper motions have been derived, revealing that the H2O masers with LSR (local standard of rest) velocities of 7–8 km s � 1 are most likely associated with VLA 4A, which is a radio counterpart of SVS 13. It is currently difficult to attribute the observed proper motions of the maser features to either the jet or the rotating circumstellar disk associated with VLA 4A, which should be investigated through future high-resolution astrometric observations of VLA 4A and other radio sources in NGC 1333.

332 citations

Journal Article
TL;DR: In this paper, a network of atomic clocks using non-local entangled states is proposed to achieve unprecedented stability and accuracy in time-keeping, as well as being secure against internal or external attack.
Abstract: A proposed network of atomic clocks—using non-local entangled states—could achieve unprecedented stability and accuracy in time-keeping, as well as being secure against internal or external attack.

247 citations

Journal ArticleDOI
TL;DR: In this article, the authors presented the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry).
Abstract: We present the initial results of multiepoch VLBI observations of 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we carried out phase-referencing VLBI astrometry, and successfully detected the annual parallax of Orion KL to be 2.29 ˙ 0.10 mas, corresponding to a distance of 437 ˙ 19 pc from the Sun. The distance to Orion KL was determined for the first time with the trigonometric parallax method in these observations. Although this value is consistent with that previously reported, 480 ˙ 80 pc, which was estimated from a statistical parallax method using the proper motions and radial velocities of the H2O maser features, our new results provide a much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.

235 citations

Journal ArticleDOI
TL;DR: In this article, the authors derived the trigonometric parallax of Orion-KL to be 2.39˙0.03mas, corresponding to a distance of 418˙6pc.
Abstract: We present results of phase-referencing VLBI observations of SiO masers in the Orion-KL region made with VERA. Using a strong maser spot in the 43 GHz v = 2 J = 1–0 emission, we derived the trigonometric parallax of Orion-KL to be 2.39˙0.03mas, corresponding to a distance of 418˙6pc, with the highest accuracy among existing parallax measurements of the source. We made a superimposed image of v = 1 J = 1–0 and v = 2 J = 1–0 maser features in Orion-KL based on absolute positions obtained from the phase-referencing astrometry with a common reference source. The maser features of both transitions show similar X-shaped distributions centered at Source I. However, in each of the four arms of the X-shape, the SiO v = 2 features tend to lie closer to Source I than the SiO v = 1 features. The radial velocities of the maser emission decrease with the distance from Source I. The spatial and radial velocity distributions of the SiO masers suggest that the SiO masers lie in the rotating materials associated with a disk around Source I, rather than a decelerating outflow.

224 citations