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Journal articles on the topic 'VLBI'

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Author: Grafiati

Published: 4 June 2021

Last updated: 20 February 2023

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1

Zhang, B., X. W. Zheng, J. L. Li, Y. Xu, and J. F. Wu. "Tropospheric correction in VLBI phase-referencing using GPS data." Proceedings of the International Astronomical Union 3, S248 (October 2007): 212–13. http://dx.doi.org/10.1017/s174392130801911x.

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AbstractComparing the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antenna, the systematic biases and standard deviations of the difference are both found to be at the level of a sub-centimeter. Based on this agreement, we used GPS data to correct the tropospheric effects in VLBI phase-referencing, resulting in close peak-to-noise ratios of images after tropospheric correction using GPS and VLBI data.

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2

Rogers, Alan E. E. "Tape recorders for VLBI." International Astronomical Union Colloquium 131 (1991): 65–75. http://dx.doi.org/10.1017/s0252921100013075.

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AbstractMagnetic tape recorders developed for VLBI have taken advantage of the best available technology to achieve high data rates and large storage capacity. The Mark I, II and III VLBI recording systems have data rates of 0.72, 4 and 224 Mbits/sec respectively. The recorder developed for the VLBA has a data rate of 256 Mbits/sec and can store 5.5 Terabits of data on one tape.

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3

Murphy, D. W. "Imaging of Extragalactic Radio Sources with the VSOP Space VLBI Mission." Symposium - International Astronomical Union 175 (1996): 531–32. http://dx.doi.org/10.1017/s0074180900081754.

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In September 1996 the first dedicated VLBI spacecraft, VSOP, will be launched. This Japanese spacecraft operating in conjunction with ground-based VLBI arrays such as the EVN or VLBA will enable observers to routinely undertake VLBI observations with maximum baseline lengths of 2.6 Earth diameters and a resolution of 55 μas at the highest operating frequency of 22 GHz. In this paper we present a brief overview of the imaging capability of the VSOP mission together with an example of an imaging simulation.

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4

Wietfeldt, R. D., and L. R. D’Addario. "Compatibility issues in VLBI." International Astronomical Union Colloquium 131 (1991): 98–101. http://dx.doi.org/10.1017/s0252921100013130.

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AbstractNew VLBI systems are under development in many countries in preparation for large dedicated arrays, including orbiting radio telescopes to be launched in the 1990s. In particular, several new recording systems and correlators are being constructed: the VLBA in the U.S., the S2 in Canada, and the K4/VSOP in Japan. This paper reviews the issues of technical compatibility among these systems in an effort to facilitate global VLBI experiments.

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5

Kemball, A. J., and P. J. Diamond. "Continuum VLBI Polarimetry of 3C454.3 at 43 GHz." Symposium - International Astronomical Union 175 (1996): 33–34. http://dx.doi.org/10.1017/s0074180900079900.

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Polarization VLBI calibration at high frequencies has traditionally been difficult due to poor sensitivity and high antenna instrumental polarization across inhomogeneous networks. The higher observing frequency and increased spatial resolution diminishes the chances of finding ideal VLBI polarization calibrators. The advent of the Very Long Baseline Array (VLBA), which has standardized feeds with low instrumental polarization, has minimized these observational difficulties. Recent work in polarization calibration has suggested that somewhat resolved sources may be used in an iterative polarization calibration scheme (Cotton 1993). A full generalization of this method has been developed by Leppanen, Zensus and Diamond (1995) in calibrating 22 GHz polarization observations with the VLBA.

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6

Kovalev, Y. Y., D. C. Gabuzda, T. P. Krichbaum, W. Alef, and A. Witzel. "A Multi-Frequency VLBI Total Intensity and Polarization Study of the BL Lacertae Object 0716+714." International Astronomical Union Colloquium 164 (1998): 173–74. http://dx.doi.org/10.1017/s0252921100045061.

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AbstractNew polarization-sensitive global VLBI and VLBA images of the BL Lacertae object 0716+714 at 6 cm (1991.4) and 3.6 cm/1.3 cm (1994.2) are presented. The source shows a compact, one-sided core–jet structure in structural position angle ~ 13°. The 3.6 cm total intensity image shows the jet beginning to curve toward the arcsecond-scale structure, which lies in position angle ~ −45°. Comparison with previously published data suggests that the apparent velocity of a 6 cm jet component is > 1.09 ± 0.15c, if the red shift of 0716+714 is z > 0.3. Linear polarization from the VLBI core was detected at all three frequencies; in addition, polarization from the innermost jet component was detected at 1.3 cm. This compact jet polarization appears to be quite high, ~ 50%, and to be aligned with the jet direction, which can be understood if it is a transverse shock. The VLBI core polarization at our earlier epoch was perpendicular to the VLBI jet direction, but had rotated to align with the jet direction by the later epoch, about a year later. This rotation may indicate the imminent birth of a new VLBI component (shock).

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7

Herrera Ruiz, N., E. Middelberg, A. Deller, V. Smolčić, R. P. Norris, M. Novak, I. Delvecchio, et al. "VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz." Astronomy & Astrophysics 616 (August 2018): A128. http://dx.doi.org/10.1051/0004-6361/201832969.

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We present very long baseline interferometry (VLBI) observations of 179 radio sources in the COSMOS field with extremely high sensitivity using the Green Bank Telescope (GBT) together with the Very Long Baseline Array (VLBA) (VLBA+GBT) at 1.4 GHz, to explore the faint radio population in the flux density regime of tens of μJy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, meaning that it is independent of X-ray or infrared properties. The milli-arcsecond resolution provided by the VLBI technique implies that the detected sources must be compact and have large brightness temperatures, and therefore they are most likely AGN (when the host galaxy is located at z ≥ 0.1). On the other hand, this technique only allows us to positively identify when a radio-active AGN is present, in other words, we cannot affirm that there is no AGN when the source is not detected. For this reason, the number of identified AGN using VLBI should be always treated as a lower limit. We present a catalogue containing the 35 radio sources detected with the VLBA+GBT, ten of which were not previously detected using only the VLBA. We have constructed the radio source counts at 1.4 GHz using the samples of the VLBA and VLBA+GBT detected sources of the COSMOS field to determine a lower limit for the AGN contribution to the faint radio source population. We found an AGN contribution of >40−75% at flux density levels between 150 μJy and 1 mJy. This flux density range is characterised by the upturn of the Euclidean-normalised radio source counts, which implies a contribution of a new population. This result supports the idea that the sub-mJy radio population is composed of a significant fraction of radio-emitting AGN, rather than solely by star-forming galaxies, in agreement with previous studies.

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8

Romney, Jonathan D. "The Very Long Baseline Array." Symposium - International Astronomical Union 129 (1988): 461–68. http://dx.doi.org/10.1017/s0074180900135272.

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The Very Long Baseline Array (VLBA) will be the world's first large-scale dedicated VLBI facility. It is planned as a multipurpose instrument, supporting continuum, spectroscopic, astrometric/geodetic, multi-frequency, bandwidth synthesis, polarization, and pulsar measurements – with a variety of bandwidths, sampling rates, quantization schemes, and multi-band modes. The Array will consist of ten new 25-m precision antennas, in an optimized geographical configuration. Extremely low-noise receivers will support observations with dual polarization in nine frequency bands from 330 MHz to 43 GHz, including almost all conventional VLBI bands. A wideband, high-density recording system will allow unattended operation at a sustained data rate of 128 Mbit/s for 24 hours, and peak rates up to 512 Mbit/s. The VLBA correlator, planned as a 20-station system to support processing of all observations involving the Array, will be by far the largest ever built for VLBI. The Array will be operated remotely, from a new operations and data-reduction center to be built in Socorro, NM.

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9

Britzen, S., R. C. Vermeulen, G. B. Taylor, R. M. Campbell, I. W. Browne, P. Wilkinson, T. J. Pearson, and A. C. S. Readhead. "CJ-F: The Kinematics of 241 AGN." Symposium - International Astronomical Union 205 (2001): 106–9. http://dx.doi.org/10.1017/s0074180900220615.

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We present first results from the analysis of multi-epoch VLBI observations of 241 sources in the CJ-F sample (Caltech Jodrell-Flat-spectrum, Taylor et al. 1996). We have been obtaining 5 GHz VLBI (global and VLBA) snapshot observations of the CJ-F sources (complete with 293 sources) since 1990 in order to create a valid database for thorough statistical tests of pc-scale jet motion in AGN. All 241 CJ-F AGN have been observed at least three times with the VLBA, which enables us to investigate jet component motions and paths. In particular, we concentrate on the analysis of those properties supposed to be essential for gamma-ray production, i.e., superluminal motion and bending.

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10

Breiding, Peter, Sarah Burke-Spolaor, Tao An, Karishma Bansal, Prashanth Mohan, Gregory B. Taylor, and Yingkang Zhang. "Deep Very Long Baseline Interferometry Observations Challenge Previous Evidence of a Binary Supermassive Black Hole Residing in Seyfert Galaxy NGC 7674." Astrophysical Journal 933, no. 2 (July 1, 2022): 143. http://dx.doi.org/10.3847/1538-4357/ac7466.

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Abstract Previous Ku-band (15 GHz) imaging with data obtained from the Very Long Baseline Array (VLBA) had shown two compact, subparsec components at the location of a presumed kiloparsec-scale radio core in Seyfert galaxy NGC 7674. It was then presumed that these two unresolved and compact components were dual radio cores corresponding to two supermassive black holes (SMBHs) accreting surrounding gas and launching radio-bright relativistic jets. However, utilizing the original VLBA data set used to claim the detection of a binary SMBH, in addition to later multiepoch/multifrequency data sets obtained from both the VLBA and the European very long baseline interferometry (VLBI) network, we find no evidence to support the presence of a binary SMBH. We place stringent upper limits to the flux densities of any subparsec-scale radio cores that are at least an order of magnitude lower than the original VLBI radio-core detections, directly challenging the original binary SMBH detection claim. With this in mind, we discuss the possible reasons for the nondetection of any VLBI radio cores in our imaging, the possibility of a binary SMBH still residing in NGC 7674, and the prospect of future observations shedding further light on the true nature of this active galactic nucleus.

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11

Minns, A. R., J. M. Riley, P. J. Warner, M. J. Rioja, and H. J. A. Röttgering. "The Nature of a Homogeneous Sample of Compact Radio Sources Variable at 151 MHz." International Astronomical Union Colloquium 164 (1998): 281–82. http://dx.doi.org/10.1017/s0252921100045565.

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AbstractGlobal VLBI snapshot observations at 1.6 GHz (5 EVN + 4 VLBA) of a sample of low-frequency variable (LFV) radio sources found using the Cambridge Low Frequency Synthesis Telescope (CLFST) are presented. Variability at 151 MHz is almost certainly due to propagation effects in our Galaxy (e.g., refractive interstellar scintillation). Theory indicates that only sources with a significant fraction of their low-frequency flux coming from components with angular sizes ≲ 30 mas will show such effects. This is confirmed by the VLBI images.

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12

Ulvestad, J. S., and R. P. Linfield. "ARISE - A Proposed Future Space VLBI Mission." International Astronomical Union Colloquium 164 (1998): 397–98. http://dx.doi.org/10.1017/s025292110004611x.

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AbstractWe are developing a future space VLBI mission, ARISE, that would involve placing a VLBA-equivalent radio telescope in a highly elliptical Earth orbit. The telescope would operate at VLBI frequencies of 5, 22, 43, and 86 GHz, as well as being capable of single-dish operation at 60 GHz. Fringe-detection thresholds will be less than 10 mJy except at 86 GHz. Key scientific goals include dual-polarization imaging of blazars on the same physical scale as their γ-ray emission, astrophysical and astrometric studies of extragalactic water megamasers, and single-dish observations of O2.

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13

Whitney, Alan R. "The Mark IV VLBI Data-Acquisition and Correlation System." Symposium - International Astronomical Union 156 (1993): 151–57. http://dx.doi.org/10.1017/s0074180900173139.

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Modern VLBI observations for both astronomy and geodesy continue to demand the utmost in sensitivity. Of the methods potentially available for increasing the sensitivity of continuum VLBI observations, increasing the recorded bandwidth is generally the most cost effective.Over the past two years a broadly-supported program has been underway at Haystack Observatory to increase the sensitivity of the Mark IIIA VLBI system by more than a factor of 2. The result is an upgrade to the existing Mark IIIA data-acquisition system, dubbed Mark IV, which increases the maximum data rate to 1024 Mbits/sec, more than quadrupling the maximum data-rate of the Mark IIIA.A new correlator, based on a new custom VLSI correlator chip is also being designed to support the 1 Gbit/sec data rates from the Mark IV data-acquisition-system. An international collaborative effort is being mounted to help defray the high costs of development.

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14

Chen, Yufan (Jerry). "Understanding Black Hole Imaging Based on Very Large Baseline Interferometry (VLBI)." Applied Science and Innovative Research 6, no. 3 (June 15, 2022): p21. http://dx.doi.org/10.22158/asir.v6n3p21.

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In 2019, the Event Horizon Telescope made history when it captured the first known image of a blackhole. The black hole, situated in the center of the Messier 87 galaxy, is more than 55 million light-years away from earth, and was only able to be captured using Very Large Baseline Interferometry (VLBI) technology and the computational imaging technologies integrated into the Event Horizon Telescope array (The Event Horizon Telescope Collaboration, 2019). Astronomical image processing and interferometry requires the collection of radio waves using a radio telescope and analyzing that data with software. VLBI collects signals with multiple telescopes simultaneously, and the resulting data can be reduced and analyzed as data collected by a telescope with the diameter equal to the largest distance between the telescopes, thus is capable of producing an image with higher angular resolution and capturing objects further away from earth. However, due to the distance between the VLBI telescopes, algorithms are needed to fill in the hole within the collected data and reduce atmospheric noise and delays in signals (Very Long Baseline Array (VLBA)). We conduct an in depth review of the algorithm and VLBI as a whole in this paper, and hope to use our findings to further push the development of this great technology.

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15

Troitskiy, I. S., D. A. Morozova, S. G. Jorstad, V. M. Larionov, A. P. Marscher, I. Agudo, D. A. Blinov, and P. S. Smith. "Multiwavelength Observations of 6 FSRQ in 2008–2012." Proceedings of the International Astronomical Union 9, S304 (October 2013): 249–51. http://dx.doi.org/10.1017/s1743921314003974.

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AbstractWe present results of 4 years of VLBA monitoring along with γ-ray and optical R-band photometric observations of 6 blazars (0420-014, 1156+295, 1222+216, PKS 1510-089, 1633+382 and CTA 102). We have analyzed total intensity images obtained with the VLBA at 43 GHz and investigated kinematic evolution of the pc-scale jets of the sources. For all sources we compare flux variations in the VLBI core and bright superluminal knots with γ-ray and optical light curves. The majority of γ-ray flares are coincident with the appearance of a new superluminal knot and/or a flare in the millimeter-wave core and at optical wavelengths. These results support the conclusion that for many flares in blazars the region of the enhanced γ-ray and optical emission is located in the vicinity or downstream of the mm-wave VLBI core.

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16

Kim, J. Y., T. P. Krichbaum, A. P. Marscher, S. G. Jorstad, I. Agudo, C. Thum, J. A. Hodgson, et al. "Spatially resolved origin of millimeter-wave linear polarization in the nuclear region of 3C 84." Astronomy & Astrophysics 622 (February 2019): A196. http://dx.doi.org/10.1051/0004-6361/201832920.

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We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular resolution of 50 μas (corresponding to ∼200Rs). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼105–6 rad m2 in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.

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17

Verrone, G., D. Picca, and C. de Martino. "Geodetic VLBI data analysis using VLBI3 software." Il Nuovo Cimento C 13, no. 5 (September 1990): 817–22. http://dx.doi.org/10.1007/bf02511998.

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18

Colomer, Francisco. "Multiline Study with VLBI of SiO Masers in Evolved Stars." Symposium - International Astronomical Union 205 (2001): 322–25. http://dx.doi.org/10.1017/s0074180900221360.

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We present preliminary results of our observations of the 7 mm (v=1 and v=2 J=1-0) and 3 mm (v=1 J=2-1) SiO masers in the circumstellar envelopes of evolved stars. The 7mm masers were observed with the Very Large Baseline Array (VLBA) and the European VLBI Network (EVN), and the 3mm ones with the Coordinated Millimeter VLBI Array (CMVA). We find similarities between the maps, despite the non-coincidence in the location of most spots in the different 7 mm lines. The visibilities found for most compact spots are compatible with maser sources as small as 0.13 mas (or 0.023 AU at the distance of R Cas).

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19

Gabuzda, D. C., A. B. Pushkarev, and T. V. Cawthorne. "3.6 cm VLBI Total Intensity and Polarization Images of BL Lacertae Objects." Symposium - International Astronomical Union 175 (1996): 51–52. http://dx.doi.org/10.1017/s0074180900079985.

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The major distinguishing features of BL Lacertae Objects are weak or absent line emission and strong and variable optical, infrared, and radio polarization (Angel and Stockman 1980; Kollgaard 1994). The radio emission and much of the optical emission is believed to be synchrotron radiation. There are now some 20 BL Lacertae objects for which VLBI polarization (VLBP) images have been made at λ = 6 cm (Gabuzda et al. 1994 and references therein). In nearly every BL Lacertae object in which polarization structure has been detected, the polarization position angles in knots in the jets are nearly parallel to the VLBI structural axis. Assuming the jet components to be optically thin, the magnetic fields inferred by this orientation are nearly perpendicular to the direction of the jet; perhaps the most natural interpretation of this is that the knots are associated with shocks that compress an initially tangled magnetic field as they propagate down the VLBI jet, enhancing the magnetic field transverse to the compression (Laing 1980; Hughes, Aller, & Aller 1989).

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20

Baek, Junhyun, Aeree Chung, Kevin Schawinski, Kyuseok Oh, O. Ivy Wong, Michael Koss, Claudio Ricci, Benny Trakhtenbrot, Krista Lynne Smith, and Yoshihiro Ueda. "BAT AGN Spectroscopic Survey – XVII. The parsec-scale jet properties of the ultrahard X-ray-selected local AGNs." Monthly Notices of the Royal Astronomical Society 488, no. 3 (July 25, 2019): 4317–28. http://dx.doi.org/10.1093/mnras/stz1995.

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ABSTRACT We have performed a very long baseline interferometry (VLBI) survey of local (z < 0.05) ultrahard X-ray (14–195 keV) selected active galactic nuclei (AGNs) from the Swift Burst Alert Telescope (BAT) using KVN, KaVA, and VLBA. We first executed fringe surveys of 142 BAT-detected AGNs at 15 or 22 GHz. Based on the result from the fringe surveys and archival data, we find 10/279 nearby AGN (∼4 per cent) VLBI have 22 GHz flux above 30 mJy. This implies that the X-ray AGNs with a bright nuclear jet are not common. Among these 10 radio-bright AGNs, we obtained 22 GHz VLBI imaging data of our own for four targets and reprocessed archival data for six targets. We find that, although our 10 AGNs observed with VLBI span a wide range of pc-scale morphological types, they lie on a tight linear relation between accretion luminosity and nuclear jet luminosity. Our result suggests that a powerful nuclear radio jet correlates with the accretion disc luminosity. We also probed the Fundamental Plane of black hole activity at VLBI scales (e.g. few milliarcsecond). The jet luminosity and size distribution among our sample roughly fit into the proposed AGN evolutionary scenario, finding powerful jets after the blow-out phase based on the Eddington ratio (λEdd)–hydrogen column density (NH) relation. In addition, we find some hints of gas inflow or galaxy–galaxy merger in the majority of our sample. This implies that gas supply via tidal interactions in galactic scale may help the central AGN to launch a powerful parsec-scale jet.

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21

Sawada-Satoh, S., M. Inoue, K. M. Shibata, S. Kameno, N. Nakai, V. Migenes, and P. J. Diamond. "HI and OH Absorption of the Nuclear Region of NGC 3079." Symposium - International Astronomical Union 205 (2001): 196–97. http://dx.doi.org/10.1017/s0074180900220949.

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We conducted an observation of weak HI and OH absorption features in the parsec-scale nuclear region of NGC 3079 using a global VLBI network; the VLBA, the VLA and the Green Bank 43-m telescope of NRAO. One mas corresponds to 0.076 pc in NGC 3079.

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22

Wietfeldt, R., J. Smith, J. Border, V. Altunin, M. Ryne, C. Lewis, J. Benson, et al. "Simulated Space VLBI Experiment using the Tidbinbilla 11-m DSN Tracking Station and SURFSAT and Simulated Satellites." International Astronomical Union Colloquium 164 (1998): 415–16. http://dx.doi.org/10.1017/s0252921100046200.

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AbstractWe describe a space VLBI experiment involving an earth-orbiting satellite (SURFSAT) and simulated satellites, a space VLBI ground tracking station, an array of ground radio telescopes, and a space VLBI correlator. The purpose of this experiment was to provide as complete as possible an end-to-end simulation of space VLBI in preparation for the first space VLBI mission, VSOP, and in particular to test the most critical aspect of space VLBI, viz. the ability to generate a stable and accurate frequency standard (clock) for the orbiting VLBI element.

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Hirabayashi, H. "Space VLBI." Symposium - International Astronomical Union 205 (2001): 422–27. http://dx.doi.org/10.1017/s0074180900221670.

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Space VLBI enables high angular resolution and high dynamic range imaging through an extension of ground-based VLBI. The TDRSS space VLBI experiments in the 1980s were followed by the first space VLBI imaging mission, VSOP, in the 1990s. The new century holds the promise of the VSOP-2 and ARISE missions, which aim for more sensitive, higher angular resolution and higher observing frequency capabilities. These missions will enable AGNs to be viewed much more clearly and will make broader science areas, including lower brightness sources, accessible. It is noted that in all space VLBI missions, international collaboration in global sense plays an important role.

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Shen, Zhi-Qiang, Philip G. Edwards, Seiji Kameno, Makoto Inoue, and James E. J. Lovell. "Space VLBI Observations of PKS 1921-293." Symposium - International Astronomical Union 205 (2001): 128–29. http://dx.doi.org/10.1017/s0074180900220706.

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We present results of the first space VLBI observations of PKS 1921-293. An inner jet component about 1.5 mas north of the core is revealed for the first time. The compact core is partially resolved, but still has a brightness temperature (at the source rest frame) of 3.0×1012 K. A spectral index map made by combining the 1.6 GHz VSOP image with the 5.0 GHz VLBA+Y image at the first epoch is also presented.

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Hada, Kazuhiro. "Relativistic Jets from AGN Viewed at Highest Angular Resolution." Galaxies 8, no. 1 (December 18, 2019): 1. http://dx.doi.org/10.3390/galaxies8010001.

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Accreting supermassive black holes in active galactic nuclei (AGN) produce powerful relativistic jets that shine from radio to GeV/TeV γ-rays. Over the past decade, AGN jets have extensively been studied in various energy bands and our knowledge about the broadband emission and rapid flares are now significantly updated. Meanwhile, the progress of magnetohydrodynamic simulations with a rotating black hole have greatly improved our theoretical understanding of powerful jet production. Nevertheless, it is still challenging to observationally resolve such flaring sites or jet formation regions since the relevant spatial scales are tiny. Observations with very long baseline interferometry (VLBI) are currently the only way to directly access such compact scales. Here we overview some recent progress of VLBI studies of AGN jets. As represented by the successful black hole shadow imaging with the Event Horizon Telescope, the recent rapid expansion of VLBI capability is remarkable. The last decade has also seen a variety of advances thanks to the advent of RadioAstron, GMVA, new VLBI facilities in East Asia as well as to the continued upgrade of VLBA. These instruments have resolved the innermost regions of relativistic jets for a number of objects covering a variety of jetted AGN classes (radio galaxies, blazars, and narrow-line Seyfert 1 galaxies), and the accumulated results start to establish some concrete (and likely universal) picture on the collimation, acceleration, recollimation shocks, magnetic field topology, and the connection to high-energy flares in the innermost part of AGN jets.

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Jauncey, DL. "VLBI in Australia ? A Review." Australian Journal of Physics 44, no. 6 (1991): 785. http://dx.doi.org/10.1071/ph910785.

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After two decades of Australian VLBI (very long baseline interferometry), high-resolution radio astronomy continues to be an active and fruitful research field. The status of Australian VLBI programs in astrophysics, astrometry and geodesy is reviewed and likely future developments are outlined. In addition to research programs with the Australian VLBI network, a number of successful collaborative projects are underway with overseas VLBI observatories. The inception of the Asia-Pacific Telescope will provide an important formal basis for fostering and extending international VLBI experiments in the Australian hemisphere. The APT will also serve a vital function in coordinating ground-based observations when the Soviet and Japanese VLBI space missions, Radioastron and VSOP, are launched in the middle of this decade. However, continued viable Australian participation in VLBI into the nineties will require new wide-bandwidth recording systems and an Australian VLBI correlator.

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Takiguchi, Hiroshi, Yasuhiro Koyama, Ryuichi Ichikawa, Tadahiro Gotoh, Atsutoshi Ishii, Thomas Hobiger, and Mizuhiko Hosokawa. "VLBI Measurements for Frequency Transfer." Proceedings of the International Astronomical Union 5, H15 (November 2009): 225. http://dx.doi.org/10.1017/s1743921310008926.

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AbstractWe carried out the intercomparison experiments between VLBI, GPS and DMTD to show the VLBI can measure the right time difference. We produced the artificial change using by line stretcher. At the artificial change part, VLBI and DMTD show good agreement, less than 10ps. The quantity and sense of VLBI results match well with DMTD results. Consequently, the geodetic VLBI technique can measure the right time difference.

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28

Kim, Jae-Young, Sang-Sung Lee, Jeffrey A. Hodgson, Juan-Carlos Algaba, Guang-Yao Zhao, Motoki Kino, Do-Young Byun, and Sincheol Kang. "Long-term millimeter VLBI monitoring of M 87 with KVN at milliarcsecond resolution: nuclear spectrum." Astronomy & Astrophysics 610 (February 2018): L5. http://dx.doi.org/10.1051/0004-6361/201732421.

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We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at ≲0.8 mas ~ 110Rs spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index α of ≳−0.37 (S ∝ ν+α) between 22 and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 and 129 GHz, which could naturally connect the 230 GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency ≳ 100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at >100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.

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MOLDÓN, J., M. RIBÓ, and J. M. PAREDES. "EVOLUTION OF THE RADIO OUTFLOW IN LS 5039." International Journal of Modern Physics: Conference Series 28 (January 2014): 1460196. http://dx.doi.org/10.1142/s2010194514601963.

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Gamma-ray binaries allow us to study physical processes such as particle acceleration up to TeV energies as well as very high energy gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with very long baseline interferometry (VLBI). We observed the gamma-ray binary LS 5039 with the VLBA at 5 GHz during five consecutive days. We present the observed radio morphological changes, which display a periodic orbital modulation. Multifrequency and multiepoch VLBI observations conducted during the last decade confirm that the morphological periodicity is stable on timescales of years. Using a simple model we show that the observed behavior is compatible with the presence of a young non-accreting pulsar with an outflow behind it.

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Fujisawa, K., K. Hachisuka, K. Sugiyama, A. Doi, M. Honma, Y. Yonekura, T. Hirota, et al. "The VLBI mapping survey of the 6.7 GHz methanol masers with the JVN/EAVN." Proceedings of the International Astronomical Union 8, S287 (January 2012): 288–89. http://dx.doi.org/10.1017/s174392131200717x.

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AbstractWe present VLBI maps of the 6.7 GHz methanol maser emission in 32 sources obtained using the Japanese VLBI Network (JVN) and the East-Asian VLBI Network (EAVN). All of the observed sources provide new VLBI maps, and the spatial morphologies have been classified into five categories similar to the results obtained from European VLBI Network observations (Bartkiewicz et al. 2009). The 32 methanol sources are being monitored to measure the relative proper motions of the methanol maser spots.

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Kochanev, P. Yu, and D. C. Gabuzda. "Serendipitous VLBI Observations of Intraday Variability in the BL Lac Objects 1334–127, 2131–021, and 2155–152." International Astronomical Union Colloquium 164 (1998): 273–74. http://dx.doi.org/10.1017/s0252921100045528.

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AbstractGlobal λ = 6 cm VLBI polarization observations of intraday variability in the three BLLacertae objects 1334–127, 2131–021, and 2155–152 are analyzed. Integrated VLA measurements during the VLBI observations show that although there were no substantial total intensity variations, the polarizations for these three sources varied significantly during the VLBI experiment. The VLBI data were divided into 2-3 hour segments in order to search for corresponding rapid variability in the VLBI polarization structure. Our analysis shows that the VLA polarization variability in 2155–152 and 2131–021 is present in the VLBI data (i.e., it occurred on mas scales), but this is not the case for 1334–127.

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32

Hirabayashi, H. "VLBI Activities in Japan and a Projected Space-VLBI Program." Symposium - International Astronomical Union 129 (1988): 449–56. http://dx.doi.org/10.1017/s0074180900135247.

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There are three institutes in Japan that are equipped with VLBI facilities. Of these, Nobeyama Radio Observatory started VLBI experiments with astrophysical and astrometric interests. The Institute of Space and Astronautical Science, the Nobeyama Radio Observatory, and the Radio Research Laboratory collaborated with JPL and other institutions for this first space-VLBI test experiment, generating fringes on ground-TDRSS satellite baselines at S band. A working group formed by scientists of the above three Japanese institutes proposed to ISAS to launch a space-VLBI satellite. The project, called VSOP, for VLBI Space Observatory Program, is described here.

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Gorgolewski, Stanislaw. "The Development of the Torun VLBI Station." Symposium - International Astronomical Union 129 (1988): 493–94. http://dx.doi.org/10.1017/s0074180900135387.

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The Torun VLBI Station is situated in Piwnice near the site of the Torun Radio Astronomical Observatory on the Nicolaus Copernicus University Land Estate, about 10 km north of Torun. The VLBI station development started in 1976, when the rubidium frequency standard was ordered and the ground was broken for the foundation of the 15-m equatorially mounted dish. The construction of the Mark II-C VLBI system began in 1977, and the completed VLBI station was first tested in 1980. The first fringes were obtained on May 31, 1981, with the Effelsberg 100-m dish. This experiment started our work with the European VLBI Network (EVN), and later in 1984 with the Global VLBI Network. We now have five radiometers and have received funding for the construction of a new VLBI station, with a 32-m dish, which is now in the final design stage.

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34

Asadi, Saghar, Erik Zackrisson, Eskil Varenius, Emily Freeland, John Conway, and Kaj Wiik. "The case against gravitational millilensing in the multiply-imaged quasar B1152+199." Monthly Notices of the Royal Astronomical Society 492, no. 1 (December 9, 2019): 742–48. http://dx.doi.org/10.1093/mnras/stz3450.

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ABSTRACT Previous very long baseline interferometry (VLBI) observations of the quasar B1152+199 at 5 GHz has revealed two images of a strongly lensed jet with seemingly discordant morphologies. Whereas the jet appears straight in one of the images, the other exhibits slight curvature on milliarcsecond scales. This is unexpected from the lensing solution and has been interpreted as possible evidence for secondary, small-scale lensing (millilensing) by a compact object with a mass of 105–107 M⊙ located close to the curved image. The probability for such a superposition is extremely low unless the millilens population has very high surface number density. Here, we revisit the case for millilensing in B1152+199 by combining new global–VLBI data at 8.4 GHz with two data sets from the European VLBI Network (EVN) at 5 GHz (archival), and the previously published 5 GHz Very Long Baseline Array (VLBA) data. We find that the new data with a more circular synthesized beam, exhibits no apparent milliarcsecond-scale curvature in image B. Various observations of the object spanning ∼15 yr apart enable us to improve the constraints on lens system to the point that the only plausible explanation left for the apparent curvature is the artefact due to the shape of the synthesized beam.

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35

Akiyama, K., M. Kino, B. Sohn, S. Lee, S. Trippe, and M. Honma. "Long-term monitoring of Sgr A* at 7 mm with VERA and KaVA." Proceedings of the International Astronomical Union 9, S303 (October 2013): 288–92. http://dx.doi.org/10.1017/s1743921314000751.

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AbstractWe present the results of radio monitoring observations of Sgr A* at 7 mm (i.e. 43 GHz) with the VLBI Exploration of Radio Astrometry (VERA), which is a VLBI array in Japan. VERA provides angular resolution on millisecond scales, resolving structures within 100 Schwarzschild radii of Sgr A* , similar to the Very Large Baseline Array (VLBA). We performed multi-epoch observations of Sgr A* in 2005 - 2008, and started monitoring it again with VERA from 2013 January to trace the current G2 encounter event. Our preliminary results in 2013 show that Sgr A* on mas scales has been in an ordinary state as of August 2013, although some fraction of the G2 cloud already passed the pericenter of Sgr A* in April 2013. We will continue monitoring Sgr A* with VERA and the newly developed KaVA (KVN and VERA Array).

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36

Kellermann, K. I. "VLBI in the 1990s." Publications of the Astronomical Society of Australia 8, no. 04 (1990): 327–29. http://dx.doi.org/10.1017/s1323358000023651.

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Abstract Radio telescopes in more than 20 different countries on five continents are being used for VLBI observations of galactic and extragalactic radio sources, as well for applications to geodesy and geophysics. New radio telescopes and networks of radio telescopes dedicated to VLBI which are being built in the United States, the U.S.S.R., Italy, China, Poland, the U.K. as well as in Australia will greatly improve the sensitivity, resolution, image quality, and frequency coverage of the existing networks. The Mk II VLBI recording system, which is based on consumer type Video Tape Recorders, is widely used but the sensitivity is limited. Broad band recording systems are expensive, but are becoming more popular due to their much greater sensitivity. They are expected to dominate VLBI systems in the 1990s. The first tests of an earth-to-space VLBI system took place in 1986, and by the mid 1990s dedicated VLBI antennas will be launched into earth orbit by the U.S.S.R. and by Japan in collaboration with Australia and other countries throughout the world. Space VLBI missions will give an order of magnitude improvement in resolution over the ground based VLBI networks operating at the same wavelengths.

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37

Wang, Pingli, Guangli Wang, Yuping Gao, Hongbing Cai, and Na Liu. "Comparison of VLBI and GNSS common view for time transfer." International Journal of Metrology and Quality Engineering 10 (2019): 15. http://dx.doi.org/10.1051/ijmqe/2019014.

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With the rapid development of optical clock, the stability and system uncertainty of optical clocks has reached a 1.0e–18 level. Optical clocks will likely constitute the next generation of time-frequency standards for redefining the SI second. Because time and frequency transfer services that rely on satellite systems are not always reliable and currently available technologies are insufficient for comparing the next generation of frequency standards, high-precision time and transfer techniques are strongly desired. Very Long Baseline Interferometry (VLBI) is one of the space geodetic techniques that measure the arrival time delays between multiple stations utilizing radio signals from distant celestial radio sources. Not only can VLBI obtain the angle position measurement of the radio source with sub-millisecond accuracy and the station coordinate measurement with millimeter accuracy, but also, it can provide high-precision information regarding inter-station atomic clock differences. Therefore, it is theoretically feasible to use the VLBI technology to do the remote time transfer. Because of this characteristic of VLBI technology, VLBI has significant application potential in the field of remote time transfer. To confirm the suitability of VLBI to time-frequency transfer for future practical applications, the results of VLBI and GPS common view time transfer were compared using a Kunming-Urumqi baseline. The performance characteristics of time transfer based on VLBI are then analyzed. Experimental results show that VLBI technology can accurately measure the variation of clock differences between stations as same as the GPS common view time comparison technology. It briefly describes the challenges of future VLBI technology for practical applications of time transfer.

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38

Petrov, Leonid, Chris Phillips, Alessandra Bertarini, Adam Deller, Sergei Pogrebenko, and Ari Mujunen. "Use of the Long Baseline Array in Australia for Precise Geodesy and Absolute Astrometry." Publications of the Astronomical Society of Australia 26, no. 1 (2009): 75–84. http://dx.doi.org/10.1071/as08030.

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AbstractWe report the results of a successful 12-hour 22-GHz VLBI experiment using a heterogeneous network that includes radio telescopes of the Long Baseline Array (LBA) in Australia and several VLBI stations that regularly observe in geodetic VLBI campaigns. We have determined positions of three VLBI stations, atca-104, ceduna and mopra, with an accuracy of 4–30 mm using a novel technique of data analysis. These stations have never before participated in geodetic experiments. We observed 105 radio sources, and amongst them 5 objects which have not previously been observed with VLBI. We have determined positions of these new sources with the accuracy of 2–5 mas. We make the conclusion that the LBA network is capable of conducting absolute astrometry VLBI surveys with an accuracy better than 5 mas.

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39

Kovalevsky, J. "Optical-Radio Reference Ties." International Astronomical Union Colloquium 165 (1997): 447–48. http://dx.doi.org/10.1017/s0252921100046935.

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In the course of the Hipparcos data reduction, the system of coordinates obtained is the result of complicated and partly random processes. It is therefore an arbitrary system which could not be retained. It was decided to compute a rotation ϵ at epoch (1991.25) and a time-dependent rotation ω (or spin) such that, when applied, it would be a realisation of the IAU extragalactic reference system as defined in 1991 and implemented by IERS in 1995.A working group was set up by the Hipparcos Science Team with the task to provide these rotations from all possible techniques. Actually eleven independent teams provided a solution. One can group them in four categories.1.Radio techniques included VLBI observations with the US Deep Space Network, VLBA and the European VLBI Network, and connected radio interferometers MERLIN and VLA. While VLBI provided a solution for both ϵ and ω, the other techniques allowed to get only ϵ.2.Medium and small field astrometry including the optical positions of compact sources (Hamburg and USNO), and observations with the HST which both provided solutions for ϵ only and comparisons of plates taken at large intervals of time (Bonn and Potsdam) which gave solutions for ω only.3.Star catalogues refered to galaxies: Lick Northern proper motion program, Yale Southern proper motion program, and KSZ catalogue. Yale University, ARI in Heidelberg and Kiev Observatory yielded solutions for ω.4.Comparison of Earth rotation parameters as obtained by VLBI and by a reduction of ground-based determinations with Hipparcos data (Prague Observatory) gave components of ϵ and ω in the equator.

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40

Lonsdale, Colin J., and Sheperd S. Doeleman. "Techniques for Analyzing Short Coherence Time VLBI Data." International Astronomical Union Colloquium 164 (1998): 407–8. http://dx.doi.org/10.1017/s0252921100046169.

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AbstractWe have written a suite of programs designed to deal with problems peculiar to short-wavelength VLBI. The most important difference between mm-VLBI and cm-VLBI is that one usually wishes to integrate the data for times which are long compared to the atmospheric coherence time. This causes difficulties with standard VLBI data reduction techniques, and mandates the use of the algorithms implemented in our package for accurate results.

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41

Jianqing, Cai, and Xia Hanlin. "Prospects for the Application of a Mobile VLBI System in China." Symposium - International Astronomical Union 156 (1993): 421–26. http://dx.doi.org/10.1017/s0074180900173644.

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Based on the present status and projects of Chinese VLBI and SLR fixed stations, the applications of a mobile VLBI system are investigated in establishing a Chinese high accuracy geodetic VLBI network and terrestrial reference frame.

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42

Zhao, Xu, Wei-Min Zheng, Li Tong, and Mao-Li Ma. "Dual-target Signal Simulation of Very Long baseline Interferometry for Chang’E-5." Publications of the Astronomical Society of the Pacific 134, no. 1038 (August 1, 2022): 084502. http://dx.doi.org/10.1088/1538-3873/ac8579.

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Abstract Very long baseline interferometry (VLBI) is an important deep-space tracking technology from radio astronomy. A mission that uses VLBI, China’s lunar sample-return mission Chang’E-5 (CE-5) collects soil samples on the lunar surface, arrives at a rendezvous point, and docks in the lunar orbit through cooperation of the Orbiter and the Ascender. During rendezvous and docking, the Chinese VLBI network (CVN) used same beam VLBI (SBI) to observe two high-velocity probes simultaneously. However, before the launching of the CE-5, CVN lacked the capability to examine dynamic dual-target tracking ability of SBI mode because there were no similar space probes can be used in tests of upgraded VLBI data-processing system. This study proposes a method to simulate VLBI signals and acquire relevant data from the dual-target probe and calibration quasar signals recorded by each station according to the orbits and beacons of probes, station coordinates, etc. The real-time data-processing capability of the system is verified by processing the simulated data during rendezvous and docking. The consistency between simulation and VLBI data-processing was consistent to better than 0.05 ns. This method is universal and can also be used in VLBI simulation and orbital analysis of other deep-space probes.

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43

Yan, Zhen, Zhi-qiang Shen, Jian-ping Yuan, Na Wang, Helge Rottmann, and Walter Alef. "VLBI astrometry of two millisecond pulsars." Proceedings of the International Astronomical Union 8, S291 (August 2012): 562–64. http://dx.doi.org/10.1017/s174392131202488x.

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AbstractWe present astrometric results on two millisecond pulsars, PSR B1257+12 and PSR J1022+1001, as carried out through VLBI. For PSR B1257+12, a model-independent distance of 710−38+43 pc and proper motion of (μα = 46.44 ± 0.08 mas/yr, μδ=−84.87 ± 0.32 mas/yr) were obtained from 5 epochs of VLBA and 4 epochs of EVN observations, spanning about 2 years. The two dimensional proper motion of PSR J1022+1001 (μα~−10.13 mas/yr, μδ~16.89 mas/yr) was also estimated, using 3 epochs of EVN observations. Based on our results, the X-ray efficiency of PSR B1257+12 should be in the same range as other millisecond pulsars, and not as low as previously thought.

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44

Rushton, A., R. E. Spencer, M. Strong, R. M. Campbell, S. Casey, R. P. Fender, M. A. Garrett, et al. "First e-VLBI observations of GRS 1915+105." Proceedings of the International Astronomical Union 2, S238 (August 2006): 437–38. http://dx.doi.org/10.1017/s1743921307005832.

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AbstractWe present results from the first successful open call e-VLBI science run, observing the X-ray binary GRS 1915+105. e-VLBI science allows the rapid production of VLBI radio maps, within hours of an observation rather than weeks. A total of 6 telescopes observing at 5 GHz across the European VLBI Network (EVN) were correlated in real time at the Joint Institute for VLBI in Europe (JIVE). Throughout this, GRS 1915+105 was observed for a total of 5.5 hours, producing 2.8 GB of visibilities of correlated data. The peak brightness was 10.2 mJy per beam, with a total integrated radio flux of 11.1 mJy.

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45

Schilizzi, R. T. "New Developments in European VLBI." Symposium - International Astronomical Union 129 (1988): 469–73. http://dx.doi.org/10.1017/s0074180900135284.

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Five major areas of development in European VLBI are reviewed: new telescope millimeter-wavelength VLBI; Mk2 and Mk3 correlators, the European Data Processing Facility for the 1990's and a 5 GHz receiver for the RADIOASTRON space VLBI program.

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46

Pashchenko, Ilya N., and Alexander V. Plavin. "Inferring the jet parameters of active galactic nuclei using Bayesian analysis of VLBI data with a non-uniform jet model." Monthly Notices of the Royal Astronomical Society 488, no. 1 (July 13, 2019): 939–53. http://dx.doi.org/10.1093/mnras/stz1677.

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ABSTRACTThe physical parameters of the jets of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI) are usually inferred from core-shift measurements or from the flux and size measured at the peak frequency of the synchrotron spectrum. Both methods are preceded by modelling the observed VLBI jet structure with simple Gaussian templates. Here we infer the jet parameters using an inhomogeneous jet model directly, bypassing the modelling of the source structure with a Gaussian template or image deconvolution. We apply Bayesian analysis to multifrequency VLBA observations of radio galaxy NGC 315 and find that its parsec-scale jet is well described by an inhomogeneous conical model. Our results favour an electron–positron jet. We also detect a component as a part of a counter jet. Its position implies the presence of an external absorber with a steep density gradient close (r = 0.1 pc) to the central engine.

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47

Zheng, Dawei, Shifang Luo, and J. R. Mackay. "An Optimal Identification of Clock Behaviour Model for VLBI." Symposium - International Astronomical Union 129 (1988): 525–28. http://dx.doi.org/10.1017/s0074180900135508.

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The accuracy of VLBI measument has reached centimeters (Carter et al., 1985). But in VLBI observation the observable are broken sometimes because of the variations of the clock phases or frequencies. This is the first problem met in VLBI data processing.

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48

Reynolds, J. E. "VLBI Astrometry." Symposium - International Astronomical Union 158 (1994): 437–44. http://dx.doi.org/10.1017/s0074180900108149.

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VLBI is at present the most accurate technique for measuring radiosource positions and the only method capable of high precision for a reasonable number of sources. The applications of VLBI astrometry in stellar, Galactic and extra-Galactic regimes are reviewed. In particular, substantial progress has been made in the last few years towards a global reference frame of extragalactic radiosource positions. The status of this frame, and of the link to the optical reference frames is also described.

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Likhachev, S. F., and R. M. Hjellming. "High Resolution Imaging of Compact Radio Sources by Gridding with Regularization." International Astronomical Union Colloquium 164 (1998): 411–12. http://dx.doi.org/10.1017/s0252921100046182.

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AbstractThe problem of VLBI image reconstruction is a classical example of an ill-posed problem. A new procedure of gridding with regularization has been developed. This procedure was used in traditional methods (CLEAN, Hybrid) to improve the quality of compact radio source images. A few sources (GRO J1655–40, RY Scuti and Cyg X-1), observed with the VLA and VLBA, were processed with this procedure.

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Kameya, Osamu. "Distribution of ionized gas density measured by differential VLBI Observations of pulsars." Proceedings of the International Astronomical Union 8, S291 (August 2012): 417. http://dx.doi.org/10.1017/s1743921312024337.

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AbstractDifferential VLBI observations of pulsars in our Galaxy can derive trigonometric parallax of them. Distance to pulsars derived by the parallax are very important to estimate some mean density of ionized gas between pulsars and the earth using rotation measures of them. Some preliminary results of distribution of the ionized gas density in our galaxy by using previous VLBI results are shown. Possibility of VLBI observations of pulsars using VERA and the other VLBI antennas will be described.

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