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• 1.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Combined-Array Imaging of Extra-Galactic Radio Sources1994In: Astronomy with Millimeter and Submillimeter Wave Interferometry: IAU Colloquium 140 : Meeting held 5-9 October 1992, Hakone, Japan / [ed] M. Ishiguro & Wm. J. Welch, San Francisco: Astronomical Society of the Pacific , 1994, p. 123-124Conference paper (Refereed)
• 2.
Laboratorio de Astrofísica Espacial y Física Fundamental, Madrid, Spain; Instituto de Astrofísica de Andalucía CSIC, Granada, Spain.
Max Planck Institut für Radioastronomie, Bonn, Germany. Max Planck Institut für Radioastronomie, Bonn, Germany. IRAM, Domaine Universitaire, Grenoble, France. IRAM, Domaine Universitaire, Grenoble, France. Departamento de Astronomía Universitat de Valéncia, Spain. Onsala Space Observatory. Onsala Space Observatory. Onsala Space Observatory; Observatorio de Yebes, Guadalajara, Spain. NEROC, Haystack Observatory, Westford, Massachusetts, USA. Boston University, Boston, MA, USA. NEROC, Haystack Observatory, Westford, Massachusetts, USA. IRAM, Domaine Universitaire, Grenoble, France; Institut für Weltraumsensorik (DLR), Berlin, Germany. Max Planck Institut für Radioastronomie, Bonn, Germany.
The high-frequency compact radio structure of the peculiar quasar 4C 39.251997In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 327, no 2, p. 513-521Article in journal (Refereed)

We present new high angular resolution images of the compact non-thermal radio source 4C 39.25 obtained from VLBI observations at λ1.3cm, λ7mm, and λ3mm wavelengths. These maps and Gaussian model-fits show that the milli-arcsecond to sub-milliarcsecond structure of 4C 39.25 consists of a complex bent core-jet structure with embedded moving and stationary VLBI components. Facilitated by the small observing beams and high angular resolutions obtained at mm-wavelengths, we measured the relative positions of the jet components with an accuracy of a few hundred micro-arcseconds. This allows the detailed followup of the ongoing merging process of a westward superluminally moving component (b_) with a stationary component a_, located at ~2.9 mas east of the putative core d_. In contrast to the other components of the structure with steeper spectra, the westernmost component d_ exhibits an inverted spectrum peaking between λ7mm and λ3mm, thus further supporting its identification as the VLBI core, self-absorbed at longer wavelengths. From two VLBI maps obtained nearly simultaneously at λ7mm and λ1.3cm, we made the first spectral index map of 4C 39.25 in this wavelength regime. The main characteristics of the spectral index distribution of the jet are pronounced changes of the spectral index between orientations parallel and transverse to the jet axis. Near the merging components a_ and b_ the spectral index steepens with increasing separation from d_. However, in the bridge of emission c_, which connects d_ with a_ and b_, the spectral index gradient has a direction transverse to the jet axis, suggesting a frequency dependent jet curvature and edge-brightening. A brief discussion of this behaviour within current jet models is presented.

• 3.
Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA; Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Astrophysical Observatory, Cambridge, MA, USA.
Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA; Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Astrophysical Observatory, Cambridge, MA, USA. NEROC, Haystack Observatory, Westford, MA, USA. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA & Max-Planck-Institut für Radioastronomie, Bonn, Germany. NEROC, Haystack Observatory, Westford, MA, USA. NEROC, Haystack Observatory, Westford, MA, USA. NEROC, Haystack Observatory, Westford, MA, USA. Steward Observatory, Tucson, AZ, USA. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
The Compact Radio Source 2021+614: Simultaneous 2.3 and 8.3 GHz Mark III VLBI Observations1984In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 279, no 1, p. 116-121Article in journal (Refereed)

Second-epoch VLBI observations of the flat-spectrum radio source 2021+614 made simultaneously at 2.3 and 8.3 GHz with the Mark III system are reported. The maps derived from these observations reveal a complex, frequency-dependent radio structure on the milliarcsecond scale. The object has four nearly collinear components oriented at a position angle of about 35 degrees and embedded in an extended region. Two of these are optically thin, one has a flat spectrum, and the other appears to be synchrotron self-absorbed in the frequency range 2.3-8.3 GHz. No significant structural variation is found at either frequency between epochs separated by about three years. The formal estimate of the transverse velocity between two components, one with a flat and the other with an inverted spectrum, is v/c = 0.7 + or - 2.3. Remarkable similarities between 2021+614 and the unusual source 0316+413 are discussed.

• 4.
University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank, Cheshire, United Kingdom.
University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank, Cheshire, United Kingdom. University of Manchester, Nuffield Radio Astronomy Laboratories, Jodrell Bank, Cheshire, United Kingdom. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
High-resolution observations of quasars from the Parkes +/- 4 deg. sample1979In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 188, p. 159-163Article in journal (Refereed)

VLBI observations of 20 compact quasars have been made between Jodrell Bank and Onsala at a frequency of 1666 MHz. Twelve of the quasars have inverted or peaked spectra at centimetre wavelengths and these are all unresolved, having angular diameters of less than 0.015 arcsec. Two out of five quasars with overall flat spectra are partially resolved on this scale size, as are three steep-spectrum quasars.

• 5.
Onsala Space Observatory, Onsala, Sweden.
mm VLBI1993In: VLBI Technology – Progress and Future Observational Possibilities: Proceedings of the International Symposium Held at Kyoto International Conference Hall on September 6-10, 1993 / [ed] Tetsuo Sasao, Seiji Manabe, Osamu Kameya & Makoto Inoue, Tokyo: Terra Scientific Publishing Company , 1993, p. 70-74Chapter in book (Refereed)

Very long Baseline Interferometry (VLBI) has now made its break-through into the mm wavelength regime. A global VLBI array has produced maps of radio sources at $\lambda$3 mm since 1988 and development is under way to improve the sensitivity for VLBI also at $\lambda$1 mm. This contribution discusses the present state of mm VLBI and the future developments.

• 6.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
A closer look at active galactic nuclei — the great engines of the universe1992In: Physica scripta. T, ISSN 0281-1847, Vol. T43, p. 57-66Article in journal (Refereed)

The intensive radio emission from powerful radio galaxies and quasars Make these attractive candidates to become "standard candles" to probe the Universe. This paper discusses this possibility and the physics of the radio sources.

• 7.
Onsala Space Observatory, Onsala, Sweden.
AGN Variability and VLBI1994In: Multi-Wavelength Continuum Emission of Agn: Proceedings of the 159th Symposium of the International Astronomical Union, Held in Geneva, Switzerland, August 30 – September 3, 1993 / [ed] T.J.-L. Courvoisier & A. Blecha, Dordrecht: Kluwer Academic Publishers, 1994, p. 181-186Chapter in book (Refereed)

This contribution discusses the connection between variability in radio and optical with structural variations observed with VLBI. Structural changes do not have to start in the core, and intensity variations may be caused by components in the jet outside the core. The scenario is probably more complicated than present day theories assume.

• 8.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Astrophysical global interferometry at Onsala Space Observatory1982In: Reports from the Observatory of Lund, ISSN 0349-4217, Vol. 18, p. 23-25Article in journal (Refereed)
• 9.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Global Fringe Fitting Applied to 100 GHZ VLBI Data1991In: Frontiers of VLBI: Proceedings of the International VSOP Symposium held at the Institute of Space and Astronautical Science on December 5-7, 1989, and Proceedings of the mm-Wave VLBI Workshop held at the Nobeyama Radio Observatory on December 8-9, 1989 / [ed] H. Hirabayashi, M. Inoue & H. Kobayashi, Tokyo: Universal Academy Press , 1991, p. 353-359Chapter in book (Refereed)

Earlier epoches of mm VLBI observations relied on single baseline fit of delays and rates to find the fringes. This approach does not, however, make use of all available information and therefore is less sensitive than is necessary. Global fringe fitting makes use of all simultaneous data to find station related clock offsets and rates over a certain period of time. © Universal Academy Press, Inc.

• 10.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Global fringe fitting applied to Mk3 VLBI data1991In: Radio Interferometry: Theory, Techniques, and Applications / [ed] T.J. Cornwell & R.A. Perley, San Francisco, USA: Astronomical Society of the Pacific , 1991, p. 321-325Conference paper (Refereed)

A system has been developed where data from a Mk3 VLBI processor can be read into the AIPS-package. Multiband and single-band delays can then be fitted globally by station. The technique has been used to phasereference a background to two radiogalaxies in the Abell cluster A2634.

• 11.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
High resolution images of Blazar cores1992In: Variability of Blazars: Proceedings of a conference in honour of the 100th anniversary of the birth of academician Yrjö Väisälä, held at Tuorla Observatory, University of Turku, Finland, January 6-10, 1991 / [ed] E. Valtaoja & M. Valtonen, Cambridge, UK: Cambridge University Press, 1992, p. 196-204Conference paper (Refereed)
• 12.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
High Resolution Observations of Low Frequency Variables at 932 MHz1987In: The Impact of VLBI on Astrophysics and Geophysics: Proceedings of the 129th Symposium of the International Astronomical Union held in Cambridge, Mass., U.S.A., May 10-15, 1987 / [ed] Reid, M. J. & Moran, J. M., Dordrecht: Kluwer Academic Publishers, 1987, p. 117-118Chapter in book (Refereed)

Fifteen sources known to be varying at low frequencies have been observed at six epochs during 1983–84 with a global VLBI array. Some of the sources show structural variations similar to the superluminals. Beaming effect may therefore play an important role at low as well as at higher frequencies.

• 13.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Mapping in Practice1989In: Very Long Baseline Interferometry: Techniques and Applications / [ed] Felli, Marcello & Spencer, Ralph E., Dordrecht: Kluwer Academic Publishers, 1989, p. 199-230Chapter in book (Refereed)

The purpose of this lecture is to introduce the VLBI user to the practical details of mapping radio sources. I will concentrate on data processing and mapping with the AIPS (Thompson and D’Addario, 1982)-package. Other data reduction systems do exist, e.g. the Caltech-package and OLAF. These differ from AIPS mostly in the strategy for reaching the final and “best” map. It is more important to know the method and to be careful than what reduction procedure is used.

• 14.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Millimetre VLBI capability status1993In: Sub-arcsecond Radio Astronomy: Proceedings of the Nuffield Radio Astronomy Laboratories' conference, held in Manchester, July 20–24, 1992 / [ed] R. J. Davis & R. S. Booth, Cambridge: Cambridge University Press, 1993, p. 431-433Chapter in book (Refereed)

The development of new reveiver and data reduction techniques have now made VLBI at mm wavelengths possible. This contribution discusses the capability of the present and future VLBI networks at λ1 and 3mm and compares with radio interferometers at other wavelength regimes.

• 15.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
mm VLBI vs. VSOP1991In: Frontiers of VLBI: Proceedings of the International VSOP Symposium held at the Institute of Space and Astronautical Science on December 5-7, 1989, and Proceedings of the mm-Wave VLBI Workshop held at the Nobeyama Radio Observatory on December 8-9, 1989 / [ed] H. Hirabayashi, M. Inoue & H. Kobayashi, Tokyo: Universal Academy Press , 1991, p. 197-201Chapter in book (Refereed)

VSOP-to-ground VLBI and mm VLBI are both needed if we want to further increase our knowledge of quasars, radio galaxies and their radio jets, mm VLBI will be a better instrument to study the "central engine" while satellite VLBI will better show the structure of the jets. The two instruments complement each other, and it is important to have a close collaboration. © Universal Academy Press, Inc.

• 16.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
New millimetric VLBI results1992In: Extragalactic radio sources - from beams to jets: Proceedings of the 7th IAP Meeting held at the Institut d'Astrophysique de Paris, Paris, France 2-5 July 1991 / [ed] J. Roland, H. Sol and G. Pelletier, Cambridge: Cambridge University Press, 1992, p. 137-144Conference paper (Refereed)
• 17.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Radiogalaxer och kvasarer1990In: Astronomisk årsbok 1991, Johanneshov: Inova , 1990, p. 71-77Chapter in book (Refereed)
• 18.
Institutionen för astronomi, Lunds universitet, Lund, Sverige.
Radiospår i galaxhopar1973In: Astronomisk tidsskrift, ISSN 0004-6345, Vol. 6, no 1, p. 18-24Article in journal (Other academic)
• 19.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Results from 100 GHz VLBI1991In: Frontiers of VLBI: Proceedings of the International VSOP Symposium held at the Institute of Space and Astronautical Science on December 5-7, 1989, and Proceedings of the mm-Wave VLBI Workshop held at the Nobeyama Radio Observatory on December 8-9, 1989 / [ed] H. Hirabayashi, M. Inoue & H. Kobayashi, Tokyo: Universal Academy Press , 1991, p. 285-295Chapter in book (Refereed)

Development of receiver and data reduction techniques have now made it possible to produce hybrid maps from global VLBI experiments observing at 100 GHz. Maps are shown here of the compact radio sources 3C 273, 3C 345, 3C 84, BL Lac and OJ 287 with angular resolution of 50 μas. The component born during the 1988 outburst of 3C 273 is seen only 2 month after its birth. The jet of 3C 345 is seen to have a larger curvature than has previously been observed.A component is seen moving outwards from the core of 3C84 with a speed of ≈21000 km sec-1. © Universal Academy Press, Inc.

• 20.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Superluminal Motion in CTA 1021987In: Superluminal Radio Sources: Proceedings of a Workshop in honor of Professor Marshall H. Cohen, held at Big Bear Solar Observatory, California, October 28-30, 1986 / [ed] Zensus, J. Anton & Pearson, Timothy J., Cambridge: Cambridge University Press, 1987, p. 206-210Chapter in book (Refereed)
• 21.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
The Milliarcsecond Structure of some Extragalactic Objects1980Doctoral thesis, comprehensive summary (Other academic)
• 22.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Varying gamma in 3C2731992In: Variability of Blazars: Proceedings of a conference in honour of the 100th anniversary of the birth of academician Yrjö Väisälä, held at Tuorla Observatory, University of Turku, Finland, January 6-10, 1991 / [ed] E. Valtaoja & M. Valtonen, Cambridge: Cambridge University Press, 1992, p. 229-233Conference paper (Refereed)
• 23.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
VLBI monitoring of BL Lac objects1983In: Highlights of Astronomy: as presented at the ... General assembly of the IAU. Vol. 6, As presented at the XVIIIth General assembly of the IAU, 1982 [in Patras, Greece, 17-26 August] / [ed] Richard M. West, Dordrecht: D. Reidel Publishing Group, 1983, p. 742-742Chapter in book (Refereed)
• 24.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
VLBI Monitoring of Bl-Lacertae Objects1984In: VLBI and Compact Radio Sources: Symposium no. 110 held in Bologna, Italy, June 27-July 1, 1983 / [ed] R. Fanti, K. Kellermann & G. Setti, Dordrecht: D. Reidel Publishing Group, 1984, p. 127-130Conference paper (Refereed)

This is a report on an ongoing project designed to study structural variations on the milliarcsecond scale in four BL Lac-type objects. The observations are made at λ6 cm with a global VLBI network consisting of the combined US and European networks. Hybrid maps at three epochs are presented for each of the sources AO 0235+164,0735+178, Mk 421, and 1749+701.

• 25.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
VLBI observations at 100 GHz1990In: Parsec-Scale radio jets: proceedings of a Workshop held at the National Radio Astronomy Observatory, Socorro, New Mexico, October 17-18, 1989 / [ed] Zensus, J. Anton & Pearson, Timothy J., Cambridge: Cambridge University Press, 1990, p. 91-97Chapter in book (Refereed)

We present maps using VLBI at 100 GHz of three sources, 3C273, 3C345, and 3C84. The resolution is 50 μas, the highest yet achieved.

• 26.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
MmVLBI - High resolution imaging1991In: Radio Interferometry: Theory, Techniques, and Applications / [ed] T.J. Cornwell & R.A. Perley, San Francisco, USA: Astronomical Society of the Pacific , 1991, p. 326-329Conference paper (Refereed)

Hybrid maps at 100 GHz have been produced using VLBI data obtained at 100 GHz. The observations and global fringe fitting technique used to produce the maps are briefly described.

• 27.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Massachusetts Institute of Technology, Cambridge, MA, USA. Massachusetts Institute of Technology, Cambridge, MA, USA. Massachusetts Institute of Technology, Cambridge, MA, USA. Massachusetts Institute of Technology, Cambridge, MA, USA. Haystack Observatory, Westford, MA, USA. Haystack Observatory, Westford, MA, USA. Haystack Observatory, Westford, MA, USA. Haystack Observatory, Westford, MA, USA. Goddard Space Flight Center, Greenbelt, MD, USA. Goddard Space Flight Center, Greenbelt, MD, USA. Jet Propulsion Laboratory, Pasadena, CA, USA.
Time-Dependent Radio Fine Structure of the Compact Sources NRAO 150 and 4C 39.251980In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 86, no 3, p. 364-372Article in journal (Refereed)

Very long baseline interferometer observations at 7.85 GHz have been used to probe the milliarcsecond structure of the unidentified, very compact radio source NRAO 150 and QSO 4 C 39.25. NRAO 150 exhibited no structural variations from 1972 to the end of 1974. A model with two circular Gaussian components fits the data well. NRAO 150 had a flux density of 7.6 plus or minus 0.5 Jy in the compact component; 4 C 39.25 showed a two-component structure, the components having a separation of (2.02 plus or minus 0.05 arc sec) x 10 to the -3rd power. The upper bound on the speed of transverse separation is 0.0001 arc sec per year or less than 2.7 c. From the spectrum there are also indications of a third, larger component.

• 28.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Owens Valley Radio Observatory, Big Pine, CA, USA. Helsinki University of Technology, Radio Laboratory, Espoo, Finland. Netherlands Foundation for Radio Astronomy, Dwingenloo, The Netherlands.
The Radio Fine Structure of the BL Lacertae Objects AO0235+164, 0735+178, BL Lac, 1749+701, Mk 421 and 3C 66 A at 5 GHz1981In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 96, no 1-2, p. 316-324Article in journal (Refereed)

Very long baseline interferometer observations at 4997 MHz (lambda = 6 cm) on European and intercontinental baselines with lengths up to 1.35 108lambda have been used to probe the milliarc s structure of the BL Lacertae objects AO 0235+164, 0735+178, BL Lac, 1749+701, Mk 421, and 3C 66A. Two sets of observations are reported, made in November 1978 and March 1979. AO 0235+164 and 0735+178 both showed structures consisting of a very bright, unresolved core and a jet-like component extending ˜5".4 10-3 along position angle ˜20° and ˜4".2 10-3 along position angle ˜45°, respectively. AO 0235+164 contained more than 98% of the total flux density within a diameter of ≲ 36" 10-3. BL Lac, 1749+701 and Mk 421 all showed evidence for a complex structure of several components, including an unresolved core. 3 C 66A was elongated along position angle ˜ -45°.

• 29.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
EVN as a phase stable interferometer1991In: Radio Interferometry: Theory, Techniques, and Applications / [ed] T. J. Cornwell & R.A. Perley, San Francisco, USA: Astronomical Society of the Pacific Conference , 1991, p. 298-301Conference paper (Refereed)
• 30.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Owens Valley Observatory, Pasadena, CA, USA. Owens Valley Observatory, Pasadena, CA, USA. Northeast Radio Observatory Corp., Haystack Observatory, Westford, MA, USA. University of California, Berkeley, CA, USA. Owens Valley Observatory, Pasadena, CA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. University of California, Berkeley, CA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. University of California, Berkeley, CA, USA. UIniversity of Massachusetts, Amherst, MA, USA. NRAO, Tucson, AZ, USA. Nobeyama Radio Observatory, Minamimaki, Nagano, Japan. Owens Valley Observatory, Pasadena, CA, USA. Nobeyama Radio Observatory, Minamimaki, Nagano, Japan. Center for Astrophysics, Cambridge, MA, USA. Nobeyama Radio Observatory, Minamimaki, Nagano, Japan. NRAO, Tucson, AZ, USA. University of California, Berkeley, CA, USA. UIniversity of Massachusetts, Amherst, MA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
The microarcsecond structure of 3C 273 at 3 MM1991In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 241, no 1, p. L1-L4Article in journal (Refereed)

Recent improvements in data analysis and receiver techniques have allowed us to produce a map of the 100GHz emission from the compact radio source 3C273 with the unsurpassed resolution of 50-mu-as (microarcseconds). Our map shows that the structure within 300-mu-as (approximately 1.5.10(18).h-1 cm) has a position angle significantly different from the position angle of the jet observed at lower frequencies. There are also indications in our map that the inner structure has a more pronounced wiggling structure than has been observed on larger scales. The observations were made about 60 days from the start of the outburst of 1988. Most of the flux from the outburst is concentrated in a component which is elongated approximately (56 x 5).10(16).h-1 cm perpendicular to the overall jet-axis. The distance between this component and the core is approximately 128-mu-as, which corresponds to the distance expected from an apparent velocity of approximately 800-mu-as year-1.

• 31.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
VLBI observations of active galactic nuclei at 3 MM1992In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 257, no 1, p. 31-46Article in journal (Refereed)

Recent improvements in data analysis and receiver techniques have allowed us to produce maps of the 100 GHz emission from the compact cores of active galactic nuclei with the unsurpassed resolution of 50-mu-as (microarcseconds). We present here hybrid maps of a set of compact radio sources observed at two epochs with a global VLBI array. The high resolution enables us to show details of active galactic nuclei on size scales of 10(16)-10(17) cm. Jets are shown to be more curved in these inner parts than further out in the areas mapped with VLBI at lower frequencies. Our maps of the quasar 3C345 show that the curvature seen with lower resolution instruments continues very close to the core. New components are seen separating from the cores of 3C84 and BL Lac. We observe a component in 3C84 separating from the core with an apparent speed approximately 21000 km sec-1. The radio source OJ287 is still unresolved with our array, having a core size of less-than-or-similar-to 10(17) cm. There is no indication of any compact component in 3C279 which would be associated with the outburst in integrated flux density which happened some months before our observation. The flux density of the most compact component we observe in 3C279 agrees well with that of the quiet core as extrapolated from its radio spectrum at lower frequencies. The inner part of the radio jet of the giant elliptical galaxy M87 also shows a continuation of the structure on a larger size scale, with a structure we interpret as parts of a helical pattern. No fringes were found for 4C39.25 or Sgr A.

• 32.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Space Research Institute, Moscow, USSR. Space Research Institute, Moscow, USSR. Space Research Institute, Moscow, USSR. Crimean Astrophysical Observatory, USSR Academy of Sciences, P/O Nauchny, Crimea, USSR. National Radio Astronomy Observatory, Goddard Space Flight Center, Greenbelt, MD, USA.
High Resolution Observations of the QSO 3C 345 at 1.3 centimeters1981In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 243, p. L123-L126Article in journal (Refereed)

High-resolution VLBI observations made at a frequency of 22.235 GHz of the quasar 3C 345 are discussed. Antennas located at the Crimean Astrophysical Observatory, USSR; Onsala, Sweden; Effelsberg, West Germany; and the Haystack Observatory, Massachusetts were employed at 4-min integration times to provide baselines ranging up to 5.5 x 10 to the 8th wavelengths. About 40% of the total flux density of 7.85 Jy, observed in November 1977, and 8.05 Jy, observed in October 1978, is found to originate in an unresolved component of the quasar core in a region less than 0.1 milliarcsec in diameter. The elongated jet-like component of the quasar is observed to contain several peaks of emission extending up to 6 milliarsec from the core which decreased in extent between the two observations.

• 33.
Onsala Space Observatory, Onsala, Sweden.
Shanghai Observatory, China.
IMAGES OF THE BL LACERTAE OBJECT 1749+701 AT 5 GHZ1992In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 262, no 1, p. 1-7Article in journal (Refereed)

This paper presents images of 1749+701 from data obtained between 1980 and 1983 as a part of our VLBI programme to monitor a subsample of BL Lac objects at 5GHz. Our images show that this source has a complex structure with a jet pointing towards the north-west at a position angle of about -65-degrees. The brightest component is growing in extension along this position angle. We suggest this to be the core and that the growing extension was caused by a shock generated during an outburst and moving out along the jet. From the extension rate, we obtain an angular separation speed of 0.18 mas yr-1, which corresponds to an apparent transverse speed of 5c. The distances and position angles of two outer components seem remarkably constant. The phenomenon can be understood through a model in which a relativistic jet bends such that its axis is very close to the line of sight in two points. We also deduce that the Lorentz factor must be 4.5 less-than-or-equal-to gamma less-than-or-equal-to 6.5 and that the viewing angle must be 5-degrees less-than-or-equal-to theta less-than-or-equal-to 10-degrees. In this paper, we also present two images obtained from 5GHz MERLIN and combined MERLIN-VLBI data respectively. These maps show that there is a weak component located at a distance of 0.42 arcsec from the core at p.a -155-degrees.

• 34.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield, Cheshire, United Kingdom. Purple Mountain Observatory, Nanjing, China.
0735+178: The cosmic conspiracy1991In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 250, no 1, p. 50-56Article in journal (Refereed)

This paper discusses a radio outburst in the BL Lac-object 0735+178 which started in early 1988, ended during 1990 and was succeeded by a new outburst which is still in progress. Our image obtained with a global VLBI network shows the source to mainly consist of a core-jet like structure ending in diffuse emission region of low surface brightness located to the east of the core. We identify two new components as having been produced during the late 1980's. They are moving out with apparent superluminal motion in a viewing angle of greater-than-or-similar-to 10-degrees at a distance of less-than-or-similar-to 2 mas from the core. The core includes another new component, but our resolution is just sufficient to resolve out this component from the core at our epoch of observation. We suggest that the weak emission region is caused by an older component reaching the point at which the Doppler boosting is at its maximum and that the long time scale flux variation are due to successive events when evolved components reach that point.

• 35.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
The radio fine structure of the BL Lacertae object 0735 + 1781991In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 243, no 2, p. 328-332Article in journal (Refereed)

The BL Lacertae object 0735 + 178 has been mapped with global VLBI arrays at four epochs from 1979 to 1983. The hybrid maps presented in this paper show a complex structure with a component separating from the core with an apparent transverse velocity of almost-equal-to 7.9 c. We suggest a model in which new components are born in the core during an outburst, initially move away towards northeast and then at a distance of almost-equal-to 4.2 mas turn towards the southeast. At this point, the component does not show any discernible motion away from the core. We suggest that the jet curls towards the south and towards the observer and at this point has its minimum viewing angle, with maximum boosting and minimum apparent velocity. We associate the outburst of 1989 with the event when a superluminally moving component reaches this point.

• 36.
Purple Mountain Observatory, Chinese Academy of Science, Nanjing, China.
Onsala Space Observatory, Onsala, Sweden. Onsala Space Observatory, Onsala, Sweden. Onsala Space Observatory, Onsala, Sweden. Nuffield Radio Astronomy Laboratories, Jodrell Bank, Macclesfield, Cheshire, UK. Shanghai Observatory, Chinese Academy och Science, Shanghai, China.
AO 0235+164 - A "Heretic" BL Lac1994In: VLBI TECHNOLOGY: Progress and Future Observational Possibilities / [ed] Tetsuo Sasao, Seiji Manabe, Osamu Kameya & Makoto Inoue, Tokyo: Terra Scientific Publishing Company , 1994, p. 146-155Chapter in book (Refereed)
• 37.
National Radio Astronomy Observatory, Socorro, NM, USA.
National Radio Astronomy Observatory, Socorro, NM, USA. E. O. Hulburt Center for Space Research, Washington, DC, USA. E. O. Hulburt Center for Space Research, Washington, DC, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Max-Planck Institut für Radioastronomie, Bonn, Germany.
High Resolution Observations of the Steep Spectrum Source 2147+1451984In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 277, p. L41-L44Article in journal (Refereed)

The results of an eight station, 18-cm intercontinental very long baseline interferometry for which the synthesized beam obtained was 0.008 x 0.003 arcsec are presented for the steep spectrum compact radio source 2147+145. These observations reveal a structure similar to flat spectrum compact sources and, if the source radiates due to the synchrotron process, a very weak magnetic field. The source consists of a line of knots 0.022 arcsec long along position angle of 38 deg dominated on one side by a component of size 0.0032 arcsec measured along a position angle of 38 deg. It is suggested that 2147+145 and, perhaps, related steep spectrum objects are similar to flat spectrum, optically active sources except that the magnetic field is much weaker.

• 38.
MIT Haystack Observatory, Westford, MA, USA.
86 GHz Global VLBI Progress Report1993In: VLBI Technology: Progress and Future Observational Possibilities: Proceedings of the International Symposium Held at Kyoto International Conference Hall on September 6-10, 1993 / [ed] Tetsuo Sasao, Seiji Manabe, Osamu Kameya & Makoto Inoue, Tokyo: Terrapub , 1993, p. 89-92Chapter in book (Refereed)
• 39.
MIT, Cambridge, MA, United States.
MIT, Cambridge, MA, United States. MIT, Cambridge, MA, United States. MIT, Cambridge, MA, United States. Smithsonian Astrophysical Observatory, Cambridge, MA, USA. Chalmers University of Technology, Gothenburg, Sweden. Chalmers University of Technology, Gothenburg, Sweden. Chalmers University of Technology, Gothenburg, Sweden. Chalmers University of Technology, Gothenburg, Sweden. Institute for Space Research, Moscow, Russia. Institute for Space Research, Moscow, Russia. Institute for Space Research, Moscow, Russia. Crimean Astrophysical Observatory, USSR Academy of Sciences, P/O Nauchny, Crimea, USSR.
VLBI Studies of the W3OH Water Maser1977In: Bulletin of the American Astronomical Society, ISSN 0002-7537, E-ISSN 2330-9458, Vol. 9, p. 576-576, article id 10.07.07Article in journal (Refereed)
• 40.
High dynamic range VLBI observations of NGC 62511986In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 305, no 2, p. 684-697Article in journal (Refereed)
• 41.
Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
A 100 GHZ map of 3C 4461993In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 280, no 1, p. 117-120Article in journal (Refereed)

The first map made from 100 GHz VLBI observations of the quasar/BL Lac object 3C446 is presented. This map represents a 25-fold increase in resolution compared to earlier maps. Our main conclusions are that the core of 3C 446 is still almost unresolved (less than or similar to 30 muas) at this frequency and that a jet extends several hundred microarcseconds at position angle almost-equal-to -142-degrees. A comparison is also made with observations at other size scales.

• 42.
Departamento de Astronomía, Universitat de València, Valencia, Spain & Harvard-Smithsonian Ctr. Astrophys., Cambridge, United States .
Discovery of shell-like radio-structure in SN1993J1995In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 373, no 6509, p. 44-45Article in journal (Refereed)

SUPERNOVA explosions are poorly understood, partly because of difficulties in modelling them theoretically(1), and partly because there have been no supernovae observed in our Galaxy since the invention of the telescope. But the recent discovery(2) of supernova SN1993J in the nearby galaxy M81 offers an opportunity to investigate the evolution of the remnant, and its interaction with the surrounding interstellar medium, at high resolution. Here we present radio observations of SN1993J, made using very-long-baseline interferometry, which show the development of a shell structure. This 8-month-old radio shell is the youngest ever discovered in a supernova. The data suggest that the supernova explosion and the expanding shell of the remnant have nearly spherical symmetry, with small deviations where some parts of the shell are brighter than others. If these deviations arise because of variations in the density of the shell, this may reconcile earlier reports of symmetric radio emission(3) with the observed optical asymmetry(4,5), as the density variations could easily cause the latter. We infer that the radio emission is generated at the interface(6-9), where the surrounding gas is shocked by the ejecta.

• 43.
Massachusetts Institute of Technology, Cambridge, MA, United States.
Massachusetts Institute of Technology, Cambridge, MA, United States. Massachusetts Institute of Technology, Cambridge, MA, United States. National Radio Astronomy Observatory, Green Bank, WV, United States. Massachusetts Institute of Technology, Cambridge, MA, United States. Massachusetts Institute of Technology, Cambridge, MA, United States. Massachusetts Institute of Technology, Cambridge, MA, United States. NEROC, Haystack Observatory, Westford, MA, USA. NEROC, Haystack Observatory, Westford, MA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Max-Planck-Institut für Radioastronomie, Bonn, Germany.
A Close Look at a Close Pair of QSOs: VLBI Observations of 1038+528 A,B1982In: Bulletin of the American Astronomical Society, ISSN 0002-7537, E-ISSN 2330-9458, Vol. 14, p. 648-648Article in journal (Refereed)
• 44.
﻿Massachusetts Institute of Technology, Cambridge, MA, USA & Max-Planck-Institut für Radioastronomie, Bonn, Germany.
﻿Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. ﻿Massachusetts Institute of Technology, Cambridge, MA, USA & ﻿﻿Haystack Observatory/NEROC, Westford, MA, USA. ﻿National Radio Astronomy Observatory, Charlottesville, VA, United States. ﻿Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. Haystack Observatory/NEROC, Westford, MA, USA. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. ﻿Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. ﻿Massachusetts Institute of Technology, Cambridge, MA, USA; Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA & ﻿﻿﻿Cornell University, Ithaca, NY, USA. ﻿NASA, Goddard Space Flight Center, Greenbelt, MD, USA. Jet Propulsion Laboratory, Pasadena, CA, USA. ﻿Massachusetts Institute of Technology, Cambridge, MA, USA & Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. Haystack Observatory/NEROC, Westford, MA, USA.
The quasars 1038+528 A and B1985In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 142, no 1, p. 71-84Article in journal (Refereed)

The results of VLBI observations of the quasars 1038 + 528 A and B at 2.8, 3.6, 13, and 18 cm at various times between November 1979 and March 1981 are reported. The observations and data calibration are described, as are the mapping and astrometric techniques applied in the study. Both quasars are found to have 'core-jet' morphologies. The core of the A quasar dominates its morphology at centrimetric wavelengths with the brightness temperature of its 400 pc long jet being about 1/100 that of the core. By contrast, the 'jet' in the B quasar is very short (about 70 pc); the tail of this jet has the steepest spectral index found to date in extragalactic compact sources, indicating that high electron losses are responsible for the shortness of the jet. No evidence for appreciable morphological change in the B quasar was found over the time span of the study, whereas a new feature may be emerging from the A quasar core at superluminal speed.

• 45.
Space Research Institute, Russian Academy of Sciences, Moscow, Russia.
Max Planck Institute for Radio Astronomy, Bonn, Germany. Max Planck Institute for Radio Astronomy, Bonn, Germany. Max Planck Institute for Radio Astronomy, Bonn, Germany. Onsala Space Observatory, Onsala, Sweden. Radio Observatory, Torun, Poland.
The structure of the quasar 3C-3451992In: Soviet Astronomy Letters, ISSN 0320-0108, Vol. 18, no 6, p. 379-391Article in journal (Refereed)

We study the fine structure of the quasar 3C 345 at a wavelength of 49 cm. The apparent size of the core and low-frequency variability are due to the opacity of the surrounding ionized medium and the electron density distribution. We find the distance dependence of the size and brightness temperature of the components. Relativistic plasma is being ejected into a cone, with vertex angle approximately 40-degrees in position -110-degrees. The jet consists of narrow twisted filaments.

• 46.
Space Research Institute, Russian Academy of Sciences, Moscow, Russia.
Max-Planck-Institut Für Radioastronomie, Auf dem Hügel, Bonn, Germany. Max-Planck-Institut Für Radioastronomie, Auf dem Hügel, Bonn, Germany. Max-Planck-Institut Für Radioastronomie, Auf dem Hügel, Bonn, Germany. Onsala Space Observatory, Chalmers University of Technology, Onsala, Sweden. Torün Radio Astronomy Observatory, Nicolaus Copernicus University, Torün, Poland.
The injector and variability of the quasar 3C 3451996In: Astronomy letters, ISSN 1063-7737, E-ISSN 1562-6873, Vol. 22, no 1, p. 14-23Article in journal (Refereed)

The VLBI technique is used to study the fine structure of the quasar 3C 345 at = 49 cm. It is shown that the low-frequency radio emission from the core is feeble, whereas the bright compact component corresponds to the jet region adjacent to the core, rather than to the core itself. This region is ~6.4 mas in size, and its brightness temperature, Tb near equal 0.6 1012 K, does not go beyond the Compton limit. The core itself is an injector of relativistic particles surrounded by a wall of thermal plasma. Between 1983.9 and 1990.8, the radio flux density and angular size of the compact component increased by a factor of about 2, while its brightness temperature did not change markedly. During this period, the emission at millimeter wavelengths was reduced about twofold; the UV emission also decreased. The low-frequency variability is shown to be a result of variations in the transparency of the wall of a cocoon—the thermal plasma surrounding the jet. The electron density in the cocoon wall near the bright compact component is Ne near equal 105 cm3 and rises toward the core as ~ r 2. The polarized emission of the quasar originates mainly from the same region, where compact sources are seen athigh frequencies. Its rotation measure is RM near equal 29 rad m–2 and increases toward the core as ~ r 3, reaching 8 104 rad m–2. In the vicinity of the core, the longitudinal component of the magnetic field is ~0.1 mG.

• 47.
AN SSSR, Institut Kosmicheskikh Issledovanii, Moscow, USSR.
Max-Planck-Institut für Radioastronomie, Bonn, Germany. Max-Planck-Institut für Radioastronomie, Bonn, Germany. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Nicolaus Copernicus University, Torun, Poland.
Decimeter-wavelength structure in the quasar 3C 3451986In: Pis'ma v Astronomicheskii Zhurnal, ISSN 0320-0108, Vol. 12, no 1, p. 63-67Article in journal (Refereed)

The nuclear structure of the quasar 3C 345 has been surveyed at wavelength = 18 and 49 cm with two different VLBI networks. From the nucleus issues a jet, bending as it advances outward. Individual structure components are identified; their radio continuum and that of the nucleus indicate an optically thick source, the break in the spectrum shifting toward lower frequencies with increasing distance from the nucleus.

• 48.
Space Research Institute, Russian Academy of Sciences, Moscow, Russia.
Max-Planck-Institute für Radio Astronomie, Bonn, Germany. Max-Planck-Institute für Radio Astronomie, Bonn, Germany. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden. Torun Radio Astronomy Observatory, Torun, Poland.
Studies of the 3c 345 Quasar Structure at Dm-Wavelengths1987In: Observational evidence of activity in galaxies: proceedings of the 121st Symposium of the International Astronomical Union, held in Byurakan, Armenia, U.S.S.R., June 3-7, 1986 / [ed] E. Ye. Khachikian, K.J. Fricke & J. Melnick, Dordrecht: D. Reidel Publishing Group, 1987, Vol. 121, no 121, p. 301-304Conference paper (Refereed)

The structure of the quasar 3C 345 has been studied at 18 and 49 cm wavelengths by VLBI method. The structure has a nucleus and a jet, which changes direction with distance from the nucleus. The continuum emission of the nucleus and the jet regions correspond to a source with a large optical depth, the cut frequence goes down with increasing distance from nucleus.

• 49.
California Institute of Technology, Pasadena, CA, USA.
California Institute of Technology, Pasadena, CA, USA. California Institute of Technology, Pasadena, CA, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Superluminal Acceleration of the New Component in 3C 3451984In: VLBI and Compact Radio Sources: Symposium no. 110 held in Bologna, Italy, June 27–July 1, 1983 / [ed] R. Fanti, K. Kellermann & G. Setti, Dordrecht: D. Reidel Publishing Group, 1984, p. 109-110Conference paper (Refereed)
• 50.
Owens Valley Radio Observatory, California Institute of Technology, Pasadena, USA. Onsala Space Observatory, Chalmers University of Technology, Gothenburg, Sweden.
Superluminal Acceleration in 3C3451983In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 306, no 5938, p. 44-46Article in journal (Refereed)

The superluminal quasar 3C345 has a curved, one-sided jet-like radio structure1,2. Ejected material has been observed travelling at apparent speeds of 13–17c (ref. 3). We report here new observations at 22 GHz which show that the most recently ejected component4 is not moving radially away from the compact radio core, but along a trajectory which could be interpreted as either a curved path originating in the compact core, or a straight line, in which case the origin of ejection is not coincident with the compact radio core. The observations provide evidence of acceleration of this component. © 1983 Nature Publishing Group.

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