I. Marti-Vidal, J. M. Marcaide, A. Alberdi, J. C. Guirado, L. Lara, M. A. Perez-Torres, E. Ros, M. K. Argo, R. J. Beswick, T. W. B. Muxlow, A. Pedlar, I. I. Sha… 展开作者
I. Marti-Vidal, J. M. Marcaide, A. Alberdi, J. C. Guirado, L. Lara, M. A. Perez-Torres, E. Ros, M. K. Argo, R. J. Beswick, T. W. B. Muxlow, A. Pedlar, I. I. Shapiro, C. J. Stockdale, R. A. Sramek, K. W. Weiler, J. Vinko
We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20 February 2005 (151 days after explosion). The Very Large Array (VLA) flux density was 1.23$\pm$0.07 mJy, corresponding to an isotropic luminosity at 8.4GHz of (4.45$\pm$0.3)$\times10^{25}$ erg s$^{-1}$ Hz$^{-1}$ and a brightness temperature of (1.3$\pm$0.3)$\times10^{8}$ K. We also provide an improved source position, accurate to about 0.5 mas in each coordinate. The VLBI image shows a clear asymmetry. From model fitting of the size of the radio emission, we estimate a minimum expansion velocity of 15,700$\pm$2,000 km s$^{-1}$. This velocity is more than twice the expected mean expansion velocity estimated from a synchrotron self-absorbed emission model, thus suggesting that synchrotron self-absorption is not relevant for this supernova. With the benefit of an optical spectrum obtained 12 days after explosion, we favor an emission model which consists of two hot spots on an underlying expanding shell of width comparable to that of SN 1993J.
We summarize previous VLBI observations of Sgr A* at millimeter wavelengths and present new results from VLBI observations at 86 and at 215 GHz. At 86 GHz the measured closure phase is close to zero, consistent with a point-like or symmetric structure of 190 +/- 30 micro-arcsec size. At 215 GHz we have detected Sgr A* with a signal-to-noise ratio of 6. This yields a tentative size estimate of 50 < theta < 190 micro-arcsec, which is larger than the scattering size of 20 micro-arcsec at this frequency. The intrinsic size of Sgr A* thus appears to be a few up to a few ten Schwarzschild radii.
We use a new phase-calibration strategy to calibrate the phase of 86 GHz VLBI observations of the FR I radio galaxy NGC 4261. Instead of switching between a calibrator source and the target source, the target was observed while rapidly switching between the target frequency and a lower reference frequency. Self-calibration at the reference frequency yielded phase corrections which were multiplied with the frequency ratio and applied to the target frequency visibilities. The resulting detection of NGC 4261 is, to our knowledge, the first of NGC 4261 with 86 GHz VLBI, and it is also the weakest …
We present a detailed VLBI study of the spectral properties of the inner core region of the radio galaxy Cygnus A at 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86 GHz. Our observations include an epoch using phase-referencing at 15 GHz and 22 GHz and the first successful VLBI observations of Cygnus A at 86 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and an inverted spectrum towards the counter-jet. The inverted spectrum and the frequency-dependent jet-to-counter-jet ratio suggest that the inner counter-jet is covered by a circum-nuclear absorber as it is propo…
86 GHz Very Long Baseline Polarimetry probes magnetic field structures within the cores of Active Galactic Nuclei at higher angular resolutions and a spectral octave higher than previously achievable. Observations of 3C273 and 3C279 taken in April 2000 with the Coordinated Millimeter VLBI Array have resulted in the first total intensity (Stokes I) and linear polarization VLBI images reported of any source at 86 GHz. These results reveal the 86 GHz electric vector position angles within the jets of 3C273 and 3C279 to be orthogonal to each other, and the core of 3C273 to be unpolarized. If this …
We have observed 13 methanol maser sources associated with massive star-forming regions; W3(OH), Mon R2, S 255, W 33A, IRAS 18151-1208, G 24.78+0.08, G 29.95-0.02, IRAS 18556+0136, W 48, OH 43.8-0.1, ON 1, Cep A and NGC 7538 at 6.7 GHz using the Japanese VLBI Network (JVN). Twelve of the thirteen sources were detected at our longest baseline of $\sim$50 M$λ$, and their images are presented. Seven of them are the first VLBI images at 6.7 GHz. This high detection rate and the small fringe spacing of $\sim$4 milli-arcsecond suggest that most of the methanol maser sources have compact structure. G…
We present very long baseline interferometry (VLBI) images of ten very high redshift (z>3) quasars at 5 GHz. The sources 0004+139, 0830+101, 0906+041, 0938+119 and 1500+045 were observed in September 1992 using a global VLBI array, while 0046+063, 0243+181, 1338+381, 1428+423 and 1557+032 were observed in October 1996 with the European VLBI Network and Hartebeesthoek, South Africa. Most of the sources are resolved and show asymmetric structure. The sample includes 1428+423, the most distant radio loud quasar known to date (z=4.72). It is barely resolved with an angular resolution of about 2.0*…
Aims and Methods: We present the results of VLBI observations of nineteen GHz-Peaked-Spectrum (GPS) radio sources at 1.6 GHz. Of them, 15 sources are selected from the Parkes Half Jansky (PHJ) sample (Snellen 2002), 4 others are from our previous observation list. We aimed at imaging the structure of GPS sources, searching for Compact Symmetric Objects (CSOs) and studying the absorption for the convex radio spectra of GPS sources. Results: We obtained total intensity 1.6 GHz VLBI images of 17 sources for the first time. Of them, 80% show mini-double-lobe radio structure, indicating that they a…
We report results of pentachromatic VLBI survey for 18 GHz-peaked spectrum sources. Spectral fitting at every pixel across five frequencies allows us to illustrate distribution of optical depth in terms of free-free absorption or synchrotron self absorption. Quasars and Seyfert 1 sources show one-sided morphology with a core at the end where the optical depth peaks. Radio galaxies and Seyfert 2 show symmetric double-sided jets with a optically thick core at the center.
We present images of quasi-simultaneous VLBI observations of the GHz-Peaked-Spectrum radio source OQ 208 with the Very Long Baseline Array at 1.4, 1.7, 2.3, 5.0, 8.4, 15.4 GHz and the European VLBI Network at 6.7 GHz. The low frequency (1.4, 1.7 and 2.3 GHz) observations reveal a weak and extended steep-spectrum component at about 30 mas away in the position angle of $- 110^\circ$ which may be a remnant emission. The radio structure of OQ 208 consists of two mini-lobes at 5.0, 6.7, 8.4 and 15.4 GHz. Our spectral analysis further confirms that the southwest lobe undergoes free-free absorption a…
The VLBI observations at 22 GHz and 43 GHz of the quasar 3C273 obtained during a multi-frequency campaign in late 1992 in the radio, millimetre and X-ray bands allow us to derive the components' angular sizes, their peak fluxes and turnover frequencies. Lower limits to the Doppler factors have been derived by comparing the observed X-ray fluxes with those predicted by the Synchrotron Self-Compton model. Independent estimates of the Doppler factors were obtained through the assumption of the energy equipartition between the particles and the magnetic field. Of the five components used to model …
The radio source M81* at the core of the nearby spiral galaxy M81 is a low-luminosity active galactic nucleus. The close distance of 3.63Mpc allows its morphology to be studied in great detail. Here we present preliminary results from continuum 7 mm VLBI observations of its core, using phase-referencing techniques. These observations set constrains on the size of M81* at this frequency and enable us to test the frequency dependence on its physical properties.
A bright feature 100 pc away from the core in the powerful jet of M 87 shows mysterious properties. Earlier radio, optical and X-ray observations have shown that this feature, labelled HST-1, is superluminal, and is possibly connected with the TeV flare detected by HESS in 2005. To examine the possible blazar-like nature of HST-1, we analyzed 2 cm VLBA data from dedicated full-track observations and the 2 cm survey/MOJAVE VLBI monitoring programs observed from 2000 to 2008. Applying wide-field imaging techniques, the HST-1 region was imaged at milliarcsecond resolutions. Here we present the fi…
We describe, and present preliminary results from the analysis of 1.6 GHz VLBI observations, made in Cluster-Cluster mode between subgroups of antennas from the WSRT, VLA and MERLIN arrays. We observed for two 6-hour periods on consecutive days, simultaneously monitoring 4 compact radio sources with angular separations ranging from ~1 to 9 degrees. The data were recorded in a non-standard MkIII mode using special set-ups, and correlated at the MPIfR MkIIIa correlator. The preliminary results from standard and phase reference analyses are very encouraging. The experiment has provided a useful d…
We present the results of EVN polarization observations of fourteen GHz-Peaked-Spectrum (GPS) radio sources at 5 GHz. These sources were selected from bright GPS source samples and we aimed at finding Compact Symmetric Objects (CSOs). We have obtained full polarization 5 GHz VLBI observations of 14 sources providing information on their source structure and spectral indices. The results show that two core-jet sources 1433$-$040 and DA193, out of 14 GPS sources, exhibit integrated fractional polarizations of 3.6% and 1.0% respectively. The other 12 sources have no clear detection of pc-scale po…
We present preliminary results from the deepest VLBI observations yet conducted. VLBA+GBT 1.4 GHz observations of a region within NOAO-N, reach an r.m.s. noise level of 9 microJy per beam. Three sources are clearly detected (> 7 sigma) within the inner 2 arcmins of the GBT primary beam, including two sub-mJy sources and the ``in-beam'' calibrator. In addition, by tapering the data, we map out a much larger area of sky, reaching well beyond the half-power point of the GBT primary beam. An additional 6 sources are detected in the extended field. We comment briefly on the scientific motivation fo…
We present results from a large global VLBI(Very Long Baseline Interferometry) survey of compact radio sources at 86 GHz which started in October 2001. The main goal of the survey is to increase the total number of objects accessible for future 3mm-VLBI imaging by factors of 3-5. The survey data reach the baseline sensitivity of 0.1 Jy, and image sensitivity of better than 10 mJy/beam. To date, a total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, and only 6 sources have not been detected. Flux densities and sizes of core and jet compone…
At 147 GHz (2mm wavelength), we detected three prominent AGN (NRAO150, 3C279, 1633+382) with Very Long Baseline Interferometry (VLBI) with an angular resolution of only 18 micro-arcseconds on the baseline between two antennas in Arizona (10m HHT and 12m KittPeak) and the IRAM 30m antenna on Pico Veleta in Spain. This is a new world record in radio interferometry and astronomical imaging and opens fascinating future possibilities to directly image and study the innermost regions in Quasars and other Active Galactic Nuclei.
The results of VLBI observations of the quasar 3C273, obtained during a multi-frequency campaign in late 1992 in the radio, millimeter and X-ray bands are presented and discussed. The VLBI observations were made at 22 GHz with a Global Array and at 43 GHz with the Very Long Baseline Array. Hybrid maps and modelfits were made in order to look for any short time scale structural variations of the inner part of the radio jet. In 42 days 3C273 was observed 5 times at roughly 10 day intervals. The jet structure did not show dramatic changes in that period. However, we were able to follow the change…
We report the results of a 5-GHz southern-hemisphere snapshot VLBI observation of a sample of blazars. The observations were performed with the Southern Hemisphere VLBI Network plus the Shanghai station in 1993 May. Twenty-three flat-spectrum, radio-loud sources were imaged. These are the first VLBI images for 15 of the sources. Eight of the sources are EGRET (> 100 MeV) gamma-ray sources. The milliarcsecond morphology shows a core-jet structure for 12 sources, and a single compact core for the remaining 11. No compact doubles were seen. Compared with other radio images at different epochs and…
We present thirty VLBI images of the TeV blazar Markarian 421 (1101+384) at fifteen epochs spanning the time range from 1994 to 1997, and at six different frequencies from 2.3 to 43 GHz. The imaged observations include a high-resolution 5 GHz VLBI Space Observatory Programme (VSOP) observation with the HALCA satellite on 1997 November 14; full-track VLBA observations from 1994 April, 1996 November, and 1997 May at frequencies between 5 and 43 GHz; six epochs of VLBA snapshot observations at frequencies between 2 and 15 GHz from Radio Reference Frame studies; and five geodetic VLBI observations…
We present results of simultaneous dual-frequency (2 GHz and 8 GHz) very long baseline interferometry (VLBI) observations of 12 active galactic nuclei with prominent jets. Spectral properties of the jets and evolution of their brightness temperature are discussed. Measured sizes and brightness temperatures of VLBI features are found to be consistent with emission from relativistic shocks dominated by adiabatic losses. Physical scenarios with different magnetic field orientation in the jets are discussed.
The first high-resolution (5 mas) VLBI observations of 6.7-GHz methanol masers in DR21(OH)N, a candidate circumstellar disc around a very young massive star, are presented. Previous observations of these masers at 50 mas angular resolution revealed a rotating structure at the position of a candidate massive protostar, with a well-sampled position-velocity diagram suggesting Keplerian rotation. Observations presented here using the European VLBI Network (EVN) have provided the first high angular resolution maps of the masers, providing a test for the disc hypothesis and the Gaussian centroiding…
We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sourc…
We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz. The brightness temperature of the core at 1.6 GHz is $5 \times 10^{11}$ K. The size is near the minimum observable value in the direction of NRAO~530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of $\sim 3 \times 10^{12}$ K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic moti…
We demonstrate phase correction of 3 mm VLBI observations using the scanning 18 GHz to 26 GHz water vapour radiometer at Effelsberg and we demonstrate an absolute accuracy of 15 mm in zenith path delay by comparing with GPS and radiosondes. This accuracy should provide significant improvement in astrometric phase referencing observations. It is not good enough for geodetic VLBI to replace the tropospheric delay estimation but could be used to remove short-term path-length fluctuations and so improve the geodetic observables. We discuss lessons learned and opportunities for further improvement.
Mapping the central 5 pc region of the nearby radio galaxy NGC 6251 with a 0.2 pc resolution using VLBI at two radio frequencies, 5 GHz and 15 GHz, we have found the sub-pc-scale counter jet for the first time in this radio galaxy. This discovery allows us to investigate the jet acceleration based on the relativistic beaming model (Ghisellini et al. 1993).
Accurate measurement of the frequency-dependent shift of the self-absorbed radio core is required for multi-frequency analysis of VLBI data since absolute positional information is lost as a result of phase self-calibration. We use the cross-correlation technique of Croke & Gabuzda (2008) on the optically thin jet emission to align our VLBA images. Our results are consistent with those obtained from the phase-referencing method, as well as alignment by model-fitted optically thin jet components. Physical parameters of the compact jet regions, such as the magnetic field strength (B_core) and th…
In this paper we present new observations of the gravitational lens system JVAS B0218+357 made with a global VLBI network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 microJy/beam and with these we have been able to image much of the extended structure of the radio jet in both the A and B images at high resolution (~1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H0 determination. We are able to identify several sub-components common to both images with the expected parity reversal, including one which we identify a…
Broad-band (gamma to radio) variations of the flux density were observed in the first half of 1992 in the luminous high redshift (z = 2.172) quasar S5 0836+710. VLBI monitoring observations during 1993 -- 1996 performed at 86 GHz, 22 GHz, 15 GHz, and 8 GHz show the ejection of a new jet component, which most probably is directly related to a quasi simultaneous gamma-, X-ray, optical flaring activity which was observed in February 1992. During the period 1992 -- 1993 the flaring propagated through the radio spectrum. From several quasi-simultaneous radio spectra taken during this phase of activ…
At radio wavelengths, images of the compact radio source Sagittarius A* (Sgr A*) in the Galactic Center are scatter broadened with a lambda^2 dependence due to an intervening ionized medium. We present VLBI observations of Sgr A* at 86 GHz using a six station array including the VLBA antennas at Pie Town, Fort Davis and Los Alamos, the 12m antenna at Kitt Peak and the millimeter arrays at Hat Creek and Owens Valley. To avoid systematic errors due to imperfect antenna calibration, the data were modeled using interferometric closure information. The data are best modeled by a circular Gaussian b…
We report the results of a 5 GHz southern hemisphere VLBI survey of compact extragalactic radio sources. These observations were undertaken with the SHEVE array plus Shanghai station in November 1992. A sample of 22 sources was observed and images of 20 of them were obtained. Of the 20 sources imaged, 15 showed core-jet structure, one had a two-sided jet and 4 had only single compact cores. Eleven of the 16 core-jet (including one two-sided jet) sources show some evidence of bent jets. No compact doubles were found. A comparison with previous images and the temporal variability of the radio fl…
We report on VLBA observations of a gamma-ray bright blazar NRAO 530 at multiple frequencies (5, 8, 15, 22, 39, 43, and 45 GHz) in 1997 and 1999. These multi-epoch multi-frequency high-resolution VLBI images exhibit a consistent core-dominated morphology with a bending jet to the north of the core. The quasi-simultaneous data observed at five frequencies (5, 8, 15, 22 and 43 GHz) in February 1997 enable us to estimate the spectra of compact VLBI components in this highly variable source. Flat spectra are seen in central two components (A and B), and the most compact component A with the flatte…
We have a project to measure magnetic fields in the obscuring tori of AGNs. We observed five free-free absorbed jets in radio galaxies with polarimetric VLBI at 15 GHz to determine Faraday rotation measures. Surprisingly, all sources are unpolarized, putting severe constraints on the degree of magnetic field turbulence and the gas distribution in these objects. We have further carried out multi-frequency VLBI monitoring of the Seyfert 2 galaxy NGC 3079 and find unusually steep and inverted spectra in the radio components, and derive general properties of Seyfert galaxies using published VLBI o…
We present the basic features and the activities of Japanese VLBI network (JVN), a newly-established VLBI network with baselines ranging from 50 to 2560 km spreading across the Japanese islands, and capable of observing at 6.7, 8.4, and 22 GHz. We show a number of results of JVN observations: 8.4-GHz continuum images of a Giga-hertz Peaked Spectrum (GPS) source and radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s), the spatial and velocity structures of water masers in NML Cygni as well as methanol masers in Cep A, and demonstrative observations with the bigradient phase referencing.
We present the first 86 GHz VLBI observations of the radio galaxy 3C~120, together with contemporaneous 43 and 22 GHz polarimetric VLBA observations. The very high angular resolution obtained at 86 GHz provides an upper limit to the size of the core of 54 $μ$as (0.025 $h^{-1}$pc). This represents a direct determination of the base of the jet which is independent of variability arguments (which depend on uncertain estimates of the Doppler factor), and places it below approximately one light-month. Comparison with previous VLBA observations after a one-year interval shows pronounced changes in t…
We present pc-scale images of ten Compact Symmetric Objects (CSO) candidates observed with the European VLBI network (EVN). Five radio sources have been observed at 1.6 GHz, and five more at 2.3/8.4 GHz, the latter subsample with the inclusion in the VLBI array of 3 antennae normally used for geodesy. These objects were selected from existing samples of GHz Peaked Spectrum (GPS) radio sources with the purpose to find and/or confirm the CSO classification. These new VLBI observations allow us to confirm the classification of two CSO candidates, and to find a few new ones. The association of GPS…
We present multi-frequency VLBI observations of the Compact Steep-spectrum Quasar 3C147 (0538+498) made with the VLBA at the four frequencies in the available 5 GHz band and at 8.4 GHz (still under analysis), from which we derived millarcsecond-resolution images of the total intensity, polarization, and rotation measure distributions. The source shows a core-jet structure, with a compact feature and a jet, extending about 200 mas to the South-West. We detect polarized emission in two bright features in the inner jet; the rotation measure of this features (aprox -1630 rad m(-2), aprox -540 rad …
Global Positioning System (GPS) satellites broadcast at frequencies of 1,575.42 MHz (L1) and of 1,227.60 MHz (L2). The dispersive property of the ionosphere can be used to combine independent measurements at the two frequencies to estimate the total electron content (TEC) between a GPS receiver site and a broadcasting satellite. Such measurements, made at sites near to Very Long Baseline Interferometry (VLBI) sites, can be used to estimate the ionospheric contribution to VLBI observables. For our 1991.9 astrometric VLBI experiment in which we obtained group-delay observations in the 8.4 and 2.…
The Caltech-Jodrell Bank VLBI surveys of bright extragalactic radio sources north of declination 35 deg were carried out between 1990 and 1995 using the Mark-II system, achieving images with a resolution of about 1 mas at 5 GHz. The CJ1 survey (together with the older `PR' survey) includes 200 objects with 5 GHz flux density greater than 0.7 Jy; the CJ2 survey includes 193 flat-spectrum sources with 5 GHz flux density greater than 0.35 Jy; and we have defined a complete flux-density limited sample, CJF, of 293 flat-spectrum sources stronger than 0.35 Jy. We summarize the definition of the samp…
In this paper, we present new VLBA observations of the radio jet in the quasar S5 0836+710 at 8 and 22 GHz. The identification of the ridge lines allow us to interpret the jet structure in terms of Kelvin-Helmholtz instability. Combined with previous epochs of VLBA and VSOP data at 1.6 and 5 GHz, these new observations will allow us to study the evolution of the instabilities in the jet. We have detected signatures of possible jet disruption in the jet due to the growth of instabilities, which points towards a possible morphological classification of the source as an FRI, contrary to what prev…
We present and discuss the results of VLBI (EVN) observations of three low-luminosity (P(5 GHz)<10^25 W/Hz) Broad Emission Line AGNs carefully selected from a sample of flat spectrum radio sources (CLASS). Based on the total and the extended radio power at 5 GHz and at 1.4 GHz respectively, these objects should be technically classified as radio-quiet AGN and thus the origin of their radio emission is not clearly understood. The VLBI observations presented in this paper have revealed compact radio cores which imply a lower limit on the brightness temperature of about 3X10^8 K. This result rule…
We present the first VLBI Space Observatory Programme (VSOP) observations of the gamma-ray blazar 3C 279 at 1.6 and 5 GHz. The combination of the VSOP and VLBA-only images at these two frequencies maps the jet structure on scales from 1 to 100 mas. On small angular scales the structure is dominated by the quasar core and the bright secondary component `C4' located 3 milliarcseconds from the core (at this epoch). On larger angular scales the structure is dominated by a jet extending to the southwest, which at the largest scale seen in these images connects with the smallest scale structure seen…
High resolution studies of circular polarization allow us see where it arises in a jet, study its local fractional level and spectrum, and compare these results to local measures of linear polarization and Faraday rotation. Here we not only review past results from Very Long Baseline Array (VLBA) circular polarization studies, but we also present preliminary new results on two quasars. In the core of PKS 0607-157, we find strong circular polarization at 8 GHz and much weaker levels at 15 GHz. Combined with the linear polarization data, we favor a simple model where the circular is produced by …
Maps of the radio source 3C 120 obtained from VLBA+ observations at 8.4 GHz at five epochs in January - September 2002 are presented. The images were reconstructed using the maximum entropy method and the Pulkovo VLBImager software package for VLBI mapping. Apparent superluminal motions of the brightest jet knots have been estimated. The speeds of jet knots decreases with distance from the core, changing from 5.40+-0.48c $ to 2.00+-0.48c over 10 mas (where c is the speed of light) for a Hubble constant of 65 km/s/Mpc. This can be explained by interaction of the jet with the medium through whic…
We summarize the present status of VLBI experiments at 3 mm (86 GHz), 2 mm (129-150 GHz) and 1.3 mm (215-230 GHz). We present and discuss a new 3 mm VLBI map of M87 (Virgo A), which has a spatial resolution of only approx. 20 Schwarzschild radii. We discuss recent VLBI results for SgrA* and argue in favor of new observations within an extended European mm-VLBI network, in order to search for variability. We discuss the possibilities to image the `event horizon' of a super-massive black hole at wavelengths < 2mm, and conclude that the addition of large and sensitive millimetre telescopes such a…
We present the results of phase-referenced VLBA+Effelsberg observations at five frequencies of the double-image gravitational lens JVAS B0218+357, made to establish the precise registration of the A and B lensed image positions. The motivation behind these observations is to investigate the anomalous variation of the image flux density ratio (A/B) with frequency - this ratio changes by almost a factor of two over a frequency range from 1.65 GHz to 15.35 GHz. We investigate whether frequency dependent image positions, combined with a magnification gradient across the image field, could give ris…
We investigate class II methanol masers and the environment in which they form with the Long Baseline Array (LBA). Using full polarisation VLBI, we're able to measure the magnetic field directions so as to distinguish between the two main models of the environment in which methanol masers form: disks or shocks. We present polarised images of the methanol maser source G339.88-1.2, made with the LBA at 6.7-GHz. With these first polarisation maps made with the LBA, which successfully reproduce observations with the ATCA confirming the new AIPS code, a new technique for Southern VLBI is opened. Th…
Very long baseline interferometry (VLBI) provides the resolution needed to make precision measurements of a pulsar's parallax and proper motion. In making these measurements, the astronomer is faced with difficult calibration problems and a paucity of strong point-like calibrators that ultimately limit the accuracy of parallax measurements of even the brightest pulsars. A new technique to calibrate away the effects of the ionosphere, the dominant source of phase error at frequencies below 5 GHz, has led to the new measurements of nine pulsar parallaxes to 100 microarcsecond or better precision…
We have developed a new phase referencing technique for high frequency VLBI observations. In conventional phase referencing, one interleaves short scans on a nearby phase calibrator between the target source observations. In fast frequency switching described here, one observes the target source continuously while switching rapidly between the target frequency and a lower reference frequency. We demonstrate that the technique allows phase calibration almost reaching the thermal noise limit and present the first detection of the AGN in the FR I radio galaxy NGC 4261 at 86 GHz. Although point-li…
We present the first multi-frequency phase-referenced observations of Cygnus A done with the VLBA at 15 and 22 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and a highly inverted spectrum towards the counter-jet. The inverted spectrum and the frequency dependent jet to counter-jet ratio suggest an obscuring torus in front of the counter-jet. From 14 epochs of 15 GHz VLBA data we accurately derive the jet and counter-jet kinematics. For the inner jet (r <= 5 mas) we measure motions of beta_app ~ 0.2-0.5 h^{-1} and on the counter-jet side we find beta_ ap…
We demonstrate that at 1.4 GHz the combined response of sources detected serendipitously in deep, wide-field VLBI images is sufficient to permit self-calibration techniques to be employed. This technique of "full-beam" VLBI self-calibration permits coherent VLBI observations to be successfully conducted in any random direction on the sky, thereby enabling very faint radio sources to be detected. Via full-beam self-calibration, Global VLBI observations can equal and indeed surpass the level of sensitivity achieved by connected arrays. This technique will enable large-scale VLBI surveys of the f…
The Seyfert 2 galaxy NGC3079 is one of the nearest radio-weak AGN. We have carried out multi-frequency, multi-epoch VLBI observations of this object and detected new components, proper motion of the order of 0.1 c, strong spectral variability, and free-free absorption. Based on phase-referenced astrometry, we suggest a new identification of components seen at 1.7 GHz. Our results suggest that the nucleus in NGC3079 is embedded in a dense medium which strongly affects the radio appearance of the AGN ejecta. We supplement our work with Seyfert VLBI observations taken from the literature.
According to a generally accepted paradigm, small intrinsic sizes of Compact Steep Spectrum (CSS) radio sources are a direct consequence of their youth, but in later stages of their evolution they are believed to become large-scale sources. However, this notion was established mainly for strong CSS sources. In this series of papers we test this paradigm on 60 weaker objects selected from the VLA FIRST survey. They have 5-GHz flux densities in the range 150 < S_{5GHz} < 550 mJy and steep spectra in the range 0.365 \le ν\le 5 GHz. The present paper is focused on sources that fulfill the above cr…
Supermassive black holes, with masses of 10^6 to more than 10^9 solar masses, are among the most spectacular objects in the Universe, and are laboratories for physics in extreme conditions. The primary goal of ARISE (Advanced Radio Interferometry between Space and Earth) is to use the technique of Space VLBI to increase our understanding of black holes and their environments, by imaging the havoc produced in the near vicinity of the black holes by their enormous gravitational fields. The mission will be based on a 25-meter space-borne radio telescope operating at frequencies between 8 and 86 G…
We present the first result of VLBI observations of the blueshifted water maser emission from the type 2 Seyfert galaxy NGC5793, which we combine with new and previous VLBI observations of continuum emission at 1.7, 5.0, 8.4, 15, and 22 GHz. Maser emission was detected earlier in single-dish observations and found to have both red- and blueshifted features relative to the systemic velocity. We could image only the blueshifted emission, which is located 3.6 pc southwest of the 22 GHz continuum peak. The blueshifted emission was found to originate in two clusters that are separated by 0.7 millia…
We present proper motions of water masers toward the Onsala 1 star forming region, observed with the Japanese VLBI network at three epochs spanning 290 days. We found that there are two water maser clusters (WMC1 and WMC2) separated from each other by 1".6 (2900 AU at a distance of 1.8 kpc). The proper motion measurement reveals that WMC1 is associated with a bipolar outflow elongated in the east-west direction with an expansion velocity of 69+-11 km/s. WMC1 and WMC2 are associated with two 345 GHz continuum dust emission sources, and located 2" (3600 AU) east from the core of an ultracompact …
We report on two epochs of very-long-baseline interferometry (VLBI) observations of the Type Ib/c supernova SN 2008D, which was associated with the X-ray outburst XRF 080109. At our first epoch, at t = 30 days after the explosion, we observed at 22 and 8.4 GHz, and at our second, at t = 133 days, at 8.4 and 5.0 GHz. The VLBI observations allow us to accurately measure the source's size and position at each epoch, and thus constrain its expansion velocity and proper motion. We find the source at best marginally resolved at both epochs, allowing us to place a 3sigma upper limit of ~0.75c on the …
