scholarly journals Microwave flux density variations of compact radio sources monitored by real-time very long baseline interferometry

Radio Science ◽  
2001 ◽  
Vol 36 (2) ◽  
pp. 223-235 ◽  
Author(s):  
Yasuhiro Koyama ◽  
Tetsuro Kondo ◽  
Noriyuki Kurihara
Keyword(s):  
Real Time ◽  
Flux Density ◽  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Long Baseline

scholarly journals VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz

2018 ◽  
Vol 616 ◽  
pp. A128 ◽  
Author(s):  
N. Herrera Ruiz ◽  
E. Middelberg ◽  
A. Deller ◽  
V. Smolčić ◽  
R. P. Norris ◽  
...  
Keyword(s):  
Radio Source ◽  
Flux Density ◽  
Lower Limit ◽  
High Sensitivity ◽  
Host Galaxy ◽  
Radio Sources ◽  
Source Population ◽  
Star Forming ◽  
Brightness Temperatures ◽  
Long Baseline

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.

Very long baseline interferometry polarization studies of quasars and active galactic nuclei

1986 ◽  
Vol 64 (4) ◽  
pp. 434-439 ◽  
Author(s):  
J. F. C. Wardle ◽  
D. H. Roberts
Keyword(s):  
Active Galactic Nuclei ◽  
Linear Polarization ◽  
Very Long Baseline Interferometry ◽  
Galactic Nuclei ◽  
Radio Sources ◽  
Long Baseline ◽  
First Results ◽  
Polarization Studies ◽  
Theoretical Issues

We present some first results of a program to map the distribution of linear polarization in compact radio sources with milliarcsecond resolution. We show first-epoch maps of 3C345 and 0735 + 178 and first- and second-epoch maps of OJ287. In general, the polarization is mainly associated with optically thin (jet) components. In the case of OJ287, polarization maps made 1 year apart are strikingly different. We also discuss some of the theoretical issues raised by these observations.

scholarly journals The Astrometric Possibilities of Very-Long-Baseline Interferometry

1986 ◽  
Vol 109 ◽  
pp. 143-155
Author(s):  
D. S. Robertson
Keyword(s):  
Plane Wave ◽  
Arrival Time ◽  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Arrival Times ◽  
Long Baseline ◽  
The Difference ◽  
Different Sources

In the application of Very-Long-Baseline Interferometry (VLBI) to astrometric problems the fundamental observable is the difference in the arrival times of a wavefront at two widely separated receiving stations. Since the radio sources being observed are sufficiently distant that the arriving wavefront can be considered to be a plane wave, the differential arrival time is a measure of the component of the baseline in the direction of the source. Equivalently, if the baseline is known, the differential arrival time is sufficient to determine a circle on the sky containing the source. It is easy to show that a minimum of ten observations distributed among three different sources is sufficient to determine all of the source coordinates and the baseline coordinates simultaneously (Robertson, 1975).

scholarly journals Development of a Radio-Astrometric Catalog by Means of Very Long Baseline Interferometry Observations

1980 ◽  
Vol 56 ◽  
pp. 351-357
Author(s):  
J. L. Fanselow ◽  
O. J. Sovers ◽  
J. B. Thomas ◽  
F. R. Bletzacker ◽  
T. J. Kearns ◽  
...  
Keyword(s):  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Radio Interferometry ◽  
Long Baseline ◽  
California Institute Of Technology ◽  
The Status ◽  
Institute Of Technology ◽  
Global Accuracy ◽  
Celestial Sphere ◽  
Interplanetary Navigation

AbstractThe Jet Propulsion Laboratory of the California Institute of Technology has been developing a radio-astrometric catalogue for use in the application of radio interferometry to interplanetary navigation and geodesy. The catalogue consists of approximately 100 compact extragalactic radio sources whose relative positions have formal uncertainties of the order of 0”.01. The sources cover nearly all of the celestial sphere above -40° declination. By using the optical counterparts of many of these radio sources, we have tied this radio reference frame to the FK4 optical system with a global accuracy of approximately 0”.01. This paper describes the status of this work.

scholarly journals Measuring Cosmological Parameters with Very Long Baseline Interferometry

1999 ◽  
Vol 183 ◽  
pp. 67-67 ◽  
Author(s):  
L.I. Gurvits ◽  
K.I. Kellermann ◽  
S. Frey
Keyword(s):  
Cosmological Model ◽  
Very Long Baseline Interferometry ◽  
Cosmological Parameters ◽  
Angular Size ◽  
Linear Size ◽  
Radio Sources ◽  
Selection Effects ◽  
Extended Source ◽  
Long Baseline ◽  
5 Ghz

Very Long Baseline Interferometry (VLBI) allows us to study a core of AGN with a sub-parsec resolution. We analyze the dependencies “apparent angular size – redshift” and “apparent motion – redshift” which contain an imprint of the source's properties and cosmology. We present data on the “angular size – redshift” relation obtained with VLBI at 5 GHz on a sample of 300 AGN distributed over the widest available range of redshifts 0.016 ≤ z ≤ 4.5. The sample exceeds those used in similar studies earlier by Kellermann (1993, 79 sources) and Wilkinson et al. (1997, 160 sources). Unlike extended source, the angular size-redshift for compact radio sources appears consistent with the predictions of standard Friedmann world models with qo ≃ 0.5 without taking into account evolutionary effects or selection effects due to a “linear size – luminosity” or “linear size – spectral index” dependences. We discuss different approaches allowing us to disentangle intrinsic evolutionary properties of sources and parameters of the cosmological model. Recent estimates of parameters of the cosmological model are given. We also discuss a perspective of conclusive cosmological tests using the VLBI technique.

scholarly journals Models for Source Structure Corrections

2000 ◽  
Vol 180 ◽  
pp. 29-39
Author(s):  
P. Charlot
Keyword(s):  
Reference Frame ◽  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Time Varying ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Spatially Extended ◽  
Astrometric Data ◽  
Celestial Reference Frame ◽  
Source Structure

AbstractAt the milliarcsecond scale, most of the extragalactic radio sources exhibit spatially-extended intrinsic structures which are variable in both time and frequency. Such radio structures set limits on the accuracy of source positions determined with the Very Long Baseline Interferometry (VLBI) technique unless their effects in the astrometric data can be accounted for. We review the modeling scheme for calculating source structure corrections and discuss the magnitude and impact of these effects for the sources that are part of the International Celestial Reference Frame (ICRF). Results obtained by applying source structure corrections to actual VLBI observations on the time-varying source 4C39.25 (0923 + 392) are also presented.

Magnetic-field configurations in compact extragalactic jets

1986 ◽  
Vol 64 (4) ◽  
pp. 463-465 ◽  
Author(s):  
T. W. Jones
Keyword(s):  
Magnetic Field ◽  
Very Long Baseline Interferometry ◽  
Radio Sources ◽  
Relativistic Jets ◽  
Detailed Model ◽  
Model Calculations ◽  
Long Baseline ◽  
Kolmogorov Turbulence ◽  
Extragalactic Jets ◽  
The Magnetic Field

Multifrequency and multitime polarimetry of active, compact radio sources strongly suggest that the magnetic-field structures in these sources are largely disordered or turbulent. Some initial, detailed model calculations of relativistic jets with turbulent fields have been performed in an effort to better understand the relationships between the observed polarization properties and physical structures of sources. Simulations are described involving nearly steady, isothermal, constant velocity jets with magnetic fields having approximately a Kolmogorov turbulence spectrum that is carried along the jet. They produce structures similar to those actually observed, including a core-jet appearance with superluminal knots. Likewise, the polarization has a behavior in frequency and time that is at least qualitatively similar to that observed. In addition, although individual portions of the model sources are highly polarized, as also seen with very long baseline interferometry, the integrated polarizations are relatively small, as observed.

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