The VLBA Calibrator Survey—VCS1

Beasley, A. J.; Gordon, D.; Peck, A. B.; Petrov, Leonid; MacMillan, D. S.; Fomalont, Ed; Ma, C.

doi:10.1086/339806

Cited by 336 publications

(361 citation statements)

References 33 publications

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“…Consequently, although there are more MPS sources than USS sources, neither of these methods would have selected more than ∼ 18 per cent of the sources in the unbiased sub-sample. Interestingly, four of the sources (J0324−2918, J0906+6930, J1606+3124 and J2102+6015) that were excluded from the unbiased sub-sample were excluded because they were selected for VLBI observation based on having flat two-point spectral indices (Beasley et al 2002;Romani et al 2004;Petrov et al 2006). However, all four sources actually have peaked spectra and only appeared to have flat spectra because their spectral indices were determined close to the spectral peak.…”

Section: Discussionmentioning

confidence: 99%

Radio spectra of bright compact sources at z>4.5

Coppejans

Velzen

Intema

et al. 2017

Mon. Not. R. Astron. Soc.

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High-redshift quasars are important to study galaxy and active galactic nuclei (AGN) evolution, test cosmological models, and study supermassive black hole growth. Optical searches for high-redshift sources have been very successful, but radio searches are not hampered by dust obscuration and should be more effective at finding sources at even higher redshifts. Identifying high-redshift sources based on radio data is, however, not trivial. Here we report on new multi-frequency Giant Metrewave Radio Telescope (GMRT) observations of eight z > 4.5 sources previously studied at high angular resolution with very long baseline interferometry (VLBI). Combining these observations with those from the literature, we construct broad-band radio spectra of all 30 z > 4.5 sources that have been observed with VLBI. In the sample we found flat, steep and peaked spectra in approximately equal proportions. Despite several selection effects, we conclude that the z > 4.5 VLBI (and likely also non-VLBI) sources have diverse spectra and that only about a quarter of the sources in the sample have flat spectra. Previously, the majority of high-redshift radio sources were identified based on their ultra-steep spectra (USS). Recently a new method has been proposed to identify these objects based on their megahertz-peaked spectra (MPS). Neither method would have identified more than 18 per cent of the high-redshift sources in this sample. More effective methods are necessary to reliably identify complete samples of high-redshift sources based on radio data.

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Section: Discussionmentioning

confidence: 99%

Radio spectra of bright compact sources at z>4.5

Coppejans

Velzen

Intema

et al. 2017

Mon. Not. R. Astron. Soc.

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“…This is very relevant for the comparison to the optical solution, since the three groups are expected to have similar formal accuracies in the Gaia solution, whereas they have very different accuracies in the radio positions. The division of the non-defining sources depends on whether the source was only observed as part of the Very Long Baseline Array (VLBA) Calibrator Survey (VCS, Beasley et al 2002;Kovalev et al 2007;Petrov et al 2008) or not. The three groups of sources in ICRF2 are as follows:…”

Section: Radio Positions Of Icrf Sourcesmentioning

confidence: 99%

GaiaData Release 1

Mignard

Klioner

Lindegren

et al. 2016

A&A

133

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Context. As part of the data processing for Gaia Data Release 1 (Gaia DR1) a special astrometric solution was computed, the so-called auxiliary quasar solution. This gives positions for selected extragalactic objects, including radio sources in the second realisation of the International Celestial Reference Frame (ICRF2) that have optical counterparts bright enough to be observed with Gaia. A subset of these positions was used to align the positional reference frame of Gaia DR1 with the ICRF2. Although the auxiliary quasar solution was important for internal validation and calibration purposes, the resulting positions are in general not published in Gaia DR1. Aims. We describe the properties of the Gaia auxiliary quasar solution for a subset of sources matched to ICRF2, and compare their optical and radio positions at the sub-mas level. Methods. Descriptive statistics are used to characterise the optical data for the ICRF sources and the optical-radio differences. The most discrepant cases are examined using online resources to find possible alternative explanations than a physical optical-radio offset of the quasars. Results. In the auxiliary quasar solution 2191 sources have good optical positions matched to ICRF2 sources with high probability. Their formal standard errors are better than 0.76 milliarcsec (mas) for 50% of the sources and better than 3.35 mas for 90%. Optical magnitudes are obtained in Gaia's unfiltered photometric G band. The Gaia results for these sources are given as a separate table in Gaia DR1. The comparison with the radio positions of the defining sources shows no systematic differences larger than a few tenths of a mas. The fraction of questionable solutions, not readily accounted for by the statistics, is less than 6%. Normalised differences have extended tails requiring case-by-case investigations for around 100 sources, but we have not seen any difference indisputably linked to an optical-radio offset in the sources. Conclusions. With less than a quarter of the data expected from the nominal mission it has been possible to obtain positions at the sub-mas level for most of the ICRF sources having an optical counterpart brighter than 20.5 mag.

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“…These sources are mainly drawn from the Radio Fundamental Catalogue (RFC) comprising all sources observed with VLBI for astrometry and geodesy programmes. They also include data from major VLBI surveys such as the VLBA Calibrator Survey (e.g., Beasley et al 2002;Petrov et al 2005;Kovalev et al 2007), the VLBI Imaging and Polarimetry Survey (Helmboldt et al 2007;Petrov & Taylor 2011), and the VLBA 2-cm Survey Kellermann et al (1998Kellermann et al ( , 2004, among others. For their sample of sources, they fit two circular Gaussian components to the self-calibrated visibilities for each source, then select the most compact component as the core component for which they determine the FWHM angular sizes.…”

Section: Published Vlbi Core Sizesmentioning

confidence: 99%

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

Koay

Macquart

Jauncey

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We investigate the relationship between 5 GHz interstellar scintillation (ISS) and 15 GHz intrinsic variability of compact, radio-selected AGNs drawn from the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory (OVRO) blazar monitoring program. We discover that the strongest scintillators at 5 GHz (modulation index, m 5 ≥ 0.02) all exhibit strong 15 GHz intrinsic variability (m 15 ≥ 0.1). This relationship can be attributed mainly to the mutual dependence of intrinsic variability and ISS amplitudes on radio core compactness at ∼ 100 µas scales, and to a lesser extent, on their mutual dependences on source flux density, arcsec-scale core dominance and redshift. However, not all sources displaying strong intrinsic variations show high amplitude scintillation, since ISS is also strongly dependent on Galactic line-of-sight scattering properties. This observed relationship between intrinsic variability and ISS highlights the importance of optimizing the observing frequency, cadence, timespan and sky coverage of future radio variability surveys, such that these two effects can be better distinguished to study the underlying physics. For the full MASIV sample, we find that Fermi-detected gamma-ray loud sources exhibit significantly higher 5 GHz ISS amplitudes than gamma-ray quiet sources. This relationship is weaker than the known correlation between gamma-ray loudness and the 15 GHz variability amplitudes, most likely due to jet opacity effects.

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The VLBA Calibrator Survey—VCS1

Cited by 336 publications

References 33 publications

Radio spectra of bright compact sources at z>4.5

Radio spectra of bright compact sources at z>4.5

GaiaData Release 1

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

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