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Start Over Subject very long baseline interferometry Validation Level 2
51. Radio source selection for the ICRF
ID:
ivo://CDS.VizieR/J/A+A/493/317
Title:
Radio source selection for the ICRF
Short Name:
J/A+A/493/317
Date:
21 Oct 2021
Publisher:
CDS
Description:
The most accurate realization of a quasi inertial reference frame, the International Celestial Reference Frame (ICRF), is made of 212 defining extragalactic radio sources whose coordinates are determined using VLBI observations. Recent studies demonstrated however that using other sets of sources could improve the frame stability. This study examines a simple radio source selection scheme to define celestial reference frame axes more stable than the ones as currently defined by the ICRF. After having derived source coordinate time series from 26 years of VLBI observations, we select the most suitable sources on the basis of their positional variability (rms and slope), and observational history. We determine the axis stability of the frame defined by the selection, as well as its suitability for global geodetic VLBI analyses, i.e., determination of Earth orientation parameters and source and station coordinates.
52. Revisited jet bending in {gamma}-ray AGNs
ID:
ivo://CDS.VizieR/J/ApJ/784/159
Title:
Revisited jet bending in {gamma}-ray AGNs
Short Name:
J/ApJ/784/159
Date:
21 Oct 2021
Publisher:
CDS
Description:
We investigate the hypothesis that {gamma}-ray-quiet active galactic nuclei (AGNs) have a greater tendency for jet bending than {gamma}-ray-loud AGNs, revisiting the analysis of Tingay et al. We perform a statistical analysis using a large sample of 351 radio-loud AGNs along with {gamma}-ray identifications from the Fermi Large Area Telescope (LAT). Our results show no statistically significant differences in jet-bending properties between {gamma}-ray-loud and {gamma}-ray-quiet populations, indicating that jet bending is not a significant factor for {gamma}-ray detection in AGNs.
53. Second Caltech-Jodrell Bank VLBI Survey. I.
ID:
ivo://CDS.VizieR/J/ApJS/95/345
Title:
Second Caltech-Jodrell Bank VLBI Survey. I.
Short Name:
J/ApJS/95/345
Date:
21 Oct 2021
Publisher:
CDS
Description:
We define the sample for the second Caltech-Jodrell Bank VLBI survey. This is a sample of 193 flat- or gigahertz-peaked-spectrum sources selected at 4850 MHz. This paper presents images of 91 sources with a resolution of ~1 mas, obtained using VLBI observations at 4992 MHz with a global array. The remaining images and the integrated radio spectra will be presented in a forthcoming paper by Henstock et al.
54. Second Caltech-Jodrell Bank VLBI Survey. II.
ID:
ivo://CDS.VizieR/J/ApJS/100/1
Title:
Second Caltech-Jodrell Bank VLBI Survey. II.
Short Name:
J/ApJS/100/1
Date:
21 Oct 2021
Publisher:
CDS
Description:
This is the second of two papers presenting the Second Caltech- Jodrell Bank VLBI survey (CJ2). The CJ2 sample consists of 193 flat- and gigahertz-peaked-spectrum sources selected at 4850 MHz. In this paper we present images of the remaining 102 sources with ~1 mas resolution, obtained using VLBI snapshot observations at 4992 MHz with a global array. We also present integrated radio spectra for the entire CJ2 sample.
55. Second epoch VLBA Calibrator Survey (VCS-II)
ID:
ivo://CDS.VizieR/J/AJ/151/154
Title:
Second epoch VLBA Calibrator Survey (VCS-II)
Short Name:
J/AJ/151/154
Date:
21 Oct 2021
Publisher:
CDS
Description:
Six very successful Very Long Baseline Array (VLBA) calibrator survey campaigns were run between 1994 and 2007 to build up a large list of compact radio sources with positions precise enough for use as very long baseline interferometry (VLBI) phase reference calibrators. We report on the results of a second epoch VLBA Calibrator Survey campaign (VCS-II) in which 2400 VCS sources were re-observed in the X and S bands in order to improve the upcoming third realization of the International Celestial Reference Frame (ICRF3) as well as to improve their usefulness as VLBI phase reference calibrators. In this survey, some 2062 previously detected sources and 324 previously undetected sources were detected and revised positions are presented. Average position uncertainties for the re-observed sources were reduced from 1.14 and 1.98mas to 0.24 and 0.41mas in R.A. and decl., respectively, or by nearly a factor of 5. Minimum detected flux values were approximately 15 and 28mJy in the X and S bands, respectively, and median total fluxes are approximately 230 and 280mJy. The vast majority of these sources are flat-spectrum sources, with ~82% having spectral indices greater than -0.5.
56. S5 0836+710 Ground and Space VLBI images
ID:
ivo://CDS.VizieR/J/A+A/641/A40
Title:
S5 0836+710 Ground and Space VLBI images
Short Name:
J/A+A/641/A40
Date:
21 Oct 2021
Publisher:
CDS
Description:
Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. The RadioAstron observations at wavelengths of 18cm, 6cm, and 1.3cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02mas to 200mas. Brightness temperatures in excess of 10^13^K are measured in the jet, requiring Doppler factors of >=100 for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin-Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be Mj~=12 and {eta}~=0.33. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.
57. Sou323 ICRF reference sample
ID:
ivo://CDS.VizieR/J/MNRAS/466/1567
Title:
Sou323 ICRF reference sample
Short Name:
J/MNRAS/466/1567
Date:
21 Oct 2021
Publisher:
CDS
Description:
We aim to evaluate the possibility of improving the International Celestial Reference System realization starting from the ICRF2 catalogue by investigating the coordinate time series of radio sources observed by the very long baseline interferometry between 1979 and 2016. Sources with long observational history are selected as the candidates and the least-squares fits with special handling of the weights are performed to derive the linear drifts of the source coordinates. Then the sources are sorted, based on the normalized linear drift (i) over the whole sky, and (ii) in four homolographic areas divided by declinations. The axial stability of the reference system and sky distribution defined by the selected sources are evaluated, which are acted as the criterion for the final source lists. With our improved source selection scheme, two groups of sources are proposed and considered suitable for defining a more stable and homogeneous celestial reference system compared to the second version of the current International Celestial Reference Frame (ICRF2). The number of sources in the final lists are 323 and 294, respectively, and the global rotations of the axes derived from apparent motion of the sources are about two times better th
58. Spectroscopy of 120 emission-line ICRF2 candidates
ID:
ivo://CDS.VizieR/J/AJ/146/10
Title:
Spectroscopy of 120 emission-line ICRF2 candidates
Short Name:
J/AJ/146/10
Date:
21 Oct 2021
Publisher:
CDS
Description:
Continuing our program of spectroscopic observations of International Celestial Reference Frame (ICRF) sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58m European Southern Observatory New Technology Telescope, the two 8.2m Gemini telescopes, the 2.5m Nordic Optical Telescope (NOT), and the 6.0m Big Azimuthal Telescope of the Special Astrophysical Observatory in Russia. The targets were selected from the International VLBI Service for Geodesy & Astrometry candidate International Celestial Reference Catalog which forms part of an observational very long baseline interferometry (VLBI) program to strengthen the celestial reference frame. We obtained spectra of the potential optical counterparts of more than 150 compact flat-spectrum radio sources, and measured redshifts of 120 emission-line objects, together with 19 BL Lac objects. These identifications add significantly to the precise radio-optical frame tie to be undertaken by Gaia, due to be launched in 2013, and to the existing data available for analyzing source proper motions over the celestial sphere. We show that the distribution of redshifts for ICRF sources is consistent with the much larger sample drawn from Faint Images of the Radio Sky at Twenty cm (FIRST) and Sloan Digital Sky Survey, implying that the ultra-compact VLBI sources are not distinguished from the overall radio-loud quasar population. In addition, we obtained NOT spectra for five radio sources from the FIRST and NRAO VLA Sky Survey catalogs, selected on the basis of their red colors, which yielded three quasars with z>4.
59. Stability of VLBI reference frame
ID:
ivo://CDS.VizieR/J/A+A/375/661
Title:
Stability of VLBI reference frame
Short Name:
J/A+A/375/661
Date:
21 Oct 2021
Publisher:
CDS
Description:
The file table 2 gives the results of various tests and noise measurements performed on time serie of extragalactic radio source coordinates. Systematic and random behaviours in time serie of individual determinations of cooordinates for 283 sources over 1987-1999 are investigated. The random variability of the source coordinates is characterized by the {chi}^2^ test (Bevington, 1969, p.187) and the "goodness of fit (gof)" (Bevington, 1969, p.188). The Allan variance method (Allan, 1966, Proc. IEEE, vol. 54, 221) is used to investigate spectral characteristics of the time series. The source structure index (Fey & Charlot, 1997ApJS..111...95F, 2000, Cat. <J/ApJS/128/17>) qualifies the level of position disturbance expected at the date of the map (1 for the less disturbed, 4 for the most disturbed). The estimation of "linear rates" in the coordinates is a qualifier of the systematic parts of the source instability. This table contains the 283 best observed sources and is divided in three parts, with the left hand side giving general information and counts, the middle part giving statistics in the source local frame, and the right hand side giving statistics in the equatorial frame.
60. Sub-milliarcsecond imaging of 3C111
ID:
ivo://CDS.VizieR/J/A+A/644/A85
Title:
Sub-milliarcsecond imaging of 3C111
Short Name:
J/A+A/644/A85
Date:
21 Oct 2021
Publisher:
CDS
Description:
Flares in radio-loud active galactic nuclei (AGNs) are thought to be associated with the injection of fresh plasma into the compact jet base. Such flares are usually strongest and appear earlier at shorter radio wavelengths. Hence, very long baseline interferometry (VLBI) at millimeter (mm)-wavelengths is the best-suited technique for studying the earliest structural changes of compact jets associated with emission flares. Aims. We study the morphological changes of the parsec-scale jet in the nearby (z=0.049) gamma-ray bright radio galaxy 3C 111 following a flare that developed into a major radio outburst in 2007. We analyse three successive observations of 3C 111 at 86 GHz with the Global mm-VLBI Array (GMVA) between 2007 and 2008 which yield a very high angular resolution of ~45 muas. In addition, we make use of single-dish radio flux density measurements from the F-GAMMA and POLAMI programmes, archival single-dish and VLBI data. We resolve the flare into multiple plasma components with a distinct morphology resembling a bend in an otherwise remarkably straight jet. The flare-associated features move with apparent velocities of ~4.0c to ~4.5c and can be traced also at lower frequencies in later epochs. Near the base of the jet, we find two bright features with high brightness temperatures up to ~10^11^K, which we associate with the core and a stationary feature in the jet. The flare led to multiple new jet components indicative of a dynamic modulation during the ejection. We interpret the bend-like feature as a direct result of the outburst which makes it possible to trace the transverse structure of the jet. In this scenario, the components follow different paths in the jet stream consistent with expectations for a spine-sheath structure, which is not seen during intermediate levels of activity. The possibility of coordinated multiwavelength observations during a future bright radio flare in 3C 111 makes this source an excellent target for probing the radio-gamma-ray connection.