A Global 86 GHZ Vlbi Survey of Compact Radio Sources

Abstract

We present results from a large 86 GHz global VLBI survey of compact radio sources. The main goal of the survey is to increase by factors of 3-5 the total number of objects accessible for future 3-mm VLBI imaging. The survey observations reach the baseline sensitivity of 0.1 Jy and image sensitivity of better than 10 mJy/beam. The total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, extending the database of the sources imaged at 86 GHz with VLBI observation by a factor of 5, and only 6 sources have not been detected. The remaining 12 objects have been detected but could not be imaged due to insufficient closure phase information. Radio galaxies are less compact than quasars and BL Lacs on submilliarcsecond scale. Flux densities and sizes of core and jet components of all imaged sources have been estimated using Gaussian model fitting. From these measurements, brightness temperatures have been calculated, taking into account resolution limits of the data. The cores of 70% of the imaged sources are resolved. The core brightness temperatures of the sources peak at ∼ 10 11 K and only 1% have brightness temperatures higher than 10 12 K. Cores of Intraday Variable (IDV) sources are smaller in angular size than non-IDV sources, and so yield higher brightness temperatures.Every source in the sample was observed for 3-4 scans of 7-minute duration (snapshot mode). Although the uv-coverage of such an experiment limits the dynamic range and structural sensitivity of images, the large number of the participating antennas gives a sufficient uv-coverage of the sources at low and high declinations ( Figure 2). The data were recorded either with 128-MHz or 64-MHz bandwidth using the MkIV VLBI system with 1-and 2-bit sampling adopted at different epochs.The observations were made in lefthand circular polarization (LCP). Three to four scans per hour were recorded, using the time between the scans for antenna focusing, pointing and calibration. The data were correlated using the MkIV correlator of the Max-Planck-Institut für Radioastronomie (MPIfR) in Bonn (Alef & Müskens 2001). Data ProcessingIn this section, we describe the post-correlation processing of the 3 mm-VLBI survey datasets. Fringes were searched in two steps using HOPS (Haystack Observatory Postprocessing System) and AIPS (The NRAO Astronomical Image Processing System). In the first step, the HOPS task fourfit was used to precisely determine phase-residuals. The first fourfit was run with a wide search window (e.g. a width of 1 µsec for singleband delay, 2 µsec for multiband delay and 500 psec sec −1 for delay rate) centered at zero in delay. Since the fourfit produces baseline-based fringe solutions, the mean and standard deviation of the detected fringe solutions on each baseline were estimated and served as the offset and width of the search window for the second fourfit. The detected fringe solutions from the second run were used to interpolate the offset of the singleband delay for nondetected scans. In the f...

Radio structure of the blazar 1156 + 295 with sub-pc resolution

“…This phenomenon seems to be common in flat-spectrum radio-loud quasars and AGNs (Lee et al 2008;Lobanov et al 2000). Since 1156+295 is still rather compact at 86 GHz, e.g.…”

“…The uncertainty in the separation of jet components to the core component C is estimated to be ΔR = W 2S NR (for the compact components from C4 to C7), or ΔR = a(1+S NR) 1/2 2S NR (for extended components C2 and C3), where W is the FWHM of the synthesized beam, a is the major-axis size of the component, and S NR is the signal-to-noise ratio for a given component (Fomalont 2004;Lee et al 2008). …”

RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz

“…Previous VSOP space VLBI observations of 0642+449 at 5 GHz put a lower limit of T b ≥ 3.3 × 10 12 K (Dodson et al 2008), while ground VLBI observations at 15 and 86 GHz resolved the compact core and yielded somewhat lower brightness temperatures of 1.2-4.3 × 10 12 K (Kovalev et al 2005) and 1.1-1.6 × 10 11 K (Lobanov et al 2000;Lee et al 2008). This implies that the compact core becomes gradually resolved out at higher frequencies, calling for comparison with the new space VLBI observations by RadioAstron.…”

Multiwavelength observations of the γ-ray flaring quasar S4 1030+61 in 2009–2014

“…Moreover, the VLBI studies of the jet samples indicate that core components are well described by elliptical Gaussians extended along the jet direction (e.g., Kovalev et al 2005;Lee et al 2008). We have analyzed if the data are significantly better described if the new C3 component is introduced instead of explaining changes in the structure by a varying core component extension and flux density.…”

“…3. The lower limit on accuracy in position of the modelled components in the image, σposition, can be estimated as (Fomalont 1999;Lee et al 2008):…”