A novel search for gravitationally lensed radio sources in wide-field VLBI imaging from the mJIVE-20 survey

We present a novel pilot search for gravitational lenses in the mJIVE-20 survey, which observed $24\,903$ radio sources selected from FIRST with the VLBA at an angular resolution of 5 mas. We have taken the visibility data for an initial $3\,640$ sources that were detected by the mJIVE-20 observations and re-mapped them to make wide-field images, selecting fourteen sources that had multiple components separated by $\geq100$ mas, with a flux-ratio of $\leq15$:$1$ and a surface brightness consistent with gravitational lensing. Two of these candidates are re-discoveries of gravitational lenses found as part of CLASS. The remaining twelve candidates were then re-observed at 1.4 GHz and then simultaneously at 4.1 and 7.1 GHz with the VLBA to measure the spectral index and surface brightness of the individual components as a function of frequency. Ten were rejected as core-jet or core-hotspot(s) systems, with surface brightness distributions and/or spectral indices inconsistent with gravitational lensing, and one was rejected after lens modelling demonstrated that the candidate lensed images failed the parity test. The final lens candidate has an image configuration that is consistent with a simple lens mass model, although further observations are required to confirm the lensing nature. Given the two confirmed gravitational lenses in the mJIVE-20 sample, we find a robust lensing-rate of $1$:($318\pm225$) for a statistical sample of 635 radio sources detected on mas-scales, which is consistent with that found for CLASS.Comment: 31 pages, 22 figures; accepted for publication in MNRA

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Contains reports on four research projects.National Aeronautics and Space Administration (Grant NsG-334)Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E

Discovering the colliding wind binary HD 93129A

HD 93129A is a binary system including an O2 If+ and probably an O3.5 V-star orbiting at a distance of about 140 AU (55 mas given the distance of 2.5 kpc), which potentially makes the system the most massive one in the Galaxy, ahead of eta-Carina. Its non-thermal radio emission was proposed to be originated by the collision between the winds of both stars. HST/FGS data have been reanalyzed to derive an accurate absolute position of the stars to compare them with the radio emission. The analysis of ATCA radio observations along several years reveals a power-lawspectrum with an increase on the radio flux density along time. We conducted an observation with the Australian Long Baseline Array (LBA) at 2.3 GHz in 2008 to resolve the radio source and its location within the stellar system. These radio data revealed a bow-shape extended emissionlocated between both stars, as expected in a wind collision region. The observed structure allows us to roughly estimate the mass-loss rate ratio for the two stars in the system, concluding that dM_b/dt = 0.7 dM_a/dt. The multiwavelength analysis points out that the detected radio emission is likely to be originated by one of the most massive collision wind binary in the Galaxy.Fil: Marcote, B.. Universidad de Barcelona; EspañaFil: Benaglia, Paula. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; ArgentinaFil: Moldon, J.. Netherlands Institute for Radio Astronomy; Países BajosFil: Nelan, E.. Space Telescope Science Institute; Estados UnidosFil: De Becker, M.. Université de Liège; BélgicaFil: Dougherty, S, M.. NRC Herzberg Astronomy And Astrophysics; CanadáFil: Koribalski, Bärbel. Australia Telescope National Facility; Australia12th European VLBI Network Symposium and Users MeetingCagliariItaliaIstituto di Radioastronomi

The LOFAR ling baseline snapshot calibrator survey

Aims:\ud An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz.\ud \ud Methods\ud We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.\ud \ud Results:\ud More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree.\ud \ud Conclusions:\ud The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator