Pilot project for a search for more astrometric calibrators using the wide field fringe search technique

Objectives of the campaign

In the past, the list of bright compact sources detected at VLA and/or MERLIN served as a pool for candidates for VLBI Calibrator surveys. Almost all these objects have been already observed with the VLBA. Since the VLBA Calibrator list is complete at the 150—200 mJy level, new calibrators are relatively faint objects. Most all-sky surveys at frequencies higher than 2 GHz are either incomplete and/or not deep enough. As a result, information about spectral indexes is sparse and unreliable. For this reason, it is more difficult to find remaining flat spectrum good candidate sources. The fainter the source is, the less reliable information about its spectral index is, if it is available at all. Although the majority of flat spectrum sources from the NVSS are AGNs, in general, it is not possible to deduce the estimate of the correlated flux density at baselines of ∼ 1000 km from the flux density at VLA baselines. A source with correlated flux density 5 mJy is not useful as a calibrator for many applications, but a source with flux density 30—100 mJy is useful.

We are going to exploit the unique capability of the JIVE correlator that is able to provide the highest output rate in the world, and that allows us to use the ultra-wide window for fringe search. This will increase the detection rate up to 10%.

New candidates to calibrators were found by analyzing 193 radioastronomical catalogues using the CATS database. The database includes NVSS, CLASS, CRATES \citep{crates} and others. We selected 854 sources which satisfy the following criteria:

According to our previous experience, the list of candidate sources selected by this way may contain up to 50% extended sources that are not detectable with VLBI.

One of the the most economic modes of making sources survey is to observe them in a ring zones in order to minimize losses for slewing. We selected the zone of interest with δ ∈ [+53° +75°], area of 1.02 srad. There is nothing specific in the selected declination zone, we picked with zone for facilitating scheduling.

We propose to exploit the much higher sensitivity of the EVN with respect to other VLBI arrays and to screen the list of candidates. One scan of 70 seconds long is sufficient to determine whether the source (or sources) has a compact component at milliarcsecond scales with the correlated flux density more than 10 mJy at baselines which include Effelsberg and with the correlated flux density more than 30 mJy at other baselines.

The goal was to test the proposed method and check the feasibility of such an approach, its efficiency to find new compact sources, to check if the observing setup and strategy are optimal. In particular, we were interested in answering the following questions:

People

Team members (in alphabetic order):

Observations:

The table of 375 observed target sources and their coordinates.

Preliminary results:

In total, 390 sources were obseved: 375 targets and 15 amplitude calibrators. Among then all, 338 targets, and 15 calibrators have been detected. Detection rate is 90%.

The table of estimates of correlated flux density among 353 detected sources is availble in both plain ascii text format and in html format. Errors in estimates are considered to have error no exceeding 15%.

Estimates of the SEFD and its adjustments 

Station: EFLSBERG SEFD_zen:    17. -+     2. Jy   Adj:  0.808 -+  0.005
Station: JODRELL2 SEFD_zen:   355. -+    10. Jy   Adj:  0.890 -+  0.007
Station: MEDICINA SEFD_zen:   135. -+     7. Jy   Adj:  0.757 -+  0.006
Station: NOTO     SEFD_zen:   764. -+   316. Jy   Adj:  1.270 -+  0.009
Station: ONSALA60 SEFD_zen:  1097. -+    10. Jy   Adj:  1.119 -+  0.015
Station: TORUN    SEFD_zen:   185. -+     7. Jy   Adj:  0.954 -+  0.007

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This page was prepared by Leonid Petrov ()
Last update: 2009.02.08_21:31:00