Issue A&A Volume 433, Number 3, April III 2005 Page(s) 897 - 909 Section Extragalactic astronomy DOI http://dx.doi.org/10.1051/0004-6361:20042078

A&A 433, 897-909 (2005)
DOI: 10.1051/0004-6361:20042078

VLBI observations of weak sources using fast frequency switching

E. Middelberg¹, A. L. Roy^{1, 2}, R. C. Walker³ and H. Falcke<sup>4

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: enno.middelberg@csiro.au;aroy@mpifr-bonn.mpg.de
²  Geodätisches Institut der Universität Bonn, Nussallee 17, 53115 Bonn, Germany
³  National Radio Astronomy Observatory, PO Box 0, Socorro, NM, 87801, USA
    e-mail: cwalker@aoc.nrao.edu
⁴  ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
    e-mail: falcke@astron.nl

(Received 28 September 2004 / Accepted 17 December 2004)

Abstract
We have developed a new phase referencing technique for high frequency VLBI observations. In conventional phase referencing, one interleaves short scans on a nearby phase calibrator between the target source observations. In fast frequency switching described here, one observes the target source continuously while switching rapidly between the target frequency and a lower reference frequency. We demonstrate that the technique allows phase calibration almost reaching the thermal noise limit and present the first detection of the AGN in the FR I radio galaxy NGC 4261 at 86 GHz. Although point-like, this is the weakest source ever detected with VLBI at this frequency.

Key words: techniques: interferometric -- techniques: high angular resolution -- methods: observational -- galaxies: active -- galaxies: jets -- galaxies: individual: NGC 4261

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