VLBI observations of weak sources using fast frequency switching

¹ 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.