VLBI observations of Jupiter with the initial test station of LOFAR and the Nançay decametric array

¹ Department of Astrophysics (IMAPP), Radboud University Nijmegen, 6535 ED Nijmegen, The Netherlands e-mail: anigl@astro.ru.nl
² LESIA, CNRS/Observatoire de Paris, 92195 Meudon, France
³ ASTRON, 7990 AA Dwingeloo, The Netherlands

Received: 31 January 2007
Accepted: 31 May 2007

Abstract

Aims.To demonstrate and test the capability of the next generation of low-frequency radio telescopes to perform high resolution observations across intra-continental baselines. Jupiter's strong burst emission is used to perform broadband full signal cross-correlations on time intervals of up to hundreds of milliseconds.

Methods.Broadband VLBI observations at about 20 MHz on a baseline of ~50 000 wavelengths were performed to achieve arcsecond angular resolution. lofar's Initial Test Station (lofar/its, The Netherlands) and the Nançay Decametric Array (nda, France) digitize the measured electric field with 12 bit and 14 bit in a 40 MHz baseband. The fine structure in Jupiter's signal was used for data synchronization prior to correlation on the time-series data.

Results.Strong emission from Jupiter was detected during snapshots of a few seconds and detailed features down to microsecond time-scales were identified in dynamic spectra. Correlations of Jupiter's burst emission returned strong fringes on 1 ms time-scales over channels as narrow as a hundred kilohertz bandwidth.

Conclusions.Long baseline interferometry is confirmed at low frequencies, in spite of phase shifts introduced by variations in ionospheric propagation characteristics. Phase coherence was preserved over tens to hundreds of milliseconds with a baseline of ~700 km. No significant variation with time was found in the correlations and an estimate for the fringe visibility of 1, suggested that the source was not resolved. The upper limit on the source region size of Jupiter Io-B S-bursts corresponds to an angular resolution of ~3 arcsec. Adding remote stations to the lofar network at baselines up to thousand kilometers will provide 10 times higher resolution down to an arcsecond.