First VLBI fringes with KAIRA & DE601

On Monday 13th May 2013, a joint test between three LOFAR-based telescopes (DE601, SE607 and KAIRA) was carried out. The target was the pulsar B0809+74. It was a complicated experiment, as there were numerous non-standard procedures in place, a variety of equipment/computing configurations and the challenges of scheduling and data transport. However, DE601 and KAIRA managed to take a short burst of co-temporal data and we managed to get the data from Kilpisjärvi to the correlator in Bonn, Germany.

Then, late last night, we received an e-mail from our German colleagues, with the news that with less than a minute of data, and with only one LOFAR "lane" (that is, a bandwidth of only 12 MHz), they had made a preliminary detection of fringes!

First-fringes! (Image: O.Wucknitz, MPIfR)

As the project coordinator, Olaf Wucknitz, explains...

The plot shows a delay/fringe-rate spectrum for 21.5 sec of the first data block with only lane 0 (12 MHz bandwidth). This is after correction for geometric delay (ca. 0.2 msec). The residual delay is about 0.5 musec, consistent with expectations for the ionosphere. The colour scale is logarithmic.

Note that the signal looks different than for unpulsed sources. For only one pulse we would have a diagonal line because of dispersion. For no dispersion, we would have a vertical line, because one short pulse does not constrain the rate. Dispersion turns this diagonal. Here we have a number of pulses, which causes the maxima at separations of 1/period in rate. This is because a full turn per period could not be detected. Ionospheric rates should be << 1 Hz, so the central peak is the real one.

It must be stressed that this is only a preliminary detection. There is still a lot of work to be done and additional verification experiments need to be carried out before we can proceed with further observational programmes. However, this is an extremely promising result and we are delighted to have made this much progress so far. Apart from advancing towards an exciting long-baseline science programme, we have also resolved numerous technical issues along the way, thus improving procedures, calibration and technique for all our other measurement campaigns.

KAIRA, Kilpisjärvi, Finland (Photo: D. McKay-Bukowski)

DE601, Effelsberg, Germany (Photo: D. McKay-Bukowski)

Of course, this initial detection is the longest baseline to have achieved this within the LOFAR system... a baseline of some 2185 km!

The location of the two stations: KAIRA (Kilpisjärvi, Finland) and DE601 (Effelsberg, Germany).

Thanks go to all the participants in this work, especially to Olaf Wucknitz (MPIfR, Bonn).