Pulsars north of the wall

After enduring many weeks north of the wall, the visiting observer at KAIRA has finally returned home to sunny Southampton. Over the next couple of posts, we will give a brief overview of her observations using KAIRA.

Poppy Martin, a PhD student at the University of Southampton, has spent the past 5 and a half weeks based at KAIRA running an observing campaign. Whilst here, her primary aim has been to develop a pioneering new method of observing the ionosphere - by using pulsar observations to look at the Faraday rotation that occurs in the ionosphere.

Pulsars are rapidly rotating, highly magnetised neutron stars that emit a beam of electromagnetic radiation that sweeps through the sky like a lighthouse, causing us to observe a pulse whenever it is pointed at the Earth. They were first discovered in 1967 by Jocelyn Bell and Antony Hewish, and because the pulses occurred so regularly, there was an early hypothesis that they were actually aliens trying to communicate with us. In fact, the first pulsar was actually nicknamed LGM-1 (Little Green Men-1). 


Pulsars have been observed using KAIRA before (see http://kaira.sgo.fi/2012/09/first-pulsar-observation-at-kaira.html). However we have developed this method, and can now observe up to eight pulsars simultaneously.

An example of a pulsar observation made using the HBA.
 (Image: P. Martin)

Text/image credit: P. Martin

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).

VLBI with KAIRA, LOFAR and the LWA

As we write this, KAIRA is carrying out its first VLBI experiment. We are recording 122 beamlets from RCU mode 4 (LBA, but with 30-80 MHz filters). The subband statistics, beamlet statistics and antenna cross-correlation files are being recorded, but it is the raw beamlet data that is significant here, as it can be used to correlate with the other VLBI stations involved. This particular experiment, organised by Olaf Wucknitz, is being done together with LOFAR stations Effelsberg (DE601), Jülich (DE605), Onsala (SE607) and the LWA in the United States. The map shows the location of these sites:

Sites used in this VLBI experiment. (Background map: ESRI/Penn)

Although this is not the first international experiment with KAIRA, it is the first with recorded raw beamlet data and, therefore, our first attempt to get radio interferometric fringes. Although this is a very ambitious project, if we do manage to get this to work it will be a major technical success.

Updates:

03:26 UTC — In the end, DE605 did not take part. DE601, KAIRA and the LWA reported in okay. No news yet, from SE607.

05:49 UTC — SE607 have now reported it. Their observations went well too.