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ET calling LOFAR?

Submitter: Olaf Wucknitz
Description: The upper part of today's image shows a signal detected by LOFAR on October 27th, 2011 between about 3:00 and 3:30 UTC, while looking for giant pulses from the Crab pulsar. After an unhealthily excessive inspection of similar diagnostic plots from several observations, this particular result reminded one of the researchers of the picture shown in the middle (from Wikipedia). It illustrates a message that was sent by the Arecibo radio telescope in 1974 towards potential civilizations in the globular star cluster M13.

Could our LOFAR detection be their response? Highly unlikely, given that by 2011 the original signal has only covered 0.15 percent of the distance to M13. The more sane astronomers among us would bet that these strange-looking signals are due to strong, man-made radio interference radio, possibly causing intermodulation products in the LOFAR system.

Our plot shows intensity of the XX autocorrelation of the coherently added Superterp as function of time and frequency. Thankfully interfering signals like this seem to be rare, but when they appear they seriously affect the scientific analysis of the data. The horizontal line near 137.1 MHz was identified as possibly originating from weather satellite NOAA-19. Other broken horizontal lines are probably due to aircraft communication. The cause for the structures resembling the Arecibo message is still unknown but is being actively investigated.

Despite the alien signals in our data, a number of giant pulses could still be identified. Examples are illustrated in the bottom section. The black, red and blue lines show total intensity, linear and circular polarisation signals, respectively, as function of time. The units of the vertical axis are arbitrary. Increments of 0.1 on the horizontal axis are equivalent to 1 second.

Olaf Wucknitz, Charlotte Sobey, Jason Hessels, Andreas Horneffer, Menno Norden, Aris Noutsos
Copyright: International LOFAR
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