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08-08-2014
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Millisecond Pulsar Scintillation Studies with LOFAR: Initial Results

Submitter: Anne Archibald
Description: Stars twinkle because the turbulent atmosphere creates interference patterns in their images. At radio frequencies, the atmosphere doesn't affect the signal (much), but the interstellar medium does. The very tenuous plasma - about one atom per cubic centimeter, as a rule of thumb - bends and delays the radio waves very slightly. Since there are a lot of centimeters between us and a pulsar, this effect leads to multi-path propagation: some of the signal from the pulsar is delayed by travelling along slightly longer paths. This effect gets stronger at lower frequencies, so as you can see in the top left image, the pulse from this pulsar gets progressively more smeared out as one moves to lower frequencies.

This smearing effect can be a problem for precision timing of pulsars because the amount of smearing varies from week to week. It is also difficult to study because the intrinsic pulse shape of the pulsar also changes with frequency. Fortunately, there is another effect of the multi-path propagation: the signals coming along different paths interfere with each other, producing a pattern of constructive and destructive interference that can be seen by looking at the brightness as a function of frequency. The top right image shows two peaks in this interference pattern. Note that they are only a few kilohertz wide! This complicates the analysis, since the pulsar's spin frequency is nearly a kilohertz.

The few bright peaks in this interference pattern are also surrounded by a great deal of noise. We therefore used an autocorrelation to effectively add up all the peaks, giving us a measurement of the average peak width, shown in the bottom left image. This average peak width is roughly the reciprocal of the amount of smearing shown in the top left panel, and its dependence on frequency (shown in the bottom right panel) can potentially tell us how the turbulent interstellar medium behaves.

This image is based on the paper Millisecond Pulsar Scintillation Studies with LOFAR: Initial Results, by Anne Archibald, Vlad Kondratiev, Jason Hessels, and Dan Stinebring.
Copyright: Anne Archibald
 
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