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Wide-band simultaneous observations of pulsars

Submitter: Tom Hassall
Description: The second LOFAR pulsar paper, "Wide-band simultaneous observations of pulsars: disentangling dispersion measure and profile variations" (Hassall et al. 2012) has recently been accepted for publication in A&A (preprint available here). In the paper, we report the results of simultaneous observations of four pulsars using LOFAR, the Lovell, and the Effelsberg telescope at observing frequencies between 48 MHz and 8 GHz.

In general, pulse profiles from radio pulsars are seen to get broader at lower observing frequencies. This can be explained by low-frequency radio emission coming from higher up in the pulsar magnetosphere. In the standard model, radio emission is produced along dipolar magnetic field lines emanating from the magnetic poles, leading to a broadening of the average pulse profile as the field lines move further apart higher in the magnetosphere. This model also predicts that pulses from lower down in the magnetosphere (at high frequencies) should take a longer time to reach us than the pulses from higher in the magnetosphere (at low frequencies), as they have further to travel.

By carefully timing when the pulses at different frequencies arrived at Earth it was possible to show that, for all four of the pulsars we observed, that their radio emission is confined to a remarkably narrow range of heights above the neutron star surface. In the case of PSR B1133+16 (pulse profiles shown in the image), this range is less than 110 km.

The pulse broadening which we observe cannot be explained by the curvature of a dipolar field in this narrow range of emission heights, suggesting that something may be wrong with our current model of pulsars. Further work needs to be done to reconcile these observations with theory. LOFAR will be vital for such studies since it probes the highest altitudes of the radio emission region.
Copyright: Hassall/Hessels
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