T.E. Hassall et al., "Differential Frequency-dependent Delay
from the Pulsar Magnetosphere", Astronomy & Astrophysics, V552,
no. A61, pp1-9, April 2013
abstract: Some radio pulsars show clear `drifting
subpulses', in which subpulses are seen to drift in pulse longitude in
a systematic pattern. Here we examine how the drifting subpulses of
PSR B0809+74 evolve with time and observing frequency. We show that
the subpulse period (P3) is constant on timescales of days,
months and years, and between 14-5100 MHz. Despite this, the shapes of
the driftbands change radically with frequency. Previous studies have
concluded that, while the subpulses appear to move through the pulse
window approximately linearly at low frequencies (< 500 MHz), a
discrete step of ~ 180 degrees in subpulse phase is observed at higher
frequencies (> 820 MHz) near to the peak of the average pulse
profile. We use LOFAR, GMRT, GBT, WSRT and Effelsberg 100-m data to
explore the frequency-dependence of this phase step. We show that the
size of the subpulse phase step increases gradually, and is observable
even at low frequencies. We attribute the subpulse phase step to the
presence of two separate driftbands, whose relative arrival times vary
with frequency - one driftband arriving 30 pulses earlier at 20 MHz
than it does at 1380 MHz, whilst the other arrives simultaneously at
all frequencies. The drifting pattern which is observed here cannot be
explained by either the rotating carousel model or the surface
oscillation model, and could provide new insight into the physical
processes happening within the pulsar magnetosphere.
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