Differential Frequency-dependent Delay from the Pulsar Magnetosphere

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.

10 pages, 6 figures, accepted for publication in Astronomy and Astrophysics

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• A census of the pulsar population observed with the international LOFAR station FR606 at low frequencies (25-80~MHz)
• Pulsar bi-drifting: implications for polar cap geometry
• Pulsars with NenuFAR: backend and pipelines
• Millisecond Pulsar Scintillation Studies with LOFAR: Initial Results
• B0943+10: low-frequency study of subpulse periodicity in the Bright mode with LOFAR
• The LOFAR Tied-Array All-Sky Survey (LOTAAS): Characterization of 20 pulsar discoveries and their single-pulse behavior
• Variations of $P_2$ in subpulse drifting pulsars
• Poynting flux transport channels formed in polar cap regions of neutron star magnetospheres
• Linear acceleration emission of pulsar relativistic streaming instability and interacting plasma bunches
• The mechanism of coherent radio emission in some classes of pulsar
• Origin of radio polarization in pulsar polar caps
• Long-term Timing Results of Ecliptic Pulsars Observed with I-LOFAR
• On the Geometry of Curvature Radiation and Implications for Subpulse Drifting