Differential Frequency-dependent Delay from the Pulsar Magnetosphere

TL;DR

This study investigates how drifting subpulses in pulsar PSR B0809+74 evolve across a wide frequency range, revealing a gradual increase in phase step size and suggesting complex magnetospheric processes beyond existing models.

Contribution

It demonstrates that the subpulse phase step increases gradually with frequency and attributes it to two separate driftbands with frequency-dependent arrival times, challenging current models.

Findings

Subpulse phase step size increases gradually with frequency.

Two separate driftbands with different frequency-dependent arrival times.

The observed pattern cannot be explained by existing models.

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.