The Subpulse Modulation Properties of Pulsars and its Frequency Dependence

TL;DR

This study analyzes the subpulse modulation properties of about two hundred pulsars at 21 and 92 cm, revealing that drifting subpulses are common, broadband, and become more organized with pulsar age, challenging previous correlations.

Contribution

It provides the first extensive observational evidence that drifting subpulses are intrinsic, broadband, and evolve with pulsar age, with new measurements at two frequencies for a large sample.

Findings

Drifting subpulses are detected in 57 pulsars for the first time.

Subpulse organization increases with pulsar age.

P3 values are highly correlated across frequencies.

Abstract

A large sample of about two hundred pulsars have been observed to study their subpulse modulation at an observing wavelength of (when achievable) both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting subpulses are discovered for the first time and are confirmed for many others. This leads to the conclusion that it could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. Drifting subpulses are in general found at both frequencies and the measured values of P3 at the two frequencies are highly correlated, showing the broadband nature of this phenomenon. Also the modulation indices measured at the two frequencies are clearly correlated, although at 92 cm they are on average possibly higher. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission is composed out of a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. There is no obvious correlation found between P3 and the pulsar age (or any other pulsar parameter) contrary to reports in the past.