The subpulse modulation properties of pulsars at 21 cm

We present the results of a systematic, unbiased search for subpulse modulation of 187 pulsars performed with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands at an observing wavelength of 21 cm. Using new observations and archival WSRT data we have increased the list of pulsars that show the drifting subpulse phenomenon by 42, indicating that at least one in three pulsars exhibits this phenomenon. The real fraction of pulsars that show the drifting phenomenon is likely to be larger than 55%. The majority of the analysed pulsars show subpulse modulation (170), of which the majority were not previously known to show subpulse modulation and 30 show clear systematic drifting. The large number of new drifters we have found allows us, for the first time, to do meaningful statistics on the drifting phenomenon. We find that the drifting phenomenon is correlated with the pulsar age such that drifting is more likely to occur in older pulsars. Pulsars that drift more coherently seem to be older and have a lower modulation index. There is no significant correlation found between P-3 and other pulsar parameters (such as the pulsar age), as has been reported in the past. There is no significant preference of drift direction and the drift direction is not found to be correlated with pulsar parameters. None of the four complexity parameters predicted by different emission models are shown to be inconsistent with the set of modulation indices of our sample of pulsars. Therefore none of the models can be ruled out based on our observations. We also present results on some interesting new individual sources like a pulsar that shows similar subpulse modulation in both the main- and interpulse and six pulsars with opposite drift senses in different components.