Quasi-regular variations of subpulse drifting for PSR J1857+0057.

During observations of the Galactic Plane Pulsar Snapshot survey by the Five-hundred-meter Aperture Spherical radio Telescope, varying subpulse drifting of PSR J1857+0057 is detected. The follow-up observation confirms the quasi-regular changes of the drifting rate about every 50 periods. We determine the drift rate D through a linear fit to the pulse-central longitudes of subpulses in a drifting band, and determine P 3 from the cross-points of two fitted lines at the zero longitude for two neighbouring drifting bands. The low frequency modulation of about every 50 periods is found on variations of not only pulse intensity but also drift parameters. In most of low frequency modulation cycles, the integrated pulse intensity I and the absolute drift rate | D | tend to increase first and then decrease, and the drifting periodicity P 3 varies just in the opposite. In addition, the phase-forward intensity-enhancement is observed in many modulation cycles. Based on our polarization data, the average position angle curve for pulses with a smaller | D | and larger P 3 is slightly steep in the leading edge of pulse profile compared with that of the fully averaged profile. [ABSTRACT FROM AUTHOR]