High-resolution single-pulse studies of the Vela pulsar

Abstract

We present high-resolution multi-frequency single-pulse observations of the Vela pulsar, PSR B0833 − 45, aimed at studying micro-structure, phase-resolved intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We find micro-structure in about 80 per cent of all pulses independent of observing frequency. The width of a micro-pulse seems to depend on its peak flux density, whilst quasi-periodicities observed in micro-pulses remain constant. The width of the micro-pulses may decrease with increasing frequency, but confirmation is needed at higher frequencies. The fraction of pulses showing quasi-periodic micro-pulses may become smaller at higher frequencies. We show that the micro-pulse width in pulsars has a period dependence which suggests a model of sweeping micro-beams as the origin of the micro-pulses. Like individual pulses, the micro-pulses of Vela are highly elliptically polarized. There is a strong correlation between Stokes parameters V and I in the micro-structure. We do not observe any micro-pulses where the handedness of the circular intensity changes within a micro-pulse, although flips within a pulse are not uncommon. We show that the V/I distribution is Gaussian with a narrow width and that this width appears to be constant as a function of pulse phase. The phase-resolved intensity distributions of I are best fitted with log-normal statistics. Extra emission components, i.e. ‘bump’ and ‘giant micro-pulses’, discovered by Johnston et al. at 0.6 and 1.4 GHz are also present at the higher frequency of 2.3 GHz. The bump component seems to be an extra component superposed on the main pulse profile but does not appear periodically. The giant micro-pulses are time-resolved and have significant jitter in their arrival times. Their flux density distribution is best fitted by a power-law, indicating a link between these features and ‘classical’ giant pulses as observed for the Crab pulsar (PSR B0531 + 21), PSR B1937 + 21 and PSR B1821 − 24. We find that Vela contains a mixture of emission properties representing both ‘classical’ properties of radio pulsars (e.g. micro-structure, high degree of polarization, S-like position angle swing, orthogonal modes) and features which are most likely related to high-energy emission (e.g. extra profile components, giant micro-pulses). It hence represents an ideal test case to study the relationship between radio and high-energy emission in significant detail.