Observations of pulsar radio-emission with microsecond resolution.

This thesis describes a new technique for observing pulsar micro-structure, which is a class of random fluctuations in pulsar radio-emission on timescales less than one millisecond. Using this technique observations were made with a time resolution of 0.5 ˜í¬¿s. No new phenomena were revealed by the improvement in time resolution over that used by other workers. However the first measurement of the microstructure timescale of pulsar PSR 1749-28 was obtained. The design of the equipment and the computer processing involved in the method are described in the thesis. In this method -pulsar signals in a bandwidth of 2 MHz are mixed to baseband and recorded on videotape. Subsequently selected parts of the tape are digitized using the stop action facility of the recorder and an interleaved sampling scheme. The effects of interstellar dispersion are removed by computer processing yielding the maximal time resolution 1/B where B is the bandwidth, that is, 2 MHz. This method avoids the normal restriction, imposed by the speed of the digitizer, that applies when digitizing is done in real time, namely that the bandwidth must be restricted to half the sample rate. Recordings were made of signals received by the Molonglo radiotelescope at 408 MHz from 15 pulsars. A total of 22 pulses from four different pulsars were strong enough and otherwise suitable for analysis. Each pulse involves 86000 samples, that is, a time window of 20 ms sampled at an effective rate of 4.5 MHz. The pulses were analyzed at time resolutions down to 0.5 ˜í¬¿s and the dynamic spectrum of each in an observing window 15 ms x 2 MHz was obtained. A search was made for frequency-time structures such as those produced by moving,radiating sources and by interference effects in the pulsar magnetosphere. Such structures were not observed. Auto-correlation and cross-correlation analyses were carried out on the data. The results for PSR 0950+08 agree with those obtained by other workers. A characteristic microstructure timescale of 250 ¬¨¬± 50 ˜í¬¿s was obtained from the auto-correlation of the power spectrum of PSR 1749-28. All of the observations are consistent with the amplitude-modulated noise (AMN) model of Rickett (1975) together with interstellar scintillation. One particular pulse from PSR 0950+08 was analyzed in great detail because some large unresolved intensity fluctuations were apparent. Single intense impulses are not consistent with the ANN model and would be of some astrophysical interest if found. A series of tests were applied to the data; it is concluded that such impulses were not observed. The relationship of the ANN model to pulsar emission theories and the direction of future research are discussed.