Simultaneous dual-frequency observations of giant pulses from the Crab pulsar

Simultaneous measurements of giant pulses from the Crab pulsar were taken a t two widely spaced frequencies, using the real-time detection of a giant p ulse at 1.4 GHz at the Very Large Array to trigger the observation of that same pulse at 0.6 GHz at a 25 m telescope in Green Bank, WV. Interstellar d ispersion of the signals provided the necessary time to communicate the tri gger across the country via the Internet. About 70% of the pulses are seen at both 1.4 and 0.6 GHz, implying an emission mechanism bandwidth of at lea st 0.8 GHz at 1 GHz for pulse structure on timescales of 1 to 10 mu s. The giant pulse spectral indices fall between -2.2 and -4.9, which may be compa red to the average main pulse value for this pulsar of -3.0. The arrival ti mes at both frequencies display a jitter of 100 mu s within the window defi ned by the average main pulse profile and are tightly correlated. This tigh t correlation places limits on both the emission mechanism and the frequenc y-dependent propagation within the magnetosphere. At 1.4 GHz, the giant pul ses are resolved into several closely spaced components. Simultaneous obser vations at 1.4 and 4.9 GHz show that the component splitting is frequency i ndependent. We conclude that the multiplicity of components is intrinsic to the emission from the pulsar, and reject the hypothesis that this is the r esult of multiple imaging as the signal propagates through the perturbed th ermal plasma in the surrounding nebula. At both 1.4 and 0.6 GHz, the pulses are characterized by a fast rise time and an exponential decay time that a re correlated. At 0.6 GHz, the rise time is not resolved. The rise and fall times of the 1.4 GHz components vary from component to component and from pulse to pulse. The pulse broadening, with its exponential decay form, is m ost likely the result of multipath propagation in intervening ionized gas. These decay times, and that seen in contemporaneous 0.3 GHz average pulse d ata, are large compared to normal conditions for the Crab pulsar. The most likely location for the perturbed plasma is the interface region between th e pulsar-driven synchrotron nebula and the overlying supernova ejecta.