Simultaneous Dual Frequency Observations of Giant Pulses from the Crab Pulsar

Simultaneous measurements of giant pulses from the Crab pulsar were taken at two widely spaced frequencies using the real-time detection of a giant pulse at 1.4GHz at the Very Large Array to trigger the observation of that same pulse at 0.6GHz at a 25-m telescope in Green Bank, WV. Interstellar dispersion of the signals provided the necessary time to communicate the trigger across the country via the Internet. About 70% of the pulses are seen at both 1.4GHz and 0.6GHz, implying an emission mechanism bandwidth of at least 0.8GHz at 1GHz for pulse structure on time scales of one to ten microseconds. The giant pulse spectral indices fall between −2.2 and −4.9, which may be compared to the average main pulse value for this pulsar of −3.0. The arrival times at both frequencies display a jitter of 100μs within the window defined by the average main pulse profile and are tightly correlated. This tight correlation places limits on both the emission mechanism and on frequency dependent propagation within the magnetosphere. At 1.4GHz the giant pulses are resolved into several, closely spaced components. Simultaneous observations at 1.4GHz and 4.9GHz show that the component splitting is frequency independent. We conclude that the multiplicity of components is intrinsic to the emission from the pulsar, and reject the hypothesis that this is the result of multiple imaging as the signal propagates through the perturbed thermal plasma in the surrounding nebula. At both 1.4GHz and 0.6GHz the pulses are characterized by a fast rise time and an exponential decay time which are correlated. At 0.6GHz the rise time is not resolved.