Giant Pulses from the Crab Pulsar: A Joint Radio and Gamma-Ray Study

The Crab Nebula pulsar emits bursts of radio emission as strong as 2000 times the average pulse amplitude. Using joint radio and gamma-ray observations of these giant radio pulses, we characterized intensity variations, measured absolute timing with 70 microseconds precision, and determined the spin-down model and interstellar dispersion. Fitting the flux-density histogram requires a two-component model a narrow distribution of weak (but nonzero) pulses and a power-law component for giant pulses with an index of -3.3 and a low-intensity cutoff that is 33 times the mean of the weak pulses. The lack of time delay between giant pulses and weak pulses (Δt = 6±12 μs) suggests that the two emission mechanisms operate within a 4 km region. Daily changes in the apparent rate of giant pulses are caused by propagation effects in the interstellar medium (scintillation) rather than intrinsic variability of the giant-pulse mechanism. The distribution of time separations between giant pulses implies that the mechanism is a Poisson process. We interpret giant-pulse properties in the context of temporal-modulation and beam-waver models. Only the temporal-modulation model is consistent with the data. Empirical measurements place limits on the duration, size, and rate of temporal modulations in the magnetosphere.

Gamma-ray flux increases in the 50-220 keV range are limited to less than a factor of 2.5 concurrent with radio bursts. We discuss how the lack of gamma-ray variation constrains radio coherence mechanisms, the steadiness of electron-positron outflow, and the amount of inverse-Compton scattering of radio photons to gamma rays.