The Crab pulsar was discovered by the occasional exceptionally bright radio pulses it emits, subsequently dubbed 'giant' pulses. Only two other pulsars are known to emit giant pulses. There is no satisfactory explanation for the occurrence of giant pulses, nor is there a complete theory of the pulsar emission mechanism in general. Competing models for the radio emission mechanism can be distinguished by the temporal structure of their coherent emission. Here we report the discovery of isolated, highly polarized, two-nanosecond subpulses within the giant radio pulses from the Crab pulsar. The plasma structures responsible for these emissions must be smaller than one metre in size, making them by far the smallest objects ever detected and resolved outside the Solar System, and the brightest transient radio sources in the sky. Only one of the current models--the collapse of plasma-turbulent wave packets in the pulsar magnetosphere--can account for the nanopulses we observe.

Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrowband nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The bands cannot easily be explained by any current theory of pulsar radio emission; we speculate on possible new models.

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We analyze the Crab pulsar at ten frequencies from 0.43 to 8.8 GHz using data obtained at the Arecibo Observatory and report the spectral dependence of all pulse components and the rate of occurrence of large-amplitude 'giant' pulses. Giant pulses occur only in the main-and-interpulse components that are manifest from radio frequencies to gamma-ray energies (known as the 'P1' and 'P2' components in the high-energy literature). Individual giant pulses reach brightness temperatures of at least 10 32 K in our data, which do not resolve the narrowest pulses, and are known to reach 10 37 K in nanosecond-resolution observations (Hankins et al. 2003). The Crab pulsar's pulses are therefore the brightest known in the observable universe. As such, they represent an important milestone for theories of the pulsar emission mechanism to explain. In addition, their short durations allow them to serve as especially sensitive probes of the Crab Nebula and the interstellar medium. We identify and quantify frequency structure in individual giant pulses using a scintillating, amplitude-modulated, polarized shot-noise model (SAMPSN). The frequency structure associated with multipath propagation decorrelates on a time scale ∼ 25 sec at 1.5 GHz. To produce this time scale requires multipath propagation to be strongly influenced by material within the Crab Nebula. We also show that some frequency structure decorrelates rapidly, on time scales less than one spin period, as would be expected from the shot-noise pattern of nanosecond duration pulses emitted by the pulsar. We discuss the detectability of individual giant pulses as a function of frequency and provenance. Taking into account the Crab pulsar's locality inside a bright supernova remnant, we conclude that the brightest pulse in a typical 1-hour observation would be most easily detectable in our lowest frequency band (0.43 GHz) to a distance ∼ 1.6 Mpc at 5σ. We also discuss the detection of such pulses using future instruments such as LOFAR and the SKA.

Previously unseen profile components of the Crab pulsar have been discovered in a study of the frequency-dependent behavior of its average pulse profile between 0.33 and 8.4 GHz. One new component, 36 degrees ahead of the main pulse at 1.4 GHz, is not coincident with the position of the precursor at lower frequencies. Two additional, flat-spectrum components appear after the interpulse between 1.4 and 8.4 GHz. The normal interpulse undergoes a transition in phase and spectrum by disappearing near 2.7 GHz, and reappearing 10 degrees earlier in phase at 4.8 and 8.4 GHz with a new spectral index. The radio frequency main disappears for frequencies above 4.8 GHz, even though it is seen at infrared, optical, and higher energies. The existence of the additional components at high frequency and the strange, frequency-dependent behavior is unlike anything seen in other pulsars, and cannot easily be explained by emission from a simple dipole field geometry.Comment: 13 pages. Source is single LaTeX file with 3 figures, using aaspp and epsf style files (included). To appear in The Astrophysical Journal, September 1996. Paper can also be found at http://www.ee.nmt.edu