A correlation between optical and giant radio pulse emission from the Crab pulsar was detected. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density. 

Despite more than 30 years of observation, the emission mechanism of pulsars is still a matter of debate. A broad consensus does exist: that the luminosity is powered by the rotation of the pulsar, that the pulsed radio signal comes from a coherent process, and that the optical–to–x-ray emission is incoherent synchrotron radiation, whereas the γ-ray emission is curvature radiation. What is not agreed on is the mechanism that accelerates the electrons to the energy required for synchrotron and curvature radiation, where this acceleration takes place, how coherency is maintained, and the stability of the electron-positron plasma outflow from the neutron star's surface. From the radiopulse profile at 1380 MHz and the optical profile for the Crab pulsar, two primary features can be identified: a main pulse and an interpulse. At lower energies, a radio precursor can be seen, and at higher energies in the optical, x-ray, and γ-ray regions, bridge emission can be seen between the main pulse and the interpulse. One suggestion is that the precursor represents emission from the pulsar polar cap region near the neutron star surface, similar to the radio emission from most pulsars, and that the other features come from higher in the magnetosphere. This picture is made more complex by the existence of giant radio pulses (GRPs) that occur at random intervals, in phase with either the main pulse or the interpulse, and that have energies about 1000 times as high as the mean energy. In the optical and infrared energy regions, the pulse profile is constant at the 1% level.

Any observed variation in the emitted flux, pulse morphology, or phase relations at higher energies coincident with a GRP would provide explicit constraints on pulsar (coherent/incoherent) emission physics and geometry. To investigate whether there is a link between the radio and optical emission from the Crab pulsar, simultaneous observations were carried out with the Westerbork Synthesis Radio Telescope (WSRT) and with the Transputer Instrument for Fast Image Detection (TRIFFID) optical photometer mounted on the William Herschel Telescope.

The Crab pulse profile showing the optical light curve (o), the average radio light curve at 1380 MHz (r), and a single giant pulse at 1357.5 MHz (gr). τ, time. Two periods are shown for clarity. Various pulse parameters have been identified. Also shown is the location of the precursor observed at lower frequencies and the bridge emission seen particularly at higher frequencies. On this scale, the GRP width corresponds to 0.00035 units of phase (12 µs), the radio pulse to 0.009(300 µs), and the optical pulse to 0.045 (1500 µs). The avalanche photodiode (APD) band pass for these observations was from 6000 to 7500 Å. Phase 0 corresponds to the arrival at the solar system barycenter of the peak radio pulse. The optical light curve for this plot was divided into 5000 phase bins. [ JPEG | TIFF ]

A total of 10,034 optical data sets of 41 periods each were collected. An average pulse profile was formed by folding the optical photons at the Crab's period and then averaging over all data sets (but not including the period associated with a GRP). By comparing the pulse profile formed by averaging only the optical pulses coincident with a GRP astronomers found that the giant optical pulses are on average 3% brighter than normal optical pulses. They also analyzed other pulse parameters: arrival time, pulse shape, and interpulse height. None of these parameters showed any statistically significant variation with the presence of a GRP.

The fact that only the optical pulse, which is coincident with a GRP, shows enhanced intensity suggests that the coherent (radio) and incoherent (optical) emissions produced in the Crab pulsar's magnetosphere are linked. A consistent explanation is that the optical emission is a reflection of increased plasma density that causes the GRP event. Whatever triggers the GRP phenomenon, it releases energy uniformly throughout most of the electromagnetic spectrum, as implied by the similar energies of radio and enhanced optical pulses. Changes in the pair production rate at the level of a few percent could explain the optical variations. However, an additional mechanism would be needed to account for the radio GRPs, which are orders of magnitude stronger than the average pulse level. It has been suggested that this could be achieved by local density enhancements to the plasma stream, which increase the coherent emission (n²) with little effect on the (high-energy) incoherent radiation ( n). These changes must occur on tiny time scales (<10 µs) to explain the observed change in optical flux. Whatever the mechanism, these observations demonstrate a clear link at the individual pulse level between the coherent and incoherent emission regimes in the Crab pulsar.

• Shearer, A., et al., 2003, "Enhanced Optical Emission During Crab Giant Radio Pulses", Science, 301, 493.