The Crab Pulsar

The adjacent false color image shows the Crab Nebula, which is the remains of a supernova whose light reached the Earth in A.D. 1054. It appeared on approximately July 4 of that year, and was chronicled by Chinese astronomers (see the right panel). The Crab Nebula is expanding. If its present motion is extrapolated backwards, it converges to a point approximately ninety years after A.D. 1054. This implies that the expansion of the Crab has accelerated slightly over its lifetime so that the present expansion rate is somewhat higher than its average expansion rate.
Crabs and Synchrotrons
The red color in the Crab Nebula image indicates regions where electrons are combining with protons to form neutral hydrogen. The green color indicates regions in which electrons are being accelerated in strong magnetic fields in the inner part of the expanding nebula to produce synchrotron radiation. The pronounced filament structure is not well understood. At the center of the nebula, not visible in this image, is the neutron star rotating 30 times a second.

Broadband Electromagnetic Emission of the Crab
The Crab Nebula is a strong source of electromagnetic radiation across the spectrum. The following false-color image illustrates the broad-band (that is, across many wavelength regions) emission of the Crab Nebula.

Rejuvenating the Crab
The powerful emission of the Crab Nebula across many wavelengths and the presence of strong synchrotron radiation in its central regions have several important implications. Since sychrotron radiation requires magnetic fields and energetic charged particles, its presence tells us that the nebula is permeated by magnetic fields and a plentiful supply of high-energy charged particles (primarily electrons). But expansion of the nebula should have weakened any initial magnetic fields well below the strength required to produce synchrotron radiation. Furthermore, at the rate that the Crab is emitting energy, its electrons should have radiated all of their energy within about a hundred years of its formation. But the Crab is still emitting strong synchrotron radiation almost a thousand years after its birth.

The Pulse and the Nebula

The rejuvenation of the Crab Nebula by the pulsar is not coming through the observed "pulse" of the pulsar. The energy in the pulse is hundreds of millions of times too small to account for the power of the nebula. The transfer of energy is instead being generated by the coupling of the pulsar to the surrounding nebula, primarily through the rotating magnetic field.

These observations imply that something must be replenishing both the magnetic field and the supply of high energy electrons in the Crab Nebula. From the observed emission of energy from the Crab, we may estimate that to keep the nebula running requires an input of about five times the energy produced by the Sun each year. The source of this energy, and of the continuing magnetic field, is the rotating neutron star in the center that stores energy like a flywheel and transfers it steadily to the nebula.

Powering the Crab
The preceding discussion implies that the tiny Crab Pulsar, which is not much more than ten kilometers in diameter, powers the enormous energy output of the Crab Nebula, which is ten light years in diameter. To set the relative sizes in perspective, this is as if a one- kilometer-wide volume of space were radiating strongly at many wavelengths and most of the power were being supplied by a single hydrogen atom at the center of that volume!

The Crab Pulsar is gradually slowing its rotation rate by an amount that can be measured very precisely. We may calculate the energy loss associated with the slowing rotation over a period of a year, which turns out to be about five times the amount of energy radiated by the Sun in a year. As we have noted, this is exactly the amount of energy that is required to be added to the nebula each year to maintain its luminosity. Thus, the Crab Pulsar is transferring its energy to the Crab Nebula and slowing its rotation in the process so that energy is conserved.

Coupling of the Pulsar and Nebula

The full mechanism for the coupling of the pulsar to the nebula is not known, but its basics are thought to be as follows. The lighthouse mechanism discussed earlier accelerates charged particles off the magnetic poles of the rapidly spinning pulsar. These particles (primarily electrons) produce the lighthouse beams discussed earlier, but also form a magnetosphere of charged particles that are trapped in the powerful magnetic field and dragged around with the rapid rotation of the neutron star.

At some distance from the neutron star the charged particles being dragged with the rotating magnetic field reach speeds approaching that of light. When that happens the charged particles are flung away from the pulsar in an equatorial "pulsar wind" that moves outward at significant fraction of the velocity of light, taking pieces of the magnetic field with it. At the same time, powerful jets of particles are channeled by the rotating field into emission along the polar axes of the rotation. These jets cause strong X-ray emission as they collide with the surrounding nebula. Thus, the pulsar transfers energetic charged particles to the nebula and replenishes its magnetic field through the equatorial pulsar wind and the polar X-ray jets.