The Crab Nebula and the pulsar at its center are endlessly fascinating. The pulsar is a neutron star, with the same mass as our Sun but only the size of a city. It rotates 30 times per second, flashing like a lighthouse as it does so. It is very nearly, but not quite, an ideal clock, without any outside influence to disturb it. At Jodrell Bank Observatory, astronomers have been watching the pulsar for over 40 years, timing it without missing a beat while it rotated more than 30 billion times. Putting together the results from our radio observations with data from the opposite end of the electromagnetic spectrum has proved remarkably rewarding.
The pulsar has slowed down from 30.2 to 29.7 rotations per second while astronomers have been watching it. This is not unexpected: the energy stored in the rapid rotation powers not only the pulses which we observe but the whole of the Crab Nebula. This little neutron star is acting as a huge electrical generator, spinning at the same speed as a dynamo in a terrestrial power station but with a magnetic field billions of times greater. So far so good, but as is often the case, it is the odd things that seem to be going wrong that we are really interested in.
Astronomers also discovered so-called "glitches", sudden changes in the regular sequence of pulses, showing that the rotation has suddenly sped up, then recovered and started on a new regime of slowing down. We have seen this happen a couple of dozen times. The explanation involves some very strange physics inside the star. Although it is so condensed, the whole of the inside is liquid and only a thin crust is solid. Furthermore, the inside is superfluid, which allows it to rotate independently of the crust. Some times, however, it clutches onto the crust and the whole rotation rate suddenly changes; this is what causes the glitch.
What researchers have found recently is less obvious, having taken the whole of the 40 years to show up. The radio pulse is actually double, like a lighthouse with two beams. These two beams are nearly, but not quite, in opposite directions; how are they formed? Fortunately the Fermi gamma-ray telescope has helped to understand the strange geometry of the atmosphere outside the star, where the beams are formed. The whole of this atmosphere is rotating with the star itself, swept round by the powerful magnetic field. It is called a magnetosphere, and it is forced to move so fast that it reaches relativistic speeds; the radiation appears to originate at a location so far outside the surface that it is moving with half the velocity of light. Now comes some more new physics.
The inside of a neutron star is superconducting as well as superfluid. This means that the strength and shape of the huge dipole magnetic field is fixed, or can change only very slowly. So the pattern of the magnetic field in the magnetosphere, where the radiated beams are formed, is fixed. But the star is less than 1000 years old (it was formed in a supernova explosion in the year 1054), so we have been observing it for an appreciable fraction of its lifetime. And we have indeed found a change in the pattern of the double pulse. The two parts are moving apart, at the rate of 3o in the whole lifetime of the star. What this means is that the dipole is not tidily arranged at a right angle to the rotation, as it would be in a power station dynamo, but it is tilted at around 45o and slowly moving to wards the expected orthogonal arrangement. Now the theorists have to take over and explain how that can happen!