In section 1. Letters
A non-glitch speed-up event in the Crab Pulsar
The Crab pulsar is one of the first radio pulsars ever detected, and the first one to be clearly linked to a supernova remnant. The neutron star spins at 33 ms and its rotation can be monitored both with radio telescopes and X-ray instruments, as the Crab is also a strong X-ray pulsar. The Crab pulsar is a precise clock. It is monitored monthly by radio telescope, and often by several X-ray instruments in orbit. Its spin period is known to an accuracy about 10 significant digits. As any other radio pulsar, the Crab pulsar slowly spins down due to rotational energy losses, which power the observed emission at all wavelengths. Sometimes it also shows glitches - sudden (less than a minute) increases of the spin frequency caused by some internal processes within the pulsar. In this Letter the discovery of a new type of speed-up event is reported. It is not impulsive, like a glitch, but it lasted for over two years. It is not easy to explain this new type of event. One possibility suggested by the author is in terms of an extra source of heat generated within the neutron star, resulting in a slowing down of the superfluid interior and speeding up the outer crust.