The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. Discovered in 1968, the pulsar was the first to be connected with a supernova remnant.

The optical pulsar is roughly 25 km in diameter and the pulsar "beams" rotate once every 33 milliseconds, or 30 times each second. The outflowing relativistic wind from the neutron star generates synchrotron emission, which produces the bulk of the emission from the nebula, seen from radio waves through to gamma rays. The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the surrounding nebula, forming a termination shock. The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar into the main body of the nebula. The period of the pulsar's rotation is slowing by 38 nanoseconds per day due to the large amounts of energy carried away in the pulsar wind.

The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar itself. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'crab' and 'millicrab' are sometimes used as units of flux density. A millicrab corresponds to a flux density of about 2.4x10^-11 erg s^-1 cm⁻² (2.4x10^-14 W m⁻²) in the 2–10 keV X-ray band, for a "crab-like" X-ray spectrum, which is roughly a powerlaw in photon energy, I(E)=9.5 E^-1.1. Very few X-ray sources ever exceed one crab in brightness.

History

The modern history of the Crab Pulsar begins with the identification of the central star of the nebula in optical light. Focus was made on two stars near the center of the nebula (referred to in the literature as the "north following" and "south preceding" stars). In September 1942, Walter Baade rules out the north following star but finds the evidence inconclusive for the south preceding. Rudolf Minkowski, in the same issue of Astrophysical Journal as Baade, advances spectral arguments claiming the "evidence admits, but does not prove, the conclusion that the south preceding star is the central star of the nebula".

In late 1968, David H. Staelin and Edward C. Reifenstein III reported the discovery of two pulsating radio sources "near the crab nebula that could be coincident with it" using the 300-foot Green Bank radio antenna. They were given the designations NP 0527 and NP 0532. A subsequent study by them including William D. Brundate found that the NP 0532 source is located at the Crab Nebula. A radio source was also reported coincident with the crab nebula in late 1968 by L. I. Matveenko in Soviet Astronomy.

Optical pulsations were reported by Nather, Warner, and Macfarlane in February 1969.