Crab Pulsar

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**Crab Pulsar**
The Crab Pulsar. Image combines optical data from Hubble (in red) and X-ray images from Chandra (in blue).

The Crab Pulsar (PSR B0531+21) is a relatively young neutron star located in the Crab Nebula, discovered in 1969. The 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.^[1]

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. Very few X-ray sources ever exceed one Crab in brightness.

Recent observations have suggested that the Crab Pulsar may have an unusually complex magnetic field with four poles instead of the usual two, possibly resulting from the progenitor star imploding in an asymmetric manner when the pulsar was first formed. The same set of observations suggested that since the primary radio pulse coming from the pulsar lasts only 0.4 nanoseconds it is being emitted from a cloud of plasma on the surface of the neutron star only 12 centimeters across - the smallest feature ever to be observed in astronomy.^[2]