Crab Pulsar

Lore & Background

The Crab Pulsar is one of very few pulsars to be identified optically. It was initially discovered as a radio pulsar. The optical pulsar is roughly 20 km in diameter and has a rotational period of 33.392 milliseconds, that is, the pulsar 'beams' perform 29.946 revolutions per 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 increasing 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 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 () in the 2–10 keV X-ray band, for a 'crab-like' X-ray spectrum, which is roughly power-law in photon energy: I ~ E−1.1. Very few X-ray sources ever exceed one crab in brightness. Pulsed emission up to 1.5 TeV has been detected from the Crab Pulsar. The only other known pulsar with emission in this energy range is the Vela Pulsar at 20 TeV.

Reader's Guide

The Crab Nebula was identified as the remnant of SN 1054 by 1939. Astronomers then searched for the nebula's central star. There were two candidates, referred to in the literature as the 'north following' and 'south preceding' stars. In September 1942, Walter Baade ruled out the 'north following' star but found the evidence inconclusive for the 'south preceding' star. Rudolf Minkowski, in the same issue of The Astrophysical Journal as Baade, advanced spectral arguments claiming that 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 rapidly varying radio sources 'near the crab nebula that could be coincident with it' using the 300 ft Green Bank radio antenna. They were given the designations NP 0527 and NP 0532. The period of 33 milliseconds and location of the Crab Nebula pulsar NP 0532 was discovered by Richard V. E. Lovelace and collaborators on 10 November 1968, at the Arecibo Radio Observatory. The discovery of the pulsar with such a short period proved that pulsars are rotating neutron stars (not pulsating white dwarfs, as many scientists suggested). Soon after the discovery of the Crab Pulsar, David Richards discovered (using the Arecibo Telescope) that it spins down and, therefore, loses its rotational energy. Thomas Gold has shown that the pulsar's spin-down power is sufficient to power the Crab Nebula. A subsequent study by them, including William D. Brundage, also 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 first reported by Cocke, Disney and Taylor using the 36 in telescope on Kitt Peak of the Steward Observatory of the University of Arizona. This observation had an audio tape recording the pulses and this tape also recorded the voices of John Cocke, Michael Disney and Bob McCallister (the night assistant) at the time of the discovery. Their discovery was confirmed by Nather, Warner and Macfarlane. Jocelyn Bell Burnell, who co-discovered the first pulsar PSR B1919+21 in 1967, under the supervision of Antony Hewish, relates that in the late 1950s a woman viewed the Crab Nebula source at the University of Chicago's telescope, then o

Did You Know?

• The Crab Pulsar was the first pulsar to be connected with a supernova remnant.
• The Crab Pulsar has a rotational period of 33.392 milliseconds, performing 29.946 revolutions per second.
• The Crab Pulsar was the first pulsar for which the spin-down limit was broken using LIGO data.
• In 2019 the Crab Nebula was observed to emit gamma rays in excess of 100 TeV, making it the first identified source of ultra-high-energy cosmic rays.