Crab Pulsar

Lore & Background

The Crab Nebula was identified as the remnant of SN 1054 by 1939, leading astronomers to search for its central star. In September 1942, Walter Baade ruled out one candidate but found evidence inconclusive for another; Rudolf Minkowski advanced spectral arguments that admitted but did not prove the conclusion that the south preceding star was the central star. In late 1968, David H. Staelin and Edward C. Reifenstein III reported two rapidly varying radio sources near the nebula, and 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 such a short period proved that pulsars are rotating neutron stars, not pulsating white dwarfs. Soon after, David Richards discovered that the pulsar spins down and loses rotational energy; Thomas Gold showed that this spin-down power is sufficient to power the Crab Nebula. Optical pulsations were first reported by Cocke, Disney and Taylor using the 36-inch telescope on Kitt Peak. Jocelyn Bell Burnell relates that in the late 1950s a woman viewing the Crab Nebula source at the University of Chicago's telescope noted it appeared to be flashing, but the astronomer disregarded it as scintillation. In 2007, it was reported that Charles Schisler detected a celestial source of radio emission in 1967 at the location of the Crab Nebula using a US Air Force radar system, later understood to be the Crab Pulsar, but not reported publicly for four decades due to classification.

Reader's Guide

The Crab Pulsar holds immense significance in astronomy as the first pulsar connected to a supernova remnant, confirming that pulsars are rotating neutron stars. Its precise 33-millisecond period and measurable spin-down provide a natural laboratory for studying neutron star physics, relativistic winds, and particle acceleration. The pulsar's outflowing relativistic wind generates synchrotron emission that powers the Crab Nebula across the electromagnetic spectrum, from radio to gamma rays. The Crab Nebula is frequently used as a calibration source in X-ray astronomy due to its brightness and constant flux density, with 'crab' and 'millicrab' used as units of flux density. The pulsar's pulsed emission has been detected up to 1.5 TeV, and the nebula was observed in 2019 to emit gamma rays in excess of 100 TeV, making it the first identified source of ultra-high-energy cosmic rays. The Crab Pulsar was also the first pulsar for which the spin-down limit was broken using LIGO data, proving that not all rotational energy loss is converted to gravitational waves. Its legacy includes advancing gravitational wave astronomy and providing a key calibration source for X-ray detectors.

Did You Know?

• The Crab Pulsar was the first pulsar to be connected with a supernova remnant.
• Its rotational period is 33.392 milliseconds, corresponding to 29.946 revolutions per second.
• The pulsar's spin-down rate is 38 nanoseconds per day due to energy carried away by the pulsar wind.
• In X-ray astronomy, 'crab' and 'millicrab' are used as units of flux density, with a millicrab corresponding to about 2.4×10−11 erg s−1 cm−2 in the 2–10 keV band.