The Crab Pulsar (PSR B0531+21) is a relatively young
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...
. The star is the central star in the
Crab Nebula
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. The common name comes from William Parsons, 3rd Earl of Rosse, who observed the object in 1842 u ...
, a
remnant
Remnant or remnants may refer to:
Religion
* Remnant (Bible), a recurring theme in the Bible
* Remnant (Seventh-day Adventist belief), the remnant theme in the Seventh-day Adventist Church
* ''The Remnant'' (newspaper), a traditional Catholic n ...
of the
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
SN 1054
SN 1054 is a supernova that was first observed on 1054, and remained visible until 1056.
The event was recorded in contemporary Chinese astronomy, and references to it are also found in a later (13th-century) Japanese document, and in a docu ...
, which was widely observed on Earth in the year 1054.
[Supernova 1054 – Creation of the Crab Nebula]
Discovered in 1968, the
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
was the first to be connected with a
supernova remnant
A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
.
![267641main_allsky_labeled_HI](https://upload.wikimedia.org/wikipedia/commons/e/e7/267641main_allsky_labeled_HI.jpg)
The Crab Pulsar is one of very few pulsars to be identified optically. The
optical pulsar
An optical pulsar is a pulsar which can be detected in the visible spectrum. There are very few of these known: the Crab pulsar, Crab Pulsar was detected by Stroboscopic effect, stroboscopic techniques in 1969, shortly after its discovery in radio ...
is roughly in diameter and has a rotational period of about 33 milliseconds, that is, the pulsar "beams" perform about 30 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
Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
through to
gamma rays
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
. 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 heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
. 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
nanosecond
A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds.
The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...
s per day due to the large amounts of energy carried away in the pulsar wind.
The
Crab Nebula
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. The common name comes from William Parsons, 3rd Earl of Rosse, who observed the object in 1842 u ...
is often used as a calibration source in
X-ray astronomy. It is very bright in
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s, and the
flux density
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
and
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
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 () 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.
History of observation
![Crab Nebula pulsar x-ray](https://upload.wikimedia.org/wikipedia/commons/a/af/Crab_Nebula_pulsar_x-ray.jpg)
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
''The Astrophysical Journal'', often abbreviated ''ApJ'' (pronounced "ap jay") in references and speech, is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and J ...
'' 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 pulsating radio sources "near the crab nebula that could be coincident with it" using the
Green Bank radio antenna. They were given the designations NP 0527 and NP 0532. The period and location of the Crab Nebula pulsar NP 0532 was discovered by
Richard V. E. Lovelace
Richard Van Evera Lovelace is an American astrophysicist and plasma physicist. He is best known for the discovery of the period of the pulsar in the Crab Nebula (Crab pulsar), which helped to prove that pulsars are rotating neutron stars, for d ...
and collaborators on November 10, 1968, at the
Arecibo radio observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science ...
.
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
''Astronomy Reports'' (Russian: ''Астрономический журнал'', ''Astronomicheskii Zhurnal''), is a Russian, monthly, peer reviewed, scientific journal. This journal tends to focus its publishing efforts on original research regard ...
''.
Optical pulsations were first reported by Cocke, Disney, and Taylor using the telescope on Kitt Peak of the Steward Observatory of the University of Arizona. Their discovery was confirmed by Nather, Warner, and Macfarlane.
Jocelyn Bell Burnell
Dame Susan Jocelyn Bell Burnell (; Bell; born 15 July 1943) is an astrophysicist from Northern Ireland who, as a postgraduate student, discovered the first radio pulsars in 1967. The discovery eventually earned the Nobel Prize in Physics in ...
, who co-discovered the first pulsar
PSR B1919+21
PSR B1919+21 is a pulsar with a period of 1.3373 seconds and a pulse width of 0.04 seconds. Discovered by Jocelyn Bell Burnell on 28 November 1967, it is the first discovered radio pulsar. The power and regularity of the signals wer ...
in 1967, relates that in the late 1950s a woman viewed the Crab Nebula source at the University of Chicago's telescope, then open to the public, and noted that it appeared to be flashing. The astronomer she spoke to, Elliot Moore, disregarded the effect as
scintillation, despite the woman's protestation that as a qualified pilot she understood scintillation and this was something else. Bell Burnell notes that the 30 Hz frequency of the Crab Nebula optical pulsar is difficult for many people to see.
["Beautiful Minds: Jocelyn Bell Burnell", BBC television documentary broadcast 7 April 2010.]
The Crab Pulsar was the first pulsar for which the ''spin-down limit'' was broken using several months of data of the
LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large ...
observatory. Most pulsars do not rotate at constant rotation frequency, but can be observed to slow down at a very slow rate (3.7 Hz/s in case of the Crab). This spin-down can be explained as a loss of rotation energy due to various mechanisms. The spin-down limit is a theoretical upper limit of the amplitude of
gravitational waves
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that Wave propagation, propagate as waves outward from their source at the speed of light. They were first proposed by Oliv ...
that a pulsar can emit, assuming that all the losses in energy are converted to gravitational waves. No gravitational waves observed at the expected amplitude and frequency (after correcting for the expected
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
) proves that other mechanisms must be responsible for the loss in energy. The non-observation so far is not totally unexpected, since physical models of the rotational symmetry of pulsars puts a more realistic upper limit on the amplitude of gravitational waves several orders of magnitude below the spin-down limit. It is hoped that with the improvement of the sensitivity of gravitational wave instruments and the use of longer stretches of data, gravitational waves emitted by pulsars will be observed in future. The only other pulsar for which the spin-down limit was broken so far is the
Vela Pulsar
The Vela Pulsar (PSR J0835-4510 or PSR B0833-45) is a radio, optical, X-ray- and gamma-emitting pulsar associated with the Vela Supernova Remnant in the constellation of Vela. Its parent Type II supernova exploded approximately 11,000 ...
.
In 2019 the Crab Nebula, and presumably therefore the Crab Pulsar, was observed to emit gamma rays in excess of 100 TeV, making it the first identified source of
ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray parti ...
s.
References
{{DEFAULTSORT:Crab Pulsar
Crab Nebula
Tauri, CM
Optical pulsars
Taurus (constellation)