A Crab is a standard astrophotometrical

unit Unit may refer to: General measurement * Unit of measurement, a definite magnitude of a physical quantity, defined and adopted by convention or by law **International System of Units (SI), modern form of the metric system **English units, histo ...

for measurement of the

intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...

Astrophysical X-ray source Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays. Several types of astrophysical objects emit X-rays. They include galaxy clusters, black holes in active galactic nuclei (AG ...

s. One Crab is defined as the intensity of 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 (constellation), Taurus. The common name comes from a drawing that somewhat resembled a crab with arm ...

at the corresponding X-ray

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

. The Crab Nebula, and the

Crab Pulsar The Crab Pulsar (PSR B0531+21 or Baade's Star) 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.

within it, is an intense

space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...

X-ray source. It is used as a

standard candle The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible ...

in the

calibration In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

procedure of X-ray instruments in space. However, because of the Crab Nebula's variable intensity at different X-ray energies, conversion of the Crab to another units depends on the X-ray energy range of interest. In the photon energy range from 2 to 10

keV In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in vacuum. When us ...

, 1 Crab equals 2.4 · 10⁻⁸

erg The erg is a unit of energy equal to 10−7joules (100Nano-, nJ). It is not an SI unit, instead originating from the centimetre–gram–second system of units (CGS). Its name is derived from (), a Greek language, Greek word meaning 'work' or ' ...

cm⁻² s⁻¹ = 15 keV cm⁻² s⁻¹ = 2.4 · 10⁻¹¹ W m⁻². For energies greater than ~30 keV, the Crab Nebula becomes unsuitable for calibration purposes, as its

flux 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 in physics. For transport phe ...

can no longer be characterized by a single coherent model. The unit ''mCrab'', or ''

milli ''Milli'' (symbol m) is a unit prefix in the metric system denoting a factor of one thousandth (10−3). Proposed in 1793, and adopted in 1795, the prefix comes from the Latin , meaning ''one thousand'' (the Latin plural is ). Since 1960, the pre ...

Crab'', is sometimes used instead of the Crab.

References

{{reflist, 2 Crab Nebula Nebulae Units of measurement in astronomy