Crab (unit)
Encyclopedia
A Crab is a standard astrophotometrical unit
for measurement of the intensity of Astrophysical X-ray source
s. One Crab is defined as the intensity of the Crab Nebula at the corresponding X-ray energy.
The name is derived from the Crab Nebula
. The Crab nebula, and the Crab Pulsar
within it, is an intensive space X-ray source. It is used as a standard candle in the calibration procedure of X-ray instruments in space.
Because the intensity of the Crab Nebula is various at various X-ray energies
, conversion of the Crab to another units depends on the X-ray photon
energy.
In the photon energy range from 2 to 10 keV, 1 Crab equals 2.4 · 10−8 erg 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 can no longer be characterized by a single coherent model .
The unit mCrab, or milli
Crab, is sometimes used instead of the Crab.
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...
for measurement of the intensity of Astrophysical X-ray source
Astrophysical X-ray source
Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.There are a number of types of astrophysical objects which emit X-rays, from galaxy clusters, through black holes in active galactic nuclei to galactic objects such as supernova...
s. One Crab is defined as the intensity of the Crab Nebula at the corresponding X-ray energy.
The name is derived from the Crab Nebula
Crab Nebula
The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...
. The Crab nebula, and the Crab Pulsar
Crab Pulsar
The Crab Pulsar 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 intensive space X-ray source. It is used as a standard candle in the calibration procedure of X-ray instruments in space.
Because the intensity of the Crab Nebula is various at various X-ray energies
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, conversion of the Crab to another units depends on the X-ray photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
energy.
In the photon energy range from 2 to 10 keV, 1 Crab equals 2.4 · 10−8 erg cm−2 s−1 = 15 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
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 can no longer be characterized by a single coherent model .
The unit mCrab, or milli
Milli
Milli is a prefix in the metric system denoting a factor of one thousandth . Adopted in 1795, the prefix comes from the Latin mille, meaning one thousand ....
Crab, is sometimes used instead of the Crab.