Crab (unit)

A Crab is a standard astrophotometrical unit for measurement of the intensity of Astrophysical X-ray sources.^[1][2][3][4] 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 to 2,4 · 10⁻⁸ erg cm⁻² s⁻¹ = 15 keV cm⁻² s⁻¹ = 2,4 · 10⁻¹¹ W m⁻²

The unit mCrab, or millicrab, is sometimes used instead of the Crab.

References

1. ^ M. Kirsch et al. (2005). "Crab: the standard X-ray candle with all (modern) X-ray satellites". Proc. SPIE, 5898 (2005) 22-33. http://xmm.vilspa.esa.es/docs/documents/CAL-TN-0061-1-0.pdf. Retrieved 2011-01-21. "Various X-ray satellites have used the Crab as a standard candle to perform their calibrations in the past" 
2. ^ "The Crab Nebula: standard candle no more?". European Space Agency. 2011-01-14. http://www.sci.esa.int/science-e/www/object/index.cfm?fobjectid=48124. Retrieved 2011-01-21. "These characteristics led astronomers to regard the Crab Nebula as a standard candle, up to the point that fluxes in high-energy astrophysics are customarily measured in units of the Crab's flux." 
3. ^ "The Crab Nebula Flickers!". Sky & Telescope. 1989-10-20. http://www.skyandtelescope.com/news/home/114295114.html. Retrieved 2011-01-21. "It’s routinely used for instrument calibration and even as a unit of measure, the “millicrab.” An unexpectedly varying Crab Nebula might have affected any research that used the Crab as a standard or a calibration tool." 
4. ^ "Specification of Physical Units within OGIP FITS files". NASA. 1995-05-04. http://heasarc.nasa.gov/docs/heasarc/ofwg/docs/general/ogip_93_001/ogip_93_001.html. Retrieved 2011-01-22. "the 'crab' was commonly used in the early days of X-ray astronomy"