A unit of X-ray intensity used in the early days of X-ray astronomy (late 20th century), based on the brightness of the Crab Nebula (= 1 crab). Since few astronomical X-ray sources are as bright as the Crab Nebula, the unit is usually encountered as the millicrab, often spelled milliCrab.

The use of the millicrab has been discouraged because it depends upon the X-ray spectrum of the source, assuming that it matches that of the Crab Nebula (a band pass from 2 to 11 kiloelectron-volts). Measurements in millicrabs of two sources are directly comparable only if the sources’ spectrums match. With this restriction, 1 Crab is equivalent to 1060 microJanskys. In the 2 to 10 keV band, 1 millicrab is 2.4 × 10⁻¹¹ erg per second per square centimeter (2.4 × 10⁻¹⁴ watts per square meter).

graph of Crab X-ray brightness

The various colors represent different detectors.

Courtesy Goddard Space Flight Center

In 2011, a study¹ that made use of a number of X-ray detectors determined that the X-ray output of the Crab Nebula, in several bands, had been declining since at least 2007. In the case of the 15 to 50 keV band, the decline between 2008 and 2010 amounted to about 7%, which rules out the use of the Crab Nebula as a standard.


The Goddard Space Flight Center has a video at

1. Colleen A. Wilson-Hodge, Michael L. Cherry, Gary L. Case, Wayne H. Baumgartner, Elif Beklen, P. Narayana Bhat, Michael S. Briggs, Ascension Camero-Arranz, Vandiver Chaplin, Valerie Connaughton, Mark H. Finger, Neil Gehrels, Jochen Greiner, Keith Jahoda, Peter Jenke, R. Marc Kippen, Chryssa Kouveliotou, Hans A. Krimm, Erik Kuulkers, Niels Lund, Charles A. Meegan, Lorenzo Natalucci, William S. Paciesas, Robert Preece, James C. Rodi, Nikolai Shaposhnikov, Gerald K. Skinner, Doug Swartz, Andreas von Kienlin, Roland Diehl and Xiao-Ling Zhang.
When a Standard Candle Flickers.
The Astrophysical Journal Letters, vol. 727 L40 (12 January 2011).
doi: 10.1088/2041-8205/727/2/L40


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