Fwd: Re: Fwd: The December 14 Gamma Ray Burst

[Andrew Karam <karam.1@osu.edu>]

This is the response from a friend of mine who is an astronomer at Ohio
State University.  I hope it helps.

Andy

>Hi Andy,
>
>   1 Crab = the flux from the Crab Nebula supernova remnant in the band
>of interest.
>
>   Goes back to the sounding rocket X-ray telescopes in the early 1970s.
>The Crab is the brightest non-variable X-ray source in the sky.  The actual
>brightest is Scorpius X-1, but it is variable.  Of course, within the Crab
>Nebula is a very rapidly variable source, the Crab pulsar, but the original
>observations integrate over those pulses and measure an average flux.  This
>made it the de-facto flux calibration reference: you could always see it
>and knew it was reasonably constant.  It has become the quasi-official unit
>of X-ray flux.  1 Crab of flux is real bright.
>
>   This is a hateful unit to those of us who think of fluxes in units of
>energy/sec/area/frequency-interval (or wavelength or photon energy
>interval).  You see proposals and papers that quote fluxes in milli-crabs.
>To convert, you have to know at what energy band, and then look up the
>published spectrum of the Crab in some rational X-ray flux units.  For
>example, at 10 KeV, the flux from the Crab is about 0.9 keV/s/cm^2/keV
>


The opinions expressed above are well-reasoned and insightful.  Needless to
say, they are not those of my employer.(with apologies to Michael Feldman)


Andrew Karam, CHP				"The mind is not a vessel
karam.1@osu.edu				to be filled but a fire
						to be lighted." (Plutarch)
Ohio State University 
Radiation Safety Section
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Columbus, OH  43221
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