A Snowden is a unit of soft X-ray emissivity, equivalent to 10 − 6 {\displaystyle 10^{-6}} ROSAT counts s − 1 a r c m i n − 2 {\displaystyle s^{-1}arcmin^{-2}} . These units were chosen by S. L. Snowden when working with the ROSAT mission to create the ROSAT All-Sky Survey in order to make the surface brightness values fit into a two byte integer.
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