soft X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 ...
emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
, equivalent to ROSAT counts .The Local Leo Cold Cloud and New Limits on a Local Hot Bubble /ref> 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.