soft X-ray
X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...
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.