X-rays and ultraviolets, is "Sirius B" an exoplanet and the "invisible companion" a 19th century myth?

The x-ray image of "Sirius B" is comparing the emissions from "Sirius B" with Sirius, although the radiation from Sirius is in ultraviolet leaking through the filter on the detector (source: http://chandra.harvard.edu/photo/2000/0065/). Why is UV being picked up as x-rays, is that a result of particular settings used during the photography?