2005A&A...444...69C

The search for Blank Field Sources (BFS), i.e. X-ray sources without optical counterparts, paves the way to identification of unusual objects in the X-ray sky. Here we present four BFS detected in the Brera Multiscale Wavelet catalog of ROSAT HRI observations. This sample was selected on the basis of source brightness, distance from possible counterparts at other wavelengths, point-like shape and a good estimate of the X-ray flux (f_X). The observed f_X and the limiting magnitude of the optical catalogs fix a lower limit for our BFS on f_X/f_opt∼40. This value puts them well beyond the 90% threshold for usual source classes once the HRI energy band and proper spectral shape are taken into account, leaving room for speculation on their nature. Three BFS also show evidence of transient behaviour.