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The search for Blank Field Sources (BFS), i.e. X-ray sources without opticalcounterparts, paves the way to the identification of unusual objects in theX-ray sky. Here we present four BFS detected in the Brera Multiscale Waveletcatalog of ROSAT HRI observations. This sample has been selected on the basisof source brightness, distance from possible counterparts at other wavelengths,point-like shape and good estimate of the X-ray flux (f_X). The observed f_Xand the limiting magnitude of the optical catalogs fix a lower limit for ourBFS on f_X/f_opt~40. This value puts them well beyond 90% threshold for usualsource classes once HRI energy band and proper spectral shape are taken intoaccount, leaving room for speculation on their nature. Three BFS show alsoevidence of a transient behaviour.Comment: 9 pages, 5 figures, accepted for publication on A&

Blank field sources in the ROSAT HRI Brera multiscale wavelet catalog