  We used the Aladin Virtual Obsrvatory tool and High Resolution Imager ROSAT
archival data to search for X-ray variability in scale of days in 23 young
stars in the sigma Orionis cluster and a background galaxy. Five stars
displayed unambiguous flares and had probabilities p_var >> 99% of being actual
variables. Two of the detected flares were violent and long-lasting, with
maximum duration of six days and amplitude of eight times above the quiescent
level. We classified another four stars as possible X-ray variables, including
the binary system formed by the B2Vp star sigma Ori E and its close late-type
companion. This makes a minimum frequency of high-amplitude X-ray variability
in excess of a day of 39% among sigma Orionis stars. The incidence of this kind
of X-ray variability seems to be lower among classical T Tauri stars with
mid-infrared flux excesses than among fast-rotating, disk-less young stars.
