Cool Stars 14 - Submitted Abstract # 168 This version created on 05 October 2006 X-ray variability of young PMS stars in Cygnus OB2 Juan Facundo Albacete Colombo, INAF-OAPA Marilena Caramazza, INAF-OAPA Giusi Micela, INAF-OAPA Ettore Flaccomio, INAF-OAPA Salvatore Sciortino, INAF-OAPA We use a 100 ksec CHANDRA ACIS-I observation to statistically characterize the X-ray variability of low-mass stars in the Cygnus OB2 star forming region. X-ray variability is first tested using the Kolmogorov-Smirnov test, obtaining that 13.4% (135/1003) of the sources are variable with confidence level greater than 95%. We then employ Maximum Likelihood Blocks (Wolk et al. 2005) in order to segment the X-ray lightcurve in intervals of 'constant' emission to find out flares taking into account the amplitude and the time-derivative of the source count-rate. We find that most of the flaring sources, with few exceptions, have hard spectra (Med. Energy>2 keV). We derive the energy distribution of detected flares and compare our result with previous estimates obtained for members of the Orion Nebula Cluster (Wolk et al. 2005) and of the Taurus Molecular Cloud (Stelzer et al. 2006). ----------------------------------