Characterizing X-ray variability in the CygnusOB2 region

ALBACETE-COLOMBO J.F., CARAMAZZA M., FLACCOMIO E., MICELA G. & SCIORTINO S

Pasadena, California. EEUU

Workshop; The 14th Cool Stars, Stellar Systems and the Sun workshop; 2006

NASA & ESA

We use a ~100 ksec CHANDRA ACIS-I observation to characterize the X-ray variability of low-mass stars of the Cygnus OB2 region. The detection and analysis of variability is based on the maximum likelihood block (MLB) algorithm. We find that 14.4 % (143/1003) of the sources are variable. The flare source population is biased by the sensitivity limit (log(Lx)~30 erg/s) and the relatively “short” exposure-time of the observation. In order to compare our results with those for the Orion Nebula region, we perform a statistical study of the flare activity of the COUP sources for 5 different 100 ksec segments. We use the MLB algorithm with the same parameters used for the CygOB2 region: in total, the five 100 ksec COUP segments contain 1293 flares. The variability analysis was also performed for the whole 850 ksec COUP observation. In its differential form, the distribution of flare energies in the CygOB2 and the 5x100 ksec COUP sources, follow for high energies, a power-law with index a=2.0±0.1, in agreement with that obtained for the whole 850 ksec COUP data (a=2.1±0.1). We find that the flare frequency in CygOB2 seems to be 4 times higher than observed in the COUP data. However, if we limit COUP data to PMS stars with masses ≥ 1Mo, the two frequencies are similar.