X-ray flaring from the young stars in Cygnus OB2 ^*

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: facundo@astropa.unipa.it
² Centro Universitario Regional Zona Atlantica (CURZA) – Univ. nacional del COMAHUE, Monsenor Esandi y Ayacucho (8500), Viedma (Rio Negro), Argentina

Received: 12 June 2007
Accepted: 2 August 2007

Abstract

Aims.We characterize individual and ensemble properties of X-ray flares from stars in the Cygnus OB2 and ONC star-forming regions.

Methods.We analyzed X-ray lightcurves of 1003 Cygnus OB2 sources observed with Chandra for 100 ks and of 1616 ONC sources detected in the “Chandra Orion Ultra-deep Project” 850 ks observation. We employed a binning-free maximum likelihood method to segment the light-curves into intervals of constants signal and identified flares on the basis of both the amplitude and the time-derivative of the source luminosity. We then derived and compared the flare frequency and energy distribution of Cygnus OB2 and ONC sources. The effect of the length of the observation on these results was investigated by repeating the statistical analysis on five 100 ks-long segments extracted from the ONC data.

Results.We detected 147 and 954 flares from the Cygnus OB2 and ONC sources, respectively. The flares in Cygnus OB2 have decay times ranging from 0.5 to about 10 h. The flare energy distributions of all considered flare samples are described at high energies well by a power law with index . At low energies, the distributions flatten, probably because of detection incompleteness. We derived average flare frequencies as a function of flare energy. The flare frequency is seen to depend on the source's intrinsic X-ray luminosity, but its determination is affected by the length of the observation. The slope of the high-energy tail of the energy distribution is, however, affected little. A comparison of Cygnus OB2 and ONC sources, accounting for observational biases, shows that the two populations, known to have similar X-ray emission levels, have very similar flare activity.

Conclusions.Studies of flare activity are only comparable if performed consistently and taking the observation length into account. Flaring activity does not vary appreciably between the age of the ONC (~1 Myr) and that of Cygnus OB2 (~2 Myr). The slope of the distribution of flare energies is consistent with the micro-flare explanation of the coronal heating.