2007A&A...471..645C

X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ∼10 day long X-ray observation of the Orion Nebula Cluster (ONC). Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3M_☉) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. The high energy tail of the energy distribution of flares is well described by a power-law with index ∼2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2M_☉), we establish that, at ∼1Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Our observational results are consistent with the following model/scenario: the light curves are entirely built by overlapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.