A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus molecular cloud

Context. This work is part of a systematic X-ray survey of the Taurus star-forming complex with XMM-Newton. Aims. We study the time series of all X-ray sources associated with Taurus members, to statistically characterize their X-ray variability, and compare the results to those for pre-main sequence stars in the Orion Nebula Cluster and to expectations arising from a model where all the X-ray emission is the result of a large number of stochastically occurring flares. Methods. The analysis of the light curves is based on a maximum likelihood algorithm that segments the time series in intervals of constant signal without the need of binning. Flares are defined with criteria that take into account the amplitude and the derivative of the segmented light curves. Variability statistics are evaluated for different classes of pre-main sequence stars (protostars, cTTS, wTTS, brown dwarfs), and for different spectral type ranges. Flare frequency and energy distribution are computed. Results. We find that roughly half of the detected X-ray sources show variability above our sensitivity limit, and in similar to 26% of the cases this variability is recognized as flares. Variability is more frequently detected at hard than at soft energies. The variability statistics of cTTS and wTTS are indistinguishable, suggesting a common ( coronal) origin for their X-ray emission. The frequency of large flares (E > 10(35) erg) on Taurus members is 1 event per star in 800 ks. The typical duration of these flares - probably biased by the finite observing time - is about 10 ks. We have applied a rigorous maximum likelihood method in the analysis of the number distribution of flare energies on pre-main sequence stars for the first time. In its differential form this distribution follows a power law with index alpha = 2.4 +/- 0.5, in the range typically observed for late-type stars and the Sun. Conclusions. The signature of the X-ray variability in the pre-main sequence stars in Taurus and Orion provides twofold support for coronal heating by flares: (i) the correlation between the maximum variability amplitude and the minimum emission level indicates that both flare and quiescent emissions are closely related to the coronal heating process; (ii) the power law index alpha derived for the flare energy distribution is large enough to explain the heating of stellar coronae by nano-flares (alpha > 2), albeit associated with a rather large uncertainty that leaves some doubt as to this conclusion.