  Supergiant fast X-ray transients (SFXTs) are a sub-class of supergiant high
mass X-ray binaries hosting a neutron star accreting from the stellar wind of a
massive OB companion. Compared to the classical systems, SFXTs display a
pronounced variability in X-rays that has long been (at least partly) ascribed
to the presence of clumps in the stellar wind. We report here on the first set
of results of an on-going XMM-Newton observational program aimed at searching
for spectroscopic variability during the X-ray flares and outbursts of the
SFXTs. The goal of the paper is to present the observational program and show
that the obtained results are according to expectations, with a number of
flares (between one and four) generally observed per source and per observation
(20~ks-long, on average). We base our work on a systematic and uniform analysis
method optimized to consistently search for spectral signatures of a variable
absorption column density, as well as other parameters of the spectral
continuum. Our preliminary results show that the program is successful and the
outcomes of the analysis support previous findings that most of the X-ray
flares seem associated to the presence of a massive structure approaching and
getting accreted by the compact object. However, we cannot rule out that other
mechanisms are at work together with clumps to enhance the X-ray variability of
SFXTs. This is expected according to current theoretical models. The success of
these observations shows that our observational program can be a powerful
instrument to deepen our understanding of the X-ray variability in SFXTs.
Further observations will help us in achieving a statistically robust sample.
This is required to conduct, in the future, a systematic analysis on the whole
SFXT class with the ultimate goal of disentangling the role of different
mechanisms giving rise to these events.
