  The usage of cumulative luminosity distributions, constructed thanks to the
long-term observations available through wide field hard X-ray imagers, has
been recently exploited to study the averaged high energy emission (>17 keV)
from Supergiant Fast X-ray Transients (SFXTs) and classical Supergiant High
Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term
monitorings now available with Swift/XRT to construct for the first time the
cumulative luminosity distributions of a number of SFXTs and the classical SgXB
IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray
telescope (0.3-10 keV). By complementing previous results obtained in the hard
X-rays, we found that classical SgXBs are characterized by cumulative
distributions with a single knee around $\sim$10$^{36}$-10$^{37}$ erg/s, while
SFXTs are found to be systematically sub-luminous and their distributions are
shifted at significantly lower luminosities (a factor of $\sim$10-100). As the
luminosity states in which these sources spend most of their time are typically
below the sensitivity limit of large field of view hard X-ray imagers, we
conclude that soft X-ray monitorings carried out with high sensitivity
telescopes are particularly crucial to reconstruct the complete profile of the
SFXT cumulative luminosity distributions. The difference between the cumulative
luminosity distributions of classical SgXBs and SFXTs is interpreted in terms
of accretion from a structured wind in the former sources and the presence of
magnetic/centrifugal gates or a quasi-spherical settling accretion regime in
the latter.
