We present results based on two years of intense Swift monitoring of three supergiant fast X–ray transients ( SFXTs ) , IGRÂ J16479 - 4514 , XTEÂ J1739 - 302 , and IGRÂ J17544 - 2619 , which we started in October 2007 .
Our out-of-outburst intensity-based X–ray ( 0.3–10 keV ) spectroscopy yields absorbed power laws characterized by hard photon indices ( \Gamma \sim 1– 2 ) .
The broad-band ( 0.3–150 keV ) spectra of these sources , obtained while they were undergoing new outbursts observed during the second year of monitoring , can be fit well with models typically used to describe the X–ray emission from accreting neutron stars in high-mass X–ray binaries .
We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument .
By considering our monitoring as a casual sampling of the X–ray light curves , we can infer that the time these sources spend in bright outbursts is between 3 and 5 % of the total .
The most probable X-ray flux for these sources is \sim 1 – 2 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } ( 2–10 keV , unabsorbed ) , corresponding to luminosities in the order of a few 10 ^ { 33 } to a few 10 ^ { 34 } erg s ^ { -1 } ( two orders of magnitude lower than the bright outbursts ) .
In particular , the duty-cycle of inactivity is \sim 19 , 39 , 55 % ( \sim 5 % uncertainty ) , for IGRÂ J16479 - 4514 , XTEÂ J1739 - 302 , and IGRÂ J17544 - 2619 , respectively .
We present a complete list of BAT on-board detections , which further confirms the continued activity of these sources .
This demonstrates that true quiescence is a rare state , and that these transients accrete matter throughout their life at different rates .
Variability in the X–ray flux is observed at all timescales and intensity ranges we can probe .
Superimposed on the day-to-day variability is intra-day flaring which involves flux variations up to one order of magnitude that can occur down to timescales as short as \sim 1 ks , and which can be naturally explained by the accretion of single clumps composing the donor wind with masses M _ { cl } \sim 0.3 – 2 \times 10 ^ { 19 } g. Thanks to the Swift  observations , the general picture we obtain is that , despite individual differences , common X–ray characteristics of this class are now well defined , such as outburst lengths well in excess of hours , with a multiple peaked structure , and a high dynamic range ( including bright outbursts ) , up to \sim 4 orders of magnitude .