  IGR J11215-5952 is a hard X-ray transient source discovered in April 2005
with INTEGRAL and a confirmed member of the new class of High Mass X-ray
Binaries, the Supergiant Fast X-ray Transients (SFXTs). Archival INTEGRAL data
and RXTE observations showed that the outbursts occur with a periodicity of
~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic
outbursts, possibly related to the orbital period. We performed a Target of
Opportunity observation with Swift with the main aim of monitoring the source
behaviour around the time of the fifth outburst, expected on 2007 Feb 9. The
source field was observed with Swift twice a day (2ks/day) starting from 4th
February, 2007, until the fifth outburst, and then for ~5 ks a day afterwards,
during a monitoring campaign that lasted 23 days for a total on-source exposure
of ~73 ks. This is the most complete monitoring campaign of an outburst from a
SFXT. The spectrum during the brightest flares is well described by an absorbed
power law with a photon index of 1 and N_H~1 10^22 cm^-2. A 1-10 keV peak
luminosity of ~10^36 erg s^-1 was derived (assuming 6.2 kpc, the distance of
the optical counterpart). These Swift observations are a unique data-set for an
outburst of a SFXT, thanks to the combination of sensitivity and time coverage,
and they allowed a study of IGR J11215-5952 from outburst onset to almost
quiescence. We find that the accretion phase lasts longer than previously
thought on the basis of lower sensitivity instruments observing only the
brightest flares. The observed phenomenology is consistent with a smoothly
increasing flux triggered at the periastron passage in a wide eccentric orbit
with many flares superimposed, possibly due to episodic or inhomogeneous
accretion.
