2007A&A...469L...5R

The hard X-ray transient source IGR J11215-5952 was discovered in April 2005 with INTEGRAL and is 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 have shown 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 2007 Feb. 4 until the fifth outburst, and then for ∼5ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ∼73ks. This is the most complete monitoring campaign of an outburst from an SFXT. The spectrum during the brightest flares is described well by an absorbed power law with a photon index of 1 and N_H ∼1x10²²cm^–2. A 1-10keV peak luminosity of ∼10³⁶erg/s was derived (assuming 6.2kpc, the distance of the optical counterpart). These Swift observations are a unique data-set for an outburst of an 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.