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(ABRIDGED)- The physical mechanism responsible for the short outbursts in arecently recognized class of High Mass X-ray Binaries, the Supergiant FastX-ray Transients (SFXTs), is still unknown. Two main hypotheses have beenproposed to date: the sudden accretion by the compact object of small ejectionsoriginating in a clumpy wind from the supergiant donor, or outbursts producedat (or near) the periastron passage in wide and eccentric orbits, in order toexplain the low (1E32 erg/s) quiescent emission.Neither proposed mechanismsseem to explain the whole phenomenology of these sources. Here we propose a newexplanation for the outburst mechanism, based on new X-ray observations of theunique SFXT known to display periodic outbursts, IGRJ11215-5952. We performedthree Target of Opportunity observations with Swift, XMM-Newton and INTEGRAL atthe time of the fifth outburst, expected on 2007 February 9. Swift observationsof the February 2007 outburst have been reported elsewhere. Another ToO withSwift was performed in July 2007, in order to monitor the supposed ``apastron''passage. A second unexpected outburst was discovered on 2007 July 24, afterabout 165 days from the February 2007 outburst. The new X-ray observationsallow us to propose an alternative hypothesis for the outburst mechanism inSFXTs, linked to the possible presence of a second wind component,in the formof an equatorial disk from the supergiant donor. We discuss the applicabilityof the model to the short outburst durations of all other SFXTs, where a clearperiodicity in the outbursts has not been found yet. The new outburst fromIGRJ11215-5952 observed in July suggests that the true orbital period is~165days, instead of 329days, as previously thought.

An alternative hypothesis for the outburst mechanism in Supergiant Fast X-ray Transients: the case of IGR J11215-5952