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We report on a broad-band X-ray study (0.5-250 keV) of the Supergiant FastX-ray Transient IGR J18483-0311 using archival INTEGRAL data and a new targetedXMM-Newton observation. Our INTEGRAL investigation discovered for the firsttime an unusually long X-ray activity (3-60 keV) which continuously lasted forat least 11 days, i.e. a significant fraction (about 60%) of the entire orbitalperiod, and spanned orbital phases corresponding to both periastron andapastron passages. This prolongated X-ray activity is at odds with the muchshorter durations marking outbursts from classical SFXTs especially above 20keV, as such it represents a departure from their nominal behavior and it addsa further extreme characteristic to the already extreme SFXT IGR J18483-0311.Our IBIS/ISGRI high energy investigation (100-250 keV) of archival outburstsactivity from the source showed that the recently reported hint of a possiblehard X-ray tail is not real and it is likely due to noisy background. The newXMM-Newton targeted observation did not detect any sign of strong X-rayoutburst activity from the source despite being performed close to itsperiastron passage, on the contrary IGR J18483-0311 was caught during thecommon intermediate X-ray state with a low luminosity value of 3x10^33 erg s^-1(0.5-10 keV). We discuss all the reported results in the framework of bothspherically symmetric clumpy wind scenario and quasi-spherical settlingaccretion model.

INTEGRAL discovery of unusually long broad-band X-ray activity from the Supergiant Fast X-ray Transient IGR J18483-0311