We report on a broad-band X-ray study (0.5-250 keV) of the Supergiant Fast X-ray Transient IGR J18483-0311 using archival INTEGRAL data and a new targeted XMM-Newton observation. Our INTEGRAL investigation discovered for the first time an unusually l
ong X-ray activity (3-60 keV) which continuously lasted for at least 11 days, i.e. a significant fraction (about 60%) of the entire orbital period, and spanned orbital phases corresponding to both periastron and apastron passages. This prolongated X-ray activity is at odds with the much shorter durations marking outbursts from classical SFXTs especially above 20 keV, as such it represents a departure from their nominal behavior and it adds a further extreme characteristic to the already extreme SFXT IGR J18483-0311. Our IBIS/ISGRI high energy investigation (100-250 keV) of archival outbursts activity from the source showed that the recently reported hint of a possible hard X-ray tail is not real and it is likely due to noisy background. The new XMM-Newton targeted observation did not detect any sign of strong X-ray outburst activity from the source despite being performed close to its periastron passage, on the contrary IGR J18483-0311 was caught during the common 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 both spherically symmetric clumpy wind scenario and quasi-spherical settling accretion model.
Context: IGR J16479-4514 is a fast X-ray transient known to display flares lasting typically a few hours. Recently, its counterpart has been identified with a supergiant star, therefore the source can be classified as member of the newly discovered c
lass of Supergiant Fast X-ray Transients (SFXTs), specifically it is the one with the highest duty cycle. Aims: to characterize the quiescent X-ray behaviour of the source and to compare its broad band spectrum to that during fast X-ray flares. Methods: we performed an analysis of IBIS and JEM-X data with OSA 5.1 as well as an analysis of archival Swift/XRT data. Results: we present results from a long term monitoring of IGR J16479-4514 with detailed spectral and timing informations on 19 bright fast X-ray flares, 10 of which newly discovered. We also report for the first time results on the quiescent X-ray emission; the typical luminosity value (about 10^34 erg s^-1) is about 2 orders of magnitude greater than that typical of SFXTs while its broad band X-ray spectrum has a shape very similar to that during fast X-ray transient activity, i.e. a rather steep power law with Gamma=2.6. Conclusions: IGR J16479$-$4514 is characterized by a quiescent X-ray luminosity higher than that typical of other known SFXTs but lower than persistent emission from classical SGXBs. We suggest that such source is a kind of transition object between these two systems, supporting the idea that there is a continuum of behaviours between the class of SFXTs and that of classical persistent SGXBs.
