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INTEGRAL discovery of unusually long broad-band X-ray activity from the Supergiant Fast X-ray Transient IGR J18483-0311
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 long 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.Comment: Accepted for publication on MNRAS. 10 pages, 7 figures, 1 tabl
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