An unusually long X-ray outburst from the SFXT IGR J18483-0311
INTEGRAL has inaugurated a new era in the study of Supergiant High-Mass
X-ray Binaries (SGXBs) by discovering a peculiar sub-class labelled as
Supergiant Fast X-ray Transients (SFXTs; see INTEGRAL POM of
December 2005,
February 2009,
Janaury 2011,
November 2011,
August 2013 ).
They are among the most extreme and exotic X-ray transients in our Galaxy:
a large dynamic range (typically 103-105) in
luminosities above 20 keV is covered in usually very few hours time. This
exceptional fast X-ray transient behaviour is at odds with the bright and
persistent X-ray emission characterizing their historical parent population
of wind-fed SGXBs. As such it represents a challenge from the point of view
of the accretion mechanism at work and as well as the companion wind structures.
Among the known SFXTs, IGR J18483-0311 is characterized by X-ray outbursts
whose duration can occasionally reach "extreme" values of a couple of days
(i.e., 2-3 days). A recent INTEGRAL investigation of the source by Sguera et al.
using archival IBIS/SIGRI data (18-60 keV, upper panel of revolutions sequence
figure) and JEM-X data (3-10 keV, lower panel of revolutions sequence figure)
discovered for the first time a particularly long broad-band X-ray activity which
lasted for at least 11 days, i.e., a significant fraction (about 60%) of the
entire orbital period. Interestingly, the activity spanned orbital phases
corresponding not only to periastron (as usually observed), but, as observed
for the first time, also apastron passages. This prolongated X-ray activity is
at odds with the much shorter durations (both in term of absolute time and fraction
of the orbital period) usually marking outbursts above 20 keV from classical SFXTs.
This represents a departure from their nominal behavior and it adds a further
extreme characteristic to IGR J18483-0311.
The physical mechanism driving the peculiar X-ray behaviour of SFXTs is far from
solved, although several different models have been proposed in the last decade.
The newly discovered long X-ray activity from IGR J18483-0311 is especially hard
to explain within the framework of the classical spherically symmetric clumpy wind
scenario which became the preferred mechanism since the early days of SFXTs discovery.
The newly reported results, however, are better explained within the framework of
the novel theoretical development known as 'quasi-spherical settling accretion model'.
Further INTEGRAL studies on the classical SFXTs searching for similarly unusually
long X-ray outbursts are needed, to understand if the reported peculiar characteristic
observed from IGRJ18483-0311 is exceptional or not among the class of SFXTs.
The figure shows the outburst X-ray activity of IGR J18483-0311 (encircled) in a
sequence of images as a function of Revolution as observed by IBIS/ISGRI (18-60 keV,
top) and JEM-X (3-10 keV, bottom). The effective exposures times in each Revolution
is given. In the images at the left the equatorial coordinate system is given. Note
that the IBIS/ISGRI and the JEM-X images are on a different scale. At the bottom of
each image sequence the colour coding is given in units of signal-to-noise, sigma.
Interestingly, in Revolution 907 a typical short outburst of another SFXT,
AX J1845.0-0433, also known as IGR J18450-0435, was seen (see also
INTEGRAL POM August 2013 ).
References:
"INTEGRAL discovery of unusually long broad-band X-ray activity from the Supergiant Fast X-ray Transient IGR J18483-0311"
Sguera et al. (2015), to be published in MNRAS:
http://arxiv.org/abs/1502.04580
