Integral reveals new class of 'supergiant' X-ray binary stars

Integral reveals new class of 'supergiant' X-ray binary stars

16 November 2005

ESA's Integral gamma-ray observatory has discovered a new, highly populated
class of X-ray fast 'transient' binary stars, undetected in previous
observations.

With this discovery, Integral confirms how much it is contributing to
revealing a whole hidden Universe.
The new class of double star systems is characterised by a very compact
object that produces highly energetic, recurrent and fast-growing X-ray
outbursts, and a very luminous 'supergiant' companion.
The compact object can be an accreting body such as a black hole, a neutron
star or a pulsar. Scientists have called such class of objects 'supergiant
fast X-ray transients'. 'Transients' are systems which display periods of
enhanced X-ray emission.
Before the launch of Integral, only a dozen X-ray binary stars containing
supergiants had been detected. Actually, scientists thought that such
high-mass X-ray systems were very rare, assuming that only a few of them
would exist at once since stars in supergiant phase have a very short
lifetime.
However, Integral's data combined with other X-ray satellite observations
indicate that transient supergiant X-ray binary systems are probably much
more abundant in our Galaxy than previously thought.
In particular, Integral is showing that such 'supergiant fast X-ray
transients', characterised by fast outbursts and supergiant companions, form
a wide class that lies hidden throughout the Galaxy.


Due to the transitory nature, in most cases these systems were not detected
by other observatories because they lacked the combination of sensitivity,
continuous coverage and wide field of view of Integral.
They show short outbursts with very fast rising times - reaching the peak of
the flare in only a few tens of minutes - and typically lasting a few hours
only. This makes the main difference with most other observed transient
X-ray binary systems, which display longer outbursts, lasting typically a
few weeks up to months.
In the latter case, the long duration of the outburst is consistent with a
'viscous' mass exchange between the star and an accreting compact object.
In 'supergiant fast X-ray transients', associated with highly luminous
supergiant stars, the short duration of the outburst seems to point to a
different and peculiar mass exchange mechanism between the two bodies.
This may have something to do with the way the strong radiative winds,
typical of highly massive stars, feed the compact object with stellar
material.







Scientists are now thinking about the reasons for such short outbursts. It
could be due to the supergiant donor ejecting material in a non-continuous
way. For example, a clumpy and intrinsically variable nature of a supergiant
's radiative winds may give rise to sudden episodes of increased accretion
rate, leading to the fast X-ray flares.
Alternatively, the flow of material transported by the wind may become, for
reasons not very well understood, very turbulent and irregular when falling
into the enormous gravitational potential of the compact object.
"In any case, we are pretty confident that the fast outbursts are associated
to the mass transfer mode from the supergiant star to the compact object,"
says Ignacio Negueruela, lead author of the results, from the University of
Alicante, Spain.
"We believe that the short outbursts cannot be related to the nature of the
compact companion, as we observed fast outbursts in cases where the compact
objects were very different - black holes, slow X-ray pulsars or fast X-ray
pulsars."
Studying sources such as 'supergiant fast X-ray transients', and
understanding the reasons for their behaviour, is very important to increase
our knowledge of accretion processes of compact stellar objects.
Furthermore, it is providing valuable insight into the evolution paths that
lead to the formation of high-mass X-ray binary systems.


Notes to editors:

These results were obtained from Integral data by I. Negueruela, lead
author, and J.M. Torrejon (University of Alicante, Spain), D.M. Smith
(University of California Santa Cruz, US), P. Reig (University of Crete,
Greece), S. Chaty (AIM, CEA Saclay, Gif-sur-Yvette, France). They appear in
the article 'Supergiant fast X-ray transients: a new class of high-mass
X-ray binaries unveiled by Integral', due to appear in ESA SP-604
(astro-ph/0511088).
Though the role of Integral has been key in the discovery of this class of
objects, there have been important contributions from other X-ray
satellites. In particular, the work that led to this discovery was started
by US collaborator David Smith, using Rossi XTE data on XTE J1739-302,
appearing in the article 'XTE J1739-302 as a supergiant fast X-ray transient
' by D.M. Smith, W.A. Heindl, C.B. Markwardt, J.H. Swank, I. Negueruela,
T.E. Harrison, L. Huss, 2006 ApJ, in press (astro-ph/0510658).
The authors acknowledge the important role of results presented by Sguera et
al. (V. Sguera, et al. 'INTEGRAL observations of recurrent fast X-ray
transient sources', 2005 A&A, in press (astro-ph/0509018).
Several of the X-ray sources discussed had already been seen (once or twice)
by NASA's Rossi XTE, JAXA's ASCA or ASI's BeppoSAX. The role of Integral has
been fundamental because of its capabilities to monitor weak sources.
Thanks to this, it has shown: that the sources are recurrent transients,
that is, they display repeated outbursts; that there are many sources with
similar behaviour, and the shape of the outbursts - which is very similar in
all cases.
However, in order to determine accurately their positions, observations were
needed from X-ray telescopes with better positional accuracy, either ESA's
XMM-Newton or NASA's Chandra observatories.


For more information:

Ignacio Negueruela, University of Alicante, Spain
E-mail: ignacio @ DFISTS.ua.es
Chris Winkler, ESA Integral Project Scientist
E-mail: christoph.winkler @ esa.int




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. Integral factsheet (http://www.esa.int/esaSC/SEMD9G1A6BD_index_0.html)


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Jacques :-)

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