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

ESA News
http://www.esa.int

16 November 2005

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

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

More about...

* ESA's gamma-ray astronomy mission
http://www.esa.int/SPECIALS/Integral/index.html
* Integral factsheet
http://www.esa.int/esaSC/SEMD9G1A6BD_index_0.html

Related articles

* Integral: three years of insight into the violent cosmos
http://www.esa.int/esaCP/SEMY146Y3EE_index_0.html
* Star eats companion
http://www.esa.int/esaCP/SEMWSAA5QCE_index_0.html
* Three satellites needed to bring out 'shy star'
http://www.esa.int/esaCP/SEMSOI6DIAE_index_0.html
* Integral rolls back history of Milky Way's super-massive black hole
http://www.esa.int/SPECIALS/Integral/SEMSKPO3E4E_0.html
* ESA's Integral detects closest cosmic gamma-ray burst
http://www.esa.int/SPECIALS/Integral/SEMV9P0XDYD_0.html
* ESA's Integral solves thirty-year old gamma-ray mystery
http://www.esa.int/esaCP/SEM24EX5WRD_index_0.html
* A gamma-ray burst bonanza
http://www.esa.int/esaCP/SEMIVX8YFDD_FeatureWeek_0.html
* Integral -- tracking extreme radiation across the Universe
http://www.esa.int/esaCP/ESAI0BTHN6D_index_0.html
* Observations: Seeing in the gamma-ray wavelengths
http://www.esa.int/esaSC/SEM3A2T1VED_index_0.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaCP/SEM20VJBWFE_index_1.html]
This artist's impression shows a high-mass binary system, composed of a
supergiant luminous star (in blue) and a compact stellar object, such as
a neutron star.

As discovered by ESA's Integral observatory, many of these supergiant
systems produce strong and exceptionally fast-rising X-ray outbursts
lasting a few hours only, hence their name 'supergiant fast X-ray
transients'.

The outbursts may depend on the way stellar material is exchanged
between the supergiant star and the compact object.

The light curve at the bottom-right was retrieved by Integral from the
supergiant fast X-ray transient source IGR J17544-2619 on 17 September 2003.

The curve shows a very fast X-ray outburst from the compact object,
lasting about two hours only, with very fast rise and slow decay. The
counterpart of this source is a luminous supergiant, unambiguously
identified by ESA's XMM-Newton and NASA's Chandra X-ray observatories.

Credits: ESA

[Image 2:
http://www.esa.int/esaCP/SEM20VJBWFE...html#subhead2]
This simulated sequence shows the interaction between the stellar
material carried by the wind of a supergiant star and its 'receiving'
companion -- a compact stellar object such as a neutron star. In the
vicinity of the compact object it is possible to see the development of
a turbulent shocked flow.

Credits: JM Blondin, North Carolina State University