* Astronomy

ESA’s Integral gamma-ray satellite 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 the satellite 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.


Position(2000): R.A.: 264.7500 Dec.: -30.2700

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.


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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

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. 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" - Ignacio Negueruela, University of Alicante, Spain, lead author of the results.

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.

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XTE J1739-302 as a Supergiant Fast X-ray Transient

Authors: D. M. Smith, W. A. Heindl, C. B. Markwardt, J. H. Swank, I. Negueruela, T. E. Harrison, L. Huss


Lightcurves of three individual outbursts of XTE J1739–302 from the 1E 1740.7–2942 pointings
(top three traces, with the flux from 1E 1740.7–2942 still present), a typical 1E 1740.7–2942 lightcurve when XTE J1739–302 is quiescent, and the lightcurve of XTE J1739–302 as seen at a low level of activity with Chandra. The Chandra trace is multiplied by 200 to be seen on this scale, and the 2002 outburst has been shifted upward by 150 counts s^-1.

XTE J1739-302 is a transient X-ray source with unusually short outbursts, lasting on the order of hours. Here we give a summary of X-ray observations we have made of this object in outburst with the Rossi X-ray Timing Explorer (RXTE) and at a low level of activity with the Chandra X-ray Observatory, as well as observations made by other groups. Visible and infrared spectroscopy of the mass donor of XTE J1739-302 are presented in a companion paper.
The X-ray spectrum is hard both at low levels and in outburst, but somewhat variable, and there is strong variability in the absorption column from one outburst to another. Although no pulsation has been observed, the outburst data from multiple observatories show a characteristic timescale for variability on the order of 1500-2000 s.
The Chandra localization (right ascension 17h 39m 11.58s, declination -30o 20' 37.6'', J2000) shows that despite being located less than 2 degrees from the Galactic Centre and highly absorbed, XTE J1739-302 is actually a foreground object with a bright optical counterpart.
The combination of a very short outburst timescale and a supergiant companion is shared with several other recently-discovered systems, forming a class we designate as Supergiant Fast X-ray Transients (SFXTs).
Three persistently bright X-ray binaries with similar supergiant companions have also produced extremely short, bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.

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-- Edited by Blobrana at 21:52, 2005-11-16