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Astron. Astrophys. 354, 987-994 (2000)

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The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass transfer

A.A. Esin  *  1,2, J.-P. Lasota  **  1,3 and R.I. Hynes 4,5

1 Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106-4030, USA
2 Theoretical Astrophysics, 133-30 Caltech, Pasadena CA91125, USA (aidle@tapir.caltech.edu)
3 Institut d'Astrophysique de Paris; CNRS, 98bis Bd. Arago, 75014 Paris, France (lasota@iap.fr)
4 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
5 Department of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK (rih@astro.soton.ac.uk)

Received 11 October 1999 / Accepted 11 January 2000

Abstract

We show that the 1996 outburst of the X-ray binary transient system GRO J1655-40 can be explained by the standard dwarf-nova type disc instability, followed by an episode of enhanced mass transfer from the secondary if the mass transfer rate in GRO J1655-40 is within a factor [FORMULA] of the stability limit. We argue that irradiation of the secondary during the onset of the outburst driven by the thermal instability in the outer disc can increase the mass transfer rate above the minimum value required for stable accretion. This will then produce the period of near-constant X-ray emission seen in this system. This scenario can also explain the observed anti-correlation between the optical and X-ray fluxes. It is generally accepted that optical emission in low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays. There is also strong circumstantial evidence that in order for the outer disc to see the irradiating flux, it must be warped. Depending on the warp propagation mechanism, either a burst of mass from the secondary or viscous decay are likely to decrease the degree of warping, thereby causing the decrease in the observed optical flux while the X-ray flux remains constant or even increases, exactly as observed in GRO J1655-40. Finally, the decrease of the disc warping and, therefore, irradiation will cause the disc to become unstable once again, terminating the outburst.

Key words: stars: binaries: close – stars: individual: GRO J1655-40 – accretion, accretion disks – instabilities – X-rays: general

* Chandra Fellow
** also: DARC, Observatoire de Paris, France

Send offprint requests to: A.A. Esin

Correspondence to: J.-P. Lasota (lasota@iap.fr)

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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