THE ORIGIN OF VARIABILITY OF THE INTERMEDIATE-MASS BLACK-HOLE ULX SYSTEM HLX-1 IN ESO 243-49
- Institut d'Astrophysique de Paris, UMR 7095 CNRS, UPMC Univ Paris 06, 98bis Boulevard Arago, 75014 Paris (France)
- Department of Particle Physics and Astrophysics, Faculty of Physics, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100 (Israel)
- UJF-Grenoble 1/CNRS-INSU, Institut de Planetologie et d'Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble F-38041 (France)
- Universite de Toulouse, Universite Paul Sabatier, Observatoire Midi-Pyrenees, Institut de Recherche en Astrophysique et Planetologie (IRAP), Toulouse (France)
- Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH (United Kingdom)
- NASA/Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
The ultra-luminous (L{sub X} {approx}< 10{sup 42} erg s{sup -1}) intermediate-mass black-hole (IMBH) system HLX-1 in the ESO 243-49 galaxy exhibits variability with a possible recurrence time of a few hundred days. Finding the origin of this variability would constrain the still largely unknown properties of this extraordinary object. Since it exhibits a hardness-intensity behavior characteristic of black-hole X-ray transients, we have analyzed the variability of HLX-1 in the framework of the disk instability model that explains outbursts of such systems. We find that the long-term variability of HLX-1 is unlikely to be explained by a model in which outbursts are triggered by thermal-viscous instabilities in an accretion disk. Possible alternatives include the instability in a radiation-pressure-dominated disk but we argue that a more likely explanation is a modulated mass transfer due to tidal stripping of a star in an eccentric orbit around the IMBH. We consider an evolutionary scenario leading to the creation of such a system and estimate the probability of its observation. We conclude, using a simplified dynamical model of the post-collapse cluster, that no more than 1/100 to 1/10 of M{sub .} {approx}< 10{sup 4} M{sub sun} IMBHs-formed by runaway stellar mergers in the dense collapsed cores of young clusters-could have a few x1 M{sub sun} main-sequence star evolve to an asymptotic giant branch on an orbit eccentric enough for mass transfer at periapse, while avoiding collisional destruction or being scattered into the IMBH by two-body encounters. The finite but low probability of this configuration is consistent with the uniqueness of HLX-1. We note, however, that the actual response of a standard accretion disk to bursts of mass transfer may be too slow to explain the observations unless the orbit is close to parabolic (and hence even rarer). Also, increased heating, presumably linked to the highly time-dependent gravitational potential, could shorten the relevant timescales.
- OSTI ID:
- 21578403
- Journal Information:
- Astrophysical Journal, Vol. 735, Issue 2; Other Information: DOI: 10.1088/0004-637X/735/2/89; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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