The Origin of Variability of the Intermediate-mass Black-hole ULX System HLX-1 in ESO 243-49

J. -P. Lasota (1) , T. Alexander , G. Dubus (2, 1) , D. Barret (3) , S. A. Farrell , N. Gehrels , O. Godet (3) , N. A. Webb (3)

1 IAP - Institut d'Astrophysique de Paris
2 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
3 IRAP - Institut de recherche en astrophysique et planétologie

The ultra-luminous (L_X <~ 10⁴² erg s^-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 _• <~ 10⁴ M _sun IMBHs—formed by runaway stellar mergers in the dense collapsed cores of young clusters—could have a few ×1 M _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.

Mots clés: accretion; accretion disks; binaries: close; galaxies: star clusters: general; instabilities; stars: kinematics and dynamics; X-rays: individual: ESO 243-49 HLX-1; Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - Astrophysics of Galaxies

Domaines: Planète et Univers [physics]

J. -P. Lasota, T. Alexander, G. Dubus, D. Barret, S. A. Farrell, et al.. The Origin of Variability of the Intermediate-mass Black-hole ULX System HLX-1 in ESO 243-49. The Astrophysical Journal, 2011, 735, ⟨10.1088/0004-637X/735/2/89⟩. ⟨insu-03624339⟩