Testing Slim-Disk Models on the Thermal Spectra of LMC X-3

Slim-disk models describe accretion flows at high luminosit ies, while reducing to the standard thin disk form in the low luminosity limit. We have developed a new spectral model,slimbb, within the framework ofXSPEC, which describes fully relativistic slim-disk accretion and includes photon ray-tracing that starts from the disk photosphere, rather than the equatorial plane. We demonstrate the features of this model by applying it to RXTE spectra of the persistent black-hole X-ray binary LMC X-3 . LMC X-3 has the virtues of exhibiting large intensity variations while maintainin g itself in soft spectral states which are well described usi ng accretion-disk models, making it an ideal candidate to test the aptness ofslimbb. Our results demonstrate consistency between the low-luminosity (thin-disk) and high luminosity (slim-disk) regimes. We also show that X-ray continuum-fitting in the high accretion ra te regime can powerfully test black-hole accretion disk models. Accretion powers the most luminous objects in the universe by converting gravitational potential energy into highly ene rgetic radiation. In order to understand this mechanism it is cruci al to study the primary engine of accreting black-hole systems: the accretion disk. Accretion disks form when gaseous matter, usually an assembly of free electrons and various types of ions, spirals onto a central gravitating body by gradually losing its initial angular momentum as a result of viscous and magnetic stresses. The simplest analytical model of an accretion disk is the standard thin disk model (Shakura & Sunyaev 1973; Novikov & Thorne 1973) which assumes that at a given radius, r, all dissipated energy is released as radiation and that the emission at each ra dius is locally given by a blackbody spectrum. The innermost stable circular orbit (ISCO), beyond which the particles in orbit become dynamically unstable and plunge into the black hole, is taken to be a manifestation of the inner disk boundary. This model, however, is only self-consistent in the limit at whic h the disk is geometrically razor thin. Consequently, its val idity is restricted to luminosities below ∼ 30% of the Eddington luminosity, LEdd ≡ 1.26 × 10 38 (M/M⊙) erg/s, when radiation pressure will cause the disk to be minimally inflated (see e.g .