Testing slim-disk models on the thermal spectra of LMC X-3

¹ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
e-mail: odele@camk.edu.pl
² Astronomical Institute, Academy of Sciences of the Czech Republic, Boční II 1401/1a, 141-31 Prague, Czech Republic
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁴ Department of Physics, Göteborg University, 412-96 Göteborg, Sweden
⁵ Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA 02139, USA

Received: 31 May 2011
Accepted: 4 August 2011

Abstract

Slim-disk models describe advective accretion flows at high luminosities, while reducing to the standard thin disk form in the low luminosity limit. We have developed a new spectral model, slimbb, within the framework of XSPEC, 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 maintaining itself in soft spectral states which are well described using accretion-disk models, making it an ideal candidate to test the aptness of slimbb. Our results demonstrate consistency between the low-luminosity (thin-disk) and high luminosity (slim-disk) regimes. The results also illustrate that advection alone does not solve the problem of the origin of the surprisingly soft high-luminosity spectra in LMC X-3. We show that X-ray continuum-fitting in the high accretion rate regime can powerfully test black-hole accretion disk models.