Full radiative coupling in two-phase models for accreting black holes

¹ Centre d'Étude Spatiale des Rayonnements, CNRS-UPS, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France e-mail: malzac@cesr.fr
² Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
³ LUTH, UMR 8102 (CNRS/Université Paris 7), Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France

Received: 21 January 2003
Accepted: 21 July 2004

Abstract

The emission from galactic black holes and Seyfert galaxies is generally understood in term of two-phase models (Haardt & Maraschi 1991, 1993). Such models postulate that a hot plasma (~10⁹ K) coexists with relatively colder material (~10⁶ K) in the inner part of the accretion flow. We present the first simulated broad-band spectra produced by such a system and accounting simultaneously for energy balance and Comptonisation in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. This was made possible by coupling three radiative transfer codes: a non-linear Monte-Carlo code (NLMC), a photo-ionization code TITAN and a linear Monte-Carlo code NOAR. The equilibrium comptonisation spectrum appears to be sensitive to the shape of the reprocessed spectrum that, in turn, depends on the ionization parameter, but also on the structure of the irradiated cold material. This is illustrated by a comparison of simulations assuming constant density or a constant pressure in the cold phase. We also compare our results with simplified models where reprocessing is approximated by a blackbody spectrum. Our detailed treatment leads to noticeably different spectral energy distributions (SEDs) characterised by harder X-ray spectra. Even at low ionization parameters ( erg s^-1 cm) the commonly used blackbody approximation is poor, leading to X-ray spectra that are too soft. The effect, however, seems not to be strong enough to reconcile the slab corona model with the hardest observed spectra, unless the reflector has a constant density and the ionization parameter is large, of the order of 10⁴ erg s^-1 cm.