Open AccessSpectral variability in transonic discs around black holes
Author(s) -
Zampieri Luca,
Turolla Roberto,
Szuszkiewicz Ewa
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04554.x
Subject(s) - physics , astrophysics , black body radiation , transonic , accretion (finance) , spectral line , radiative transfer , instability , radius , radiation , astronomy , optics , mechanics , computer security , computer science , aerodynamics
Transonic discs with accretion rates relevant to intrinsically bright Galactic X‐ray sources (L≈10 38 –10 39 erg s −1 ) exhibit a time‐dependent cyclic behaviour due to the onset of a thermal instability driven by radiation pressure. In this paper we calculate radiation spectra emitted from thermally unstable discs to provide detailed theoretical predictions for observationally relevant quantities. The emergent spectrum has been obtained by solving self‐consistently the vertical structure and radiative transfer in the disc atmosphere. We focus on four particular stages of the disc evolution, the maximal evacuation stage and three intermediate stages during the replenishment phase. The disc is found to undergo rather dramatic spectral changes during the evolution, emitting mainly in the 1–10 keV band during outburst and in the 0.1–1 keV band off‐outburst. Local spectra, although different in shape from a blackbody at the disc effective temperature, may be characterized in terms of a hardening factor f . We have found that f is more or less constant, both in radius and in time, with a typical value ∼ 1.65.