The slimming effect of advection on black-hole accretion flows

¹ Institut d’Astrophysique de Paris, CNRS et Sorbonne Universités, UPMC Paris 06, UMR 7095, 98bis Bd Arago, 75014 Paris, France
e-mail: lasota@iap.fr
² Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
³ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05508-090 São Paulo, SP, Brazil
⁴ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave, Cambridge, MA 02139, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁶ Physics Department, Gothenburg University, 412-96 Göteborg, Sweden

Received: 24 October 2015
Accepted: 23 December 2015

Abstract

Context. At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio H/R ≲ 1) or thick (H/R> 1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick.

Aims. We wish to establish under what conditions slim accretion flows become thick.

Methods. We use analytical equations, numerical 1 + 1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates.

Results. We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick.

Conclusions. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.