Leaving the innermost stable circular orbit: the inner edge of a black-hole accretion disk at various luminosities

¹ Department of Physics, Göteborg University, 412-96 Göteborg, Sweden e-mail: Marek.Abramowicz@physics.gu.se
² N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland e-mail: [as;agata]@camk.edu.pl
³ Institut d'Astrophysique de Paris, UMR 7095 CNRS, UPMC Univ Paris 06, 98bis Bd Arago, 75014 Paris, France e-mail: lasota@iap.fr
⁴ Institute of Physics, Faculty of Philosophy and Science, Silesian University in Opava, Bezručovo nám. 13, 746-01 Opava, Czech Republic
⁵ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland e-mail: mj@astrouw.edu.pl
⁶ 2-2-2 Shikanodai-Nishi, Ikoma-shi, Nara 630-0114, Japan e-mail: kato.shoji@gmail.com, kato@kusastro.kyoto-u.ac.jp
⁷ Jagiellonian University Observatory, ul. Orla 171, 30-244 Kraków, Poland

Received: 19 March 2010
Accepted: 26 May 2010

Abstract

The “radiation inner edge” of an accretion disk is defined as the inner boundary of the region from which most of the luminosity emerges. Similarly, the “reflection edge” is the smallest radius capable of producing a significant X-ray reflection of the fluorescent iron line. For black hole accretion disks with very sub-Eddington luminosities these and all other “inner edges” coexist at the innermost stable circular orbit (ISCO). Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even at moderate luminosities, there is no such unique inner edge because differently defined edges are located at different places. Several of them are significantly closer to the black hole than ISCO. These differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge loses all practical significance.