Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities

The “radiation inner edge” of an accretion disk is defined as t he inner boundary of the region from which most of the luminosity emerges. Similarly, the “reflection edge” is the smallest ra dius capable of producing a significant X-ray reflection of th e fluorescent iron line. For black hole accretion disks with very sub-Eddi ngton luminosities these and all other “inner edges” locate at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner edge of the black hole accretion disk. However, even for moderate luminosities, there is no such unique inner edge as differently defined edges locate at di fferent places. Several of them are significantly closer to the black hole than ISCO. The differences grow with the increasing luminosity. For nearly Eddington luminosities, they are so huge that the notion of the inner edge losses all practical significance.