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

TL;DR

This paper investigates how the inner edge of black hole accretion disks varies with luminosity, showing that at higher luminosities, the inner edge can be significantly closer to the black hole than the ISCO, challenging previous assumptions.

Contribution

It demonstrates that the inner edge of accretion disks is not fixed at ISCO at higher luminosities, highlighting the need to reconsider models of black hole accretion.

Findings

Inner edges shift closer to the black hole at higher luminosities.

Differences between various inner edge definitions grow with luminosity.

At near Eddington luminosities, the concept of a fixed inner edge becomes impractical.

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" 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 different 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.