Advection-Dominated Accretion and the Black Hole Event Horizon

Understand

As the luminosity of an accreting black hole drops to a few percent of Eddington, the spectrum switches from the familiar soft state to a hard state that is well-described by a distended and tenuous advection-dominated accretion flow (ADAF).

• An ADAF is a poor radiator, and the ion temperature can approach 10^{12} K near the center, although the electrons are cooler, with their temperature typically capped at ~10^{9-11} K.
• The foundational papers predicted that the large thermal energy in an ADAF would drive strong winds and jets, as later observed and also confirmed in computer simulations.
• Of chief interest, however, is the accreting gas that races inward.