The Blandford-znajek Mechanism and Emission from Isolated Accreting Black Holes

In the presence of a magnetic field, rotational energy can be extracted from black holes via the Blandford-Znajek mechanism. We use self-similar advection dominated accretion (ADAF) models to estimate the efficiency of this mechanism for black holes accreting from geometrically thick disks, in the light of recent magnetohydrodynamic disk simulations, and show that the power from electromagnetic energy extraction exceeds the accretion luminosity for ADAFs at sufficiently low accretion rates. We consider the detectability of isolated stellar mass black holes accreting from the ISM, and show that for any rapidly rotating holes the efficiency of energy extraction, BZ ≡ LBZ/Ṁc, could reach 10−2. The estimated total luminosity would be consistent with the tentative identification of some EGRET sources as accreting isolated black holes, if that energy is radiated primarily as gamma rays. We discuss the importance of emission from the Blandford-Znajek mechanism for the spectra of other advection dominated accretion flows, especially those in low luminosity galactic nuclei. Subject headings: accretion, accretion disks — black hole physics — magnetic fields — galaxies: nuclei — gamma rays: observations — X-rays: galaxies