Accretion onto Black Holes: the Emergent Radiation Spectrum

The luminosity and frequency spectrum of radiation resulting from interstellar gas accreting onto a black hole are calculated. The model is that of spherically symmetric, steady-state accretion onto a nonrotating black hole at rest in the interstellar medium. A fully relativistic treatment (including special- and general-relativity effects) of both the fluid mechanics and radiation processes has been used. The principal radiation mechanisms are electron-proton and electron-electron bremsstrahlung. The results for a black hole of mass Al = 1 Al0 depend on whether the interstellar medium is an H I or an H ii region. For an H I region with n0 = 1 and T0 = 100 K, the total luminosity L = 3 x 1025 ergs 5 - . The spectrum has a bremsstrahlung shape with T 10 K. An H ii region with n5 = 1 cm-3 and T0 = 1Q000 K produces L = 2 x 10 ergs O The spectrum is again bremsstrahlung in appearance with T 1011 K. In both cases only a small fraction of the available thermal energy is radiated. For different values of no, T0, and Al the spectral distribution remains the same while the total luminosity L , provided (Al!Al0)(T0!100 K) 311n0 102 for the H I region and (Al/Al0)(T0/10,000 K)-2n0 i0 for the H ii region. The possibility of detecting this radiation and thereby identifying black holes is discussed. Subject headings: black holes interstellar matter radiative transfer - relativity