Luminous hot accretion flows: the origin of X-ray emission from Seyfert galaxies and black hole binaries

We investigate accretion disc models for the X-ray emission of type 1 Seyfert galaxies and the hard state of black hole binaries. We concentrate on two hot accretion disc models: advection-dominated accretion flow( ADAF) and the recently found luminous hot accretion flow( LHAF). We solve for the global solution of both ADAF and LHAF to obtain the electron temperature, T-e, and Thomson optical depth, tau, at the radius where most of the radiation comes from. We adopt two kinds of electron energy equations. In one, only synchrotron and bremsstrahlung radiation and their Comptonization are considered. The other is parametrized by a constant Compton parameter to model the case in which thermal Comptonization of external soft photons is important. We compare the calculated T-e and with the observational values obtained by fitting the average spectra of type 1 Seyfert galaxies and black hole binaries using the thermal-Comptonization model. We find that the most favoured model is an LHAF with a parametrized electron energy equation, with ADAFs predicting a too-high T-e. Also, LHAF, but not ADAF, can explain the large luminosities in excess of 10 per cent of the Eddington luminosity seen in the hard state of transient black hole binaries.