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

We investigate accretion disc models for the X-ray emission of Seyfert-1 galaxies and the hard state of black-hole binaries. We concentrate on two hot accretion disc models: advection-dominated accretion flow (ADAF) and recently found luminous hot accretion flow (LHAF). We solve for the global solution of both ADAF and LHAF to obtain the electron temperature, Te, and Thompson optical depth, τ , 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 parameterized by a constant Compton parameter to model the case in which thermal Comptonization of external soft photon is important. We compare the calculated Te and τ with the observational values obtained by fitting the average spectra of Seyfert-1 galaxies and black-hole binaries using thermal Comptonization model. We find that the most favoured model is an LHAF with parameterized electron energy equation, with ADAFs predicting too high Te. Also, the LHAF, but not ADAF, can explain large luminosities in excess of 10 per cent of the Eddington luminosity seen in the hard state of transient black-hole binaries.