Hot one-temperature accretion flows revisited

The effectiveness of the thermal coupling of ions and electrons in optically thin, hot accretion flows is investigated in a phenomenological approach. In the limit of complete coupling, we focus on the one-temperature accretion flows around black holes. Based on a global analysis, the results are compared with two-temperature accretion flow models and with the observations of black hole sources. Many features of one-and two-temperature solutions are quite similar. That is, hot one-temperature solutions are found to exist for mass flow rates less than a critical value, i.e.,. M less than or similar to 10 alpha M-2(Edd), where M-Edd = L-Edd/c(2) is the Eddington accretion rate. When. M less than or similar to 10(-3)alpha M-2(Edd), the viscous energy is mainly balanced by the advective cooling, i.e., the solution is in the advection-dominated accretion flow ( ADAF) regime. On the other hand, when 10(-3)alpha M-2(Edd) less than or similar to M less than or similar to 10 alpha M-2(Edd), radiative cooling is effective and is mainly balanced by advective heating, placing the solution in the regime of luminous hot accretion flow ( LHAF). When. M greater than or similar to 10 alpha M-2(Edd), the accretion flow collapses at a transition radius with only the standard optically thick and geometrically thin disk solution existing in the innermost regions. We have fitted the spectra of the two black hole sources with the one-temperature models, Sgr A* and XTE J1118+ 480, which have been examined successfully with two-temperature models. It is found that the one-temperature models do not provide acceptable fits to the multi-wavelength spectra of Sgr A* nor to XTE J1118+ 480 as a result of the higher temperatures characteristic of the one-temperature models. It is concluded that the thermal coupling of ions and electrons cannot be fully effective and that a two-temperature description is required.