The global structure of optically thin advection dominated accretion flows which are composed of two-temperature plasma around black holes is calculated. We adopt the full set of basic equations including the advective energy transport in the energy equation for the electrons. The spectra emitted by the optically thin accretion flows are also investigated. The radiation mechanisms which are taken into accout are bremsstrahlung, synchrotron emission, and Comptonization. The calculation of the spectra and that of the structure of the accretion flows are made to be completely consistent by calculating the radiative cooling rate at each radius. As a result of the advection domination for the ions, the heat transport from the ions to the electrons becomes practically zero and the radiative cooling balances with the advective heating in the energy equation of the electrons. Following up on the successful work of , we applied our model to the spectrum of Sgr A*. We find that the spectrum of Sgr A* is explained by the optically thin advection dominated accretion flow around a black hole of the mass M BH = 10 6 M ⊙ . The parameter dependence of the spectrum and the structure of the accretion flows is also discussed.

Using multi-frequency spectra from TeV blazars in quiescent states, we obtain the physical parameters of the emission region of blazars within the framework of the one-zone synchrotron self-Compton (SSC) model. We numerically calculate the steady-state energy spectra of electrons by self-consistently taking into account the effects of radiative cooling with a proper account of the Klein-Nishina effects. Here electrons are assumed to be injected with a power-law spectrum and to escape on a finite time scale, which naturally leads to the existence of a break energy scale. Although we do not use time variabilities but utilize a model of electron escape to constrain the size of the emission region, the resultant size turns out to be similar to that obtained based on time variabilities. Through detailed comparison of the predicted emission spectra with observations, we find that for Mrk 421, Mrk 501, and PKS 2155-304, the energy density of relativistic electrons is about an order of magnitude larger than that of magnetic fields with an uncertainty within a factor of a few.

The second-order Fermi acceleration (Fermi-II) driven by turbulence may be responsible for the electron acceleration in blazar jets. We test this model with time-dependent simulations. The hard electron spectrum predicted by the Fermi-II process agrees with the hard photon spectrum of 1ES 1101-232. For other blazars that show softer spectra, the Fermi-II model requires radial evolution of the electron injection rate and/or diffusion coefficient in the outflow. Such evolutions can yield a curved electron spectrum, which can reproduce the synchrotron spectrum of Mrk 421 from the radio to the X-ray regime. The photon spectrum in the GeV energy range of Mrk 421 is hard to fit with a synchrotron self-Compton model. However, if we introduce an external radio photon field with a luminosity of 4.9 ×10 38 erg s −1 , GeV photons are successfully produced via inverse Compton scattering. The temporal variability of the diffusion coefficient or injection rate causes flare emission. The observed synchronicity of X-ray and TeV flares implies a decrease of the magnetic field in the flaring source region.

We present global models of optically thin, advection-dominated two-temperature accretion flows onto black holes, paying careful attention to transonic properties of the flows. We treat the physical quantities integrated over the vertical direction, and adopt the standard a-viscosity and the pseudo-Newtonian potential. Bremsstrahlung and synchrotron cooling amplified by Comptonization is considered as to be cooling processes of electrons. It is found that when moderate synchrotron cooling is included, the electron temperature distribution in the inner part of the disk is roughly flat with temperatures slightly lower than the electron rest mass energy. This high electron temperature is appropriate to explain the hard X-rays from X-ray sources and AGNs. It is also found that in the inner part of the disks the energy input to electrons from ions by the ion-electron Coulomb collision is negligible as the electron heating, i.e., the electron temperature is determined by the balance between advective heating and synchrotron cooling.

Multiwavelength observations of blazars such as Mrk 421 and Mrk 501 show that they exhibit strong short time variabilities in flare-like phenomena. Based on the homogeneous synchrotron self-Compton (SSC) model and assuming that time variability of the emission is initiated by changes in the injection of nonthermal electrons, we perform detailed temporal and spectral studies of a purely cooling plasma system, using parameters appropriate to blazars. One important parameter is the total injected energy E and we show how the synchrotron and Compton components respond as E varies. When the synchrotron and SSC components have comparable peak fluxes, we find that the SSC process contributes strongly to the electron cooling and the whole system is nonlinear, thus simultaneously solving electron and photon kinetic equations is necessary. In the limit of the injection-dominated situation when the cooling timescale is long, we find a unique set of model parameters that are fully constrained by observable quantities. In the limit of cooling-dominated situation, TeV emissions arise mostly from a cooled electron distribution and Compton scattering process is always in the Klein-Nishina regime, which makes the TeV spectrum having a large curvature. Furthermore, even in a single injection event, the multiwavelength light-curves do not necessarily track each other because the electrons that are responsible for those emissions might have quite different lifetimes. We discuss in detail how one could infer important physical parameters using the observed spectra. In particular, we could infer the size of the emission region by looking for exponential decay in the light curves. We could also test the basic assumption of SSC by measuring the difference in the rate of peak energy changes of synchrotron and SSC peaks. We also show that the trajectory in the photon-index-flux plane evolves clockwise or counter-clockwise depending on the value of E and observed energy bands.