Abstract
We investigate the effect of a hybrid electron population, consisting of both thermal and nonthermal particles, on the synchrotron spectrum, image size, and image shape of a hot accretion flow onto a supermassive black hole. We find two universal features in the emitted synchrotron spectrum: (1) a prominent shoulder at low (≲1011 Hz) frequencies that is weakly dependent on the shape of the electron energy distribution, and (2) an extended tail of emission at high (≳1013 Hz) frequencies whose spectral slope depends on the slope of the power-law energy distribution of the electrons. In the low-frequency shoulder, the luminosity can be up to 2 orders of magnitude greater than with a purely thermal plasma even if only a small fraction (<1%) of the steady state electron energy is in the nonthermal electrons. We apply the hybrid model to the Galactic center source, Sgr A*. The observed radio and IR spectra imply that at most 1% of the steady state electron energy is present in a power-law tail in this source. This corresponds to no more than 10% of the electron energy injected into the nonthermal electrons and hence 90% into the thermal electrons. We show that such a hybrid distribution can be sustained in the flow because thermalization via Coulomb collisions and synchrotron self-absorption are both inefficient.
Highlights
The mechanisms of particle heating and acceleration, and the emission spectra from the resulting particle energy distributions, are of great importance in the theory of collisionless hot accretion flows onto compact objects
We identify the characteristic signatures of the non-thermal electrons on the emitted radio synchrotron spectrum of an accretion flow and on its image as observed with a radio telescope
In this paper we considered hot accretion flows around supermassive black holes, using the advection-dominated accretion flow (ADAF) model as a typical example
Summary
The mechanisms of particle heating and acceleration, and the emission spectra from the resulting particle energy distributions, are of great importance in the theory of collisionless hot accretion flows onto compact objects. Quataert and Gruzinov (1999; see Gruzinov & Quataert 1999) considered two processes specific to MHD turbulence that accelerate particles in magnetic collisionless plasmas: Landau damping by electric fields parallel to the local magnetic field and transit-time damping by time-varying magnetic fields They found that the assumption of negligible electron heating/acceleration is valid only for weak magnetic fields, i.e., when the ratio of the gas pressure to total pressure βADAF is larger than a critical value βcrit. Shocks and pion decay can lead to non-thermal electrons in the accretion flow Several processes, such as Coulomb collisions and synchrotron self-absorption, can potentially lead to thermalization of particles in accretion flows (Mahadevan & Quataert 1997; Ghisellini et al 1998). We present in an Appendix approximate analytic expressions for the contribution of a non-thermal particle population to the synchrotron spectrum of an accretion flow
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