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We investigate the effect of a hybrid electron population, consisting of boththermal and non-thermal particles, on the synchrotron spectrum, image size, andimage shape of a hot accretion flow onto a supermassive black hole. We find twouniversal features in the emitted synchrotron spectrum: (i) a prominentshoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shapeof the electron energy distribution, and (ii) an extended tail of emission athigh (> 10^13 Hz) frequencies whose spectral slope depends on the slope of thepower-law energy distribution of the electrons. In the low-frequency shoulder,the luminosity can be up to two orders of magnitude greater than with a purelythermal plasma even if only a small fraction (< 1%) of the steady-stateelectron energy is in the non-thermal electrons. We apply the hybrid model tothe Galactic center source, Sgr A*. The observed radio and IR spectra implythat at most 1% of the steady-state electron energy is present in a power-lawtail in this source. This corresponds to no more than 10% of the electronenergy injected into the non-thermal electrons and hence 90% into the thermalelectrons. We show that such a hybrid distribution can be sustained in the flowbecause thermalization via Coulomb collisions and synchrotron self-absorptionare both inefficient. The presence of non-thermal electrons enlarges the sizeof the radio image at low frequencies and alters the frequency dependence ofthe brightness temperature. A purely thermal electron distributions produces asharp-edged image while a hybrid distribution causes strong limb brightening.These effects can be seen up to frequencies ~10^11 Hz and are accessible toradio interferometers.Comment: 33 pages with figures, to appear in the Astrophysical Journa

Hybrid Thermal‐Nonthermal Synchrotron Emission from Hot Accretion Flows