------------------------------------------------------------------------
From: Feryal Ozel  fozel@cfa.harvard.edu 
To: Galactic Center Newsletter <gcnews@aoc.nrao.edu>
Subject:astro-ph/0004195
MIME-Version: 1.0

%astro-ph/0004195
\title{Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion
Flows}
\author{Feryal \"Ozel\altaffilmark{1}, Dimitrios Psaltis, and Ramesh
Narayan}
\affil{Harvard-Smithsonian Center for Astrophysics, 60 Garden 
Street, Cambridge, MA 02138; \\
fozel,dpsaltis,rnarayan@cfa.harvard.edu}
\altaffiltext{1}{Physics Department, Harvard University}


\begin{abstract}
  
  We investigate the effect of a hybrid electron population,
  consisting of both thermal and non-thermal 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: (i) a prominent shoulder at low
  $(\lesssim 10^{11}~\rm{Hz})$ frequencies that is weakly dependent on
  the shape of the electron energy distribution, and (ii) an extended
  tail of emission at high $(\gtrsim 10^{13}~\rm{Hz})$ 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 two 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 non-thermal 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 non-thermal 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.
  
  The presence of non-thermal electrons enlarges the size of the radio
  image at low frequencies and alters the frequency dependence of the
  brightness temperature. A purely thermal electron distributions
  produces a sharp-edged image while a hybrid distribution causes
  strong limb brightening. These effects can be seen up to frequencies
  $\sim 10^{11}$~Hz and are accessible to radio interferometers.

\end{abstract}

\keywords{accretion, accretion flows -- black hole
physics -- radiation mechanisms: thermal and non-thermal synchrotron -- 
Galaxy: center}

\centerline{To appear in {\it The Astrophysical Journal}}


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