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%astro-ph/0603659

\documentclass[12pt,preprint]{aastex}
\begin{document}

\title{ANNIHILATION EMISSION FROM THE GALACTIC BLACK HOLE}

\author{K. S. Cheng\altaffilmark{1},  D. O. Chernyshov\altaffilmark{1,2}, 
and V. A. Dogiel\altaffilmark{1,3}}
\altaffiltext{1}{
  Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, 
China;}
  \altaffiltext{2}{ Moscow Institute of Physics and Technology, 
Institutskii lane, 141700 Moscow
Region,
   Dolgoprudnii, Russia;}
  \altaffiltext{3}{I.E.Tamm Theoretical Physics Division of P.N.Lebedev 
Institute, Leninskii pr, 53,
119991 Moscow, Russia.}

Both diffuse high energy gamma-rays and an extended electron-positron
annihilation line emission have been observed in the Galactic Center (GC)
region. Although X-ray observations indicate that the galactic black hole 
Sgr A$^*$ is inactive now, we suggest that Sgr A$^*$ can become active 
when a captured star is tidally disrupted and matter is accreted into the 
black hole.  As a consequence the galactic black hole could be a powerful 
source of relativistic protons. We are able to explain the current 
observed diffuse gamma-rays and the very detailed 511 keV annihilation 
line of secondary positrons by $p-p$ collisions of such protons, with 
appropriate injection times and energy. Relativistic protons could have 
been injected into the ambient material if the black hole captured a 
50M$_\odot$ star at several tens million years ago. An alternative 
possibility is that the black hole continues to capture stars with 
$\sim$1M$_\odot$ every hundred thousand years. Secondary positrons 
produced by $p-p$ collisions at energies $\ga 30$ MeV are cooled down to 
thermal energies by Coulomb collisions, and annihilate in the warm neutral 
and ionized phases of the interstellar medium with temperatures about 
several eV, because the annihilation cross-section reaches its maximum at 
these temperatures. It takes about ten million years for the positrons to 
cool down to thermal temperatures so they can diffuse into a very large 
extended region around the Galactic center. A much more recent star 
capture may be also able to account for recent TeV observations within 10 
pc of the galactic center as well as for the unidentified GeV gamma-ray 
sources found by EGRET at GC. The spectral difference between the GeV flux 
and the TeV flux could be explained naturally in this model as well.
\end{abstract}

\keywords{comic rays : general - Galaxy : center - Galaxies : gamma-rays - 
black hole - radiation mechanisms : %%@
nonthermal}

\end{document}