------------------------------------------------------------------------
 tagger_melia05.tex Ap.J. Letters, accepted
Date: Mon, 21 Nov 2005 12:21:02 +0100
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%astro-ph/0511520
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%\shorttitle{A model for the flares of Sagittarius A*}
%\shortauthors{Tagger and Melia}

\begin{document}
\title{A Possible Rossby Wave Instability Origin for the Flares in  
Sagittarius A*}
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\author{Michel Tagger }
\affil{Service d'Astrophysique, (UMR AstroParticules et Cosmologie),  
CEA Saclay \\ 91191 Gif-sur-Yvette, France}
\email{tagger@cea.fr}
\author{Fulvio Melia$^1$}
\affil{Departments of Physics and Astronomy, The University of  
Arizona,\\
Tucson AZ 85721, USA}
\email{fmelia@as.arizona.edu}
\altaffiltext{1}{Sir Thomas Lyle Fellow and Miegunyah Fellow.}

\begin{abstract}
In recent years, near-IR and X-ray flares have been detected from the  
Galaxy's central
radio point source, Sagittarius A* (Sgr A*), believed to be a $\sim 3 
\times 10^6\;M_\odot$ supermassive
black hole. In some cases, the transient emission appears to be  
modulated with a (quasi-)period
of $\sim 17-20$ minutes. The implied $\sim 3\,r_S$ size of the  
emitter (where $r_S\equiv
2GM/c^2$ is the Schwarzschild radius) points to an instability--- 
possibly induced by accretion---near
the Marginally Stable Orbit (MSO) of a slowly spinning object. But  
Sgr A* is not accreting
via a large, `standard' disk; instead, the low density environment  
surrounding it apparently
feeds the black hole with low angular momentum clumps of plasma that  
circularize within
$\sim 10-300\,r_S$ and merge onto a compact, hot disk. In this {\it  
Letter}, we follow the
evolution of the disk following such an event, and show that
a Rossby wave instability, particularly in its magnetohydrodynamic  
form, grows rapidly
and produces a period of enhanced accretion lasting several hours.  
Both the amplitude of
this response, and its duration, match the observed flare  
characteristics rather well.
\end{abstract}
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\keywords{accretion---black hole physics---Galaxy: center--- 
magnetohydrodynamics---plasmas---Instabilities}
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astro-ph/0511520, to be published in Ap.J. Letters

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