------------------------------------------------------------------------
 flare.tex ApjL, submitted
X-Enigmail-Version: 0.86.0.0
X-Enigmail-Supports: pgp-inline, pgp-mime
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
X-Authenticated-Id: ste
X-MailScanner-Information: Please contact postmaster@aoc.nrao.edu for more information
X-MailScanner: Found to be clean
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 5,
	autolearn=disabled)
X-MailScanner-From: ste@mpe.mpg.de

%astro-ph/0511302
%http://www.mpe.mpg.de/~ste/ms_gillessen.pdf

\documentclass[12pt,preprint]{aastex}

\shorttitle{Spectral slope of Sgr~A* during a NIR flare}
\shortauthors{Gillessen et al.}

\begin{document}

\title{Variations in the spectral slope of Sgr~A* during a NIR flare}

\author{S.~Gillessen$^1$, F.~Eisenhauer$^1$, E.~Quataert$^2$, 
R.~Genzel$^{1,\,3}$,
T.~Paumard$^1$, S.~Trippe$^1$,
T.~Ott$^1$, R.~Abuter$^1$,
A.~Eckart$^4$, P.~O.~Lagage$^5$, M.~D.~Lehnert$^1$, L.~J.~Tacconi$^1$, 
F.~Martins$^1$}
\affil{$^1\,$Max-Planck-Institut f\"ur Extraterrestrische Physik, 85748 
Garching, Germany}
\affil{$^2\,$Astronomy Department, University of California, Berkeley, 
CA 94720, USA}
\affil{$^3\,$Physics Department, University of California, Berkeley, CA 
94720, USA}
\affil{$^4\,$1. Physikalisches Institut, Universit\"at zu K\"oln, 
Z\"ulpicher Str. 77, 50937 K\"oln, Germany}
\affil{$^5\,$UMR 7158, CEA-CNRS-Universit\'e Paris 7, DSM/DAPNIA/Service 
d'Astrophysique, CEA/Saclay, France}

\begin{abstract}
We have observed a bright flare of Sgr~A* in the near infrared
with the adaptive optics assisted integral field
spectrometer SINFONI\footnote{This work is based on observations
collected at the European Southern Observatory, Paranal, Chile. Program 
ID: 075.B-0547(B)}.
The observed spectrum is featureless and can be described
by a power law. We show
for the first time that the spectral index
is correlated with the instantaneous flux and that both
flux and spectral index
experience significant changes within less than one hour.
We argue that the near infrared flares from Sgr~A* are due to 
synchrotron emission of
transiently heated electrons, the emission being
affected by orbital dynamics and synchrotron cooling, both
acting on timescales of $\sim 20$ minutes.
The synchrotron cooling process may account for
the observed variation in spectral index, which in turn
requires a magnetic field $\sim 30\,$G, consistent with
the equipartition field in a hot accretion flow with
an accretion rate of $\sim 10^{-8}\,M_\odot/$yr.
\end{abstract}

\keywords{blackhole physics --- Galaxy: center --- infrared: stars --- 
techniques: spectroscopic}
\end{document}