------------------------------------------------------------------------ ms.tex ApJ, 2004, in press Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII 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=-5.4, required 7, BAYES_01 -5.40, USER_AGENT_PINE 0.00) %astro-ph/0401429 \documentclass[12pt,preprint]{aastex} %\usepackage{apjfonts} %\usepackage{amsmath} \catcode`\@=11 % This allows us to modify PLAIN macros. \def\@versim#1#2{\vcenter{\offinterlineskip \ialign{$\m@th#1\hfil##\hfil$\crcr#2\crcr\sim\crcr } }} \def\be{\begin{equation}} \def\ee{\end{equation}} \def\mdot{\dot{m}} \def\ergs{{\rm\,erg\,s^{-1}}} \def\msun{M_{\odot}} \def\ergscc{\rm \ \ erg \ cm^{-3} \ s^{-1}} \def\gs{\rm \,g\,s^{-1}} %\def\ergscc{\rm \,erg\,cm^{-3}\,s^{-1}} %\def\ergs{\rm \,erg\,s^{-1}} %\def\be{\begin{equation}} %\def\ee{\end{equation}} \def\lsim{\mathrel{\mathpalette\@versim<}} \def\gsim{\mathrel{\mathpalette\@versim>}} \def\mpy{M_\odot \ {\rm yr^{-1}}} \begin{document} \title{On The Nature of the Variable Infrared Emission from Sgr A*} \author{Feng Yuan\altaffilmark{1}, Eliot Quataert\altaffilmark{2}, and Ramesh Narayan\altaffilmark{3}} IN 47907; fyuan@physics.purdue.edu} Californ ia, Berkeley, CA 94720; eliot@astron.berkeley.edu} \altaffiltext{3}{Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; narayan@cfa.harvard.edu} \begin{abstract} Recent infrared (IR) observations of the center of our Galaxy indicate that the supermassive black hole source Sgr A* is strongly variable in the IR. The timescale for the variability, $\sim 30$ min, is comparable to that of the X-ray flares observed by {\em Chandra} and {\em XMM}, suggesting a common physical origin. In this paper, we investigate the nature of the IR emission in the context of models recently proposed to interpret the X-ray flares. We show that the IR emission in Sgr A* can be well explained by nonthermal synchrotron emission if a small fraction of the electrons in the innermost region of the accretion flow around the black hole are accelerated {into a broken power-law distribution}, perhaps due to transient events such as magnetic reconnection. The model predicts differences in the variability amplitudes of flares in the IR and X-rays, in general agreement with observations. It also predicts that the IR emission should be linearly polarized, as has indeed been observed during one epoch. IR and X-ray flares analogous to those observed in Sgr A* may be detectable from other accreting SMBHs, provided $L \lsim 10^{-8} L_{EDD}$; at higher luminosities the flaring emission is dominated by thermal synchrotron-self Compton emission, which is likely to be less variable. \end{abstract} \keywords{accretion, accretion disks --- black hole physics --- galaxies: active --- Galaxy: center --- radiation mechanisms: non-thermal} ------------------------------- Feng Yuan Department of Physics Purdue University 1396 Physics Building West Lafayette, IN 47907+1396 Tel: 765 4945191 -------------------------------