------------------------------------------------------------------------ ms.tex ApJ, Submitted From: Siming Liu Reply-To: liusm@lanl.gov To: gcnews@aoc.nrao.edu Content-Type: text/plain Organization: Theoretical Astrophysics Date: Mon, 06 Mar 2006 12:00:48 -0700 Message-Id: <1141671648.4464.13.camel@regge.lanl.gov> Mime-Version: 1.0 X-Mailer: Evolution 2.0.4 (2.0.4-7) Content-Transfer-Encoding: 7bit X-PMX-Version: 4.7.1.128075 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: liusm@lanl.gov %astro-ph/0603137 \documentclass[12pt,preprint]{aastex} \title{Stochastic Acceleration in the Galactic Center HESS Source} \author{Siming Liu,\altaffilmark{1} Fulvio Melia,\altaffilmark{2,3} Vah\'e Petrosian,\altaffilmark{4} and Marco Fatuzzo\altaffilmark{5}} liusm@lanl.edu} \altaffiltext{2}{Physics Department and Steward Observatory, The University of Arizona, Tucson, AZ 85721; melia@physics.arizona.edu} \altaffiltext{3}{Sir Thomas Lyle Fellow and Miegunyah Fellow.} \altaffiltext{4}{Center for Space Science and Astrophysics, Department of Applied Physics, Stanford University, Stanford, CA 94305; vahe@astronomy.stanford.edu} \altaffiltext{5}{Physics Department, Xavier University, Cincinnati, OH 45207} \begin{abstract} Stochastic acceleration of charged particles interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s mm and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated {\it in situ}. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air {\v C}erenkov telescopes, notably HESS, have detected TeV emission from within $1^\prime$ of the Galactic Center, with characteristics hinting at a pp-induced pion decay process for the $\gamma$-ray emission. Given (i) that we now know the size of this acceleration region, where Sagittarius A*'s 7-mm wavelength emission originates, and (ii) that we can now map the wind-injected ISM within $\sim 3$ pc of the nucleus using the diffuse X- rays detected with {\it Chandra}, it is feasible to test the idea that protons accelerated within $\sim 20$ Schwarzschild radii of the black hole produce the TeV emission farther out. We show that the diffusion length of these particles away from their source guarantees a majority of TeV protons scattering at least once within $\sim 3$ pc of Sagittarius A*, and we demonstrate that the proton power ($\sim 10^{37}$ ergs s$^{-1}$) produced in concert with the 7-mm radio emission matches the TeV luminosity well. The particle cascade generated by the pp scatterings produces GeV $\gamma$-rays from $\pi^0$ decays, and bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey. This model explains why the TeV source is unresolved, yet does not vary on a time scale of a year or less, and it also accounts for the high-energy emission while retaining consistency with Sgr A*'s well- studied cm and mm characteristics. \end{abstract} \end{document} -- Siming Liu T-6, MS B227 Los Alamos National Laboratory Los Alamos, NM 87545 Tel: 505-667-3862 Fax: 505-664-0007