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\shorttitle{MID-IR TO X-RAY OBSERVATIONS OF A STRONG SGR A* FLARE}
\shortauthors{DODDS-EDEN ET AL.}

\begin{document}

\title{Evidence for X-ray synchrotron emission from simultaneous mid-IR 
to X-ray observations of a strong Sgr A* flare}
\author{K.~Dodds-Eden\altaffilmark{1}, D.~Porquet\altaffilmark{2}, 
G.~Trap\altaffilmark{3,4}, E.~Quataert\altaffilmark{5}, 
S.~Gillessen\altaffilmark{1}, N.~Grosso\altaffilmark{2}, 
E.~Pantin\altaffilmark{3,7}, H.~Falcke\altaffilmark{8,9}, 
D.~Rouan\altaffilmark{6},
A.~Goldwurm\altaffilmark{3,4}, F.~Yusef-Zadeh\altaffilmark{11},  
S.~Trippe\altaffilmark{1}, P.-O.~Lagage\altaffilmark{3,7}, 
H.~Bartko\altaffilmark{1}, F.~Eisenhauer\altaffilmark{1}, 
T.~Ott\altaffilmark{1}, T.~Paumard\altaffilmark{6}, 
G.~Perrin\altaffilmark{6}, F.~Yuan\altaffilmark{12}, 
T.K.~Fritz\altaffilmark{1}, L.~Mascetti\altaffilmark{1} }

Postfach 1312, D-85741, Garching, Germany.}
\altaffiltext{2}{Observatoire astronomique de Strasbourg, Universit{\'e} 
de Strasbourg, CNRS, INSU, 11 rue de l'Universit{\'e}, F-67000 
Strasbourg, France}
\altaffiltext{3}{CEA, IRFU, Service d'Astrophysique, Centre de Saclay, 
F-91191 Gif-surYvette, France.}
\altaffiltext{4}{AstroParticule et Cosmologie (APC), 10 rue Alice Domont 
et Leonie Duquet, F-75205 Paris, France.}
\altaffiltext{5}{Department of Astronomy, University of California, 
Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411.}
\altaffiltext{6}{LESIA, Observatoire de Paris, CNRS, UPMC, 
Universit{\'e} Paris Diderot; 5 Place Jules Janssen, 92190 Meudon, France}
\altaffiltext{7}{Laboratoire AIM, CEA - Centre de Saclay, F-91191 
Gif-sur-Yvette, France.}
\altaffiltext{8}{Department of Astrophysics, Institute for Mathematics, 
Astrophysics and Particle Physics, Radboud University, P.O. Box 9010, 
6500 GL Nijmegen, The Netherlands}
\altaffiltext{9}{ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The 
Netherlands}
\altaffiltext{10}{Department of Physics, University of California, 
Berkeley, 366 Le Comte Hall, Berkeley, CA 94720-7300}
\altaffiltext{11}{Department of Physics and Astronomy, Northwestern 
University, Evanston, Il. 60208}
\altaffiltext{12}{Shanghai Astronomical Observatory, Shanghai 200030, China}


\keywords{accretion, accretion disks --- black hole physics --- 
infrared: general --- radiation mechanisms: general --- Galaxy: center 
--- X-rays: general}

\begin{abstract}
This paper reports measurements of Sgr A* made with NACO in L$'$-band 
(3.80 $\mu$m), Ks-band (2.12 $\mu$m) and H-band (1.66 $\mu$m) and with 
VISIR in N-band (11.88 $\mu$m) at the ESO VLT\footnote{The Very Large 
Telescope (VLT) at the European Southern Observatory (ESO) on Paranal, 
Chile: Program IDs 179.B-0261(A) and 60.A-9234(A).}, as well as with 
XMM-Newton at X-ray (2-10 keV) wavelengths. On 4 April, 2007, a very 
bright flare was observed from Sgr A* simultaneously at L$'$-band and 
X-ray wavelengths. No emission was detected using VISIR. The resulting 
SED has a blue slope ($\beta > 0$ for $\nu L_\nu \propto \nu^{\beta}$, 
consistent with $\nu L_\nu \propto \nu^{0.4}$) between 12 micron and 3.8 
micron.

For the first time our high quality data allow a detailed comparison of 
infrared and X-ray light curves with a resolution of a few minutes. The 
IR and X-ray flares are simultaneous to within 3 minutes. However the IR 
flare lasts significantly longer than the X-ray flare (both before and 
after the X-ray peak) and prominent substructures in the 3.8 micron 
light curve are clearly not seen in the X-ray data. From the shortest 
timescale variations in the L$'$-band lightcurve we find that the 
flaring region must be no more than 1.2 $R_S$ in size.

The high X-ray to infrared flux ratio, blue $\nu L_\nu$ slope MIR to 
L$'$-band, and the soft $\nu L_\nu$ spectral index of the X-ray flare 
together place strong constraints on possible flare emission mechanisms. 
We find that it is quantitatively difficult to explain this bright X-ray 
flare with inverse Compton processes. A synchrotron emission scenario 
from an electron distribution with a cooling break is a more viable 
scenario.

\end{abstract}

\end{document}