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Astrophysical Journal 698 (2009) 676-692
Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare
K. Dodds-Eden1, D. Porquet2, G. Trap3, 4, E. Quataert, X. Haubois5, S. Gillessen1, N. Grosso2, E. Pantin4, 6, H. Falcke, D. Rouan5, R. Genzel1, G. Hasinger1, A. Goldwurm3, 4, F. Yusef-Zadeh, Y. Clenet5, S. Trippe1, P.-O. Lagage4, 6, H. Bartko1, F. Eisenhauer1, T. Ott1, T. Paumard5, G. Perrin5, F. Yuan, T. K. Fritz1, L. Mascetti

This paper reports measurements of Sgr A* made with NACO in L' -band (3.80 um), Ks-band (2.12 um) and H-band (1.66 um) and with VISIR in N-band (11.88 um) at the ESO VLT, 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 nuL_nu ~ nu^beta, consistent with nuL_nu ~ 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 nuL_nu slope MIR to L' -band, and the soft nuL_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.
1 :  MPE - Max Planck Institute for Extraterrestrial Physics
2 :  OAS - Observatoire astronomique de Strasbourg
3 :  APC - UMR 7164 - AstroParticule et Cosmologie
4 :  IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers (ex DAPNIA)
5 :  OP - Observatoire de Paris - Site de Paris
6 :  AIM - UMR 7158 - Astrophysique Interactions Multi-échelles
Physique/Astrophysique/Phénomènes cosmiques de haute energie

Planète et Univers/Astrophysique/Phénomènes cosmiques de haute energie
accretion – accretion disks – black hole physics – Galaxy: center – infrared: general – radiation mechanisms: general – X-rays: general
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