Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare

K. Dodds-Eden (1), D. Porquet (2), G. Trap (3,4), E. Quataert (5), X. Haubois (6), S. Gillessen (1), N. Grosso (2), E. Pantin (3,7), H. Falcke (8,9), D. Rouan (6), R. Genzel (1,10), G. Hasinger (1), A. Goldwurm (3,4), F. Yusef-Zadeh (11), Y. Clenet (6), S. Trippe (1), P.-O. Lagage (3,7), H. Bartko (1), F. Eisenhauer (1), T. Ott (1), T. Paumard (6), G. Perrin (6), F. Yuan (12), T.K. Fritz (1), L. Mascetti (1)

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

Paper: ApJ, accepted

EPrint Server: 09033416


This paper reports measurements of Sgr A* made with NACO in L'-band (3.80 micron ), Ks-band (2.12 micron ) and H-band (1.66 micron ) and with VISIR in N-band (11.88 micron ) at the ESO VLT 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 RS 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.

Preprints available from the authors at katie@mpe.mpg.de , or the raw TeX (no figures) if you click here.

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