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
Abstract:
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
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