  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.
