EVIDENCE FOR X-RAY SYNCHROTRON EMISSION FROM SIMULTANEOUS MID-INFRARED TO X-RAY OBSERVATIONS OF A STRONG Sgr A* FLARE

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, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On 2007 April 4, 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 spectral energy distribution has a blue slope (beta > 0 for nu L-nu proportional to nu(beta), consistent with nu L-nu proportional to nu(0.4)) between 12 mu m and 3.8 mu m. For the first time, our high-quality data allow a detailed comparison of infrared (IR) 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 mu m light curve are clearly not seen in the X-ray data. From the shortest timescale variations in the L'-band light curve, we find that the flaring region must be no more than 1.2R(S) in size. The high X-ray to IR 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.