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 μm), Ks band (2.12 μm), and H band (1.66 μm), and with VISIR in N band (11.88 μm) 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 (β>0 for νL _ν ν^β, consistent with νL _ν ν^0.4) between 12 μm and 3.8 μm. 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 μm 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 νL _ν slope MIR to L′ band, and the soft νL _ν 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.