Evidence for X-Ray Synchrotron Emission from Simultaneous Mid-Infrared to X-Ray Observations of a Strong Sgr A* Flare
Genzel, R.; Rouan, D.; Pantin, E.; Hasinger, G.; Grosso, N.; Porquet, D.; Gillessen, S.; Falcke, H.; Eisenhauer, F.; Fritz, T. K.; Quataert, E.; Goldwurm, A.; Dodds-Eden, K.; Trap, G.; Haubois, X.; Yusef-Zadeh, F.; Clenet, Y.; Trippe, S.; Lagage, P. -O.; Bartko, H.; Ott, T.; Paumard, T.; Perrin, G.; Yuan, F.; Mascetti, L.
Germany, France, United States, Netherlands, China
Abstract
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 vprop νβ, consistent with νL ν vprop ν0.4) 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 1.2RS 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.