Evidence for X-ray synchrotron emission from simultaneous mid-infrared to X-ray observations of a strong Sgr A* flare

K. Dodds-Eden, D. Porquet, G. Trap, E. Quataert, X. Haubois, S. Gillessen, N. Grosso, E. Pantin, H. Falcke, D. Rouan, R. Genzel, G. Hasinger, A. Goldwurm, F. Yusef-Zadeh, Y. Clenet, S. Trippe, P. O. Lagage, H. Bartko, F. Eisenhauer, T. OttT. Paumard, G. Perrin, F. Yuan, T. K. Fritz, L. Mascetti

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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 νβ, consistent with νL ν 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.

Original languageEnglish (US)
Pages (from-to)676-692
Number of pages17
JournalAstrophysical Journal
Issue number1
StatePublished - 2009
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Accretion, accretion disks
  • Black hole physics
  • Galaxy: center
  • Infrared: general
  • Radiation mechanisms: general
  • X-rays: general


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