First simultaneous NIR/X-ray detection of a flare from Sgr A

A. Eckart, F. K. Baganoff, M. Morris, M. W. Bautz, W. N. Brandt, G. P. Garmire, R. Genzel, T. Ott, G. R. Ricker, C. Straubmeier, T. Viehmann, R. Schödel, G. C. Bower, J. E. Goldston

Research output: Contribution to journalArticlepeer-review

159 Scopus citations


We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart associated with the massive 3-4 × 106 M⊙ black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope* and the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6 × 1033 erg/s. A fading flare of Sgr A* with >2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 h before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 h afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices αNIR/X-ray (Sν ν) of both states are quite comparable with a value of ∼1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 min.

Original languageEnglish (US)
Pages (from-to)1-11
Number of pages11
JournalAstronomy and Astrophysics
Issue number1
StatePublished - Nov 2004

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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