The flare activity of Sagittarius A* New coordinated mm to X-ray observations

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3-4 × 10⁶ M_⊙ black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from Sgr A*. Methods. 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 as well as the Submillimeter Array SMA** on Mauna Kea, Hawaii, and the Very Large Array*** in New Mexico. Results. We detected one moderately bright flare event in the X-ray domain and 5 events at infrared wavelengths. The X-ray flare had an excess 2-8 keV luminosity of about 33 × 10³³ erg/s. The duration of this flare was completely covered in the infrared and it was detected as a simultaneous NIR event with a time lag of ≤10 min. Simultaneous infrared/X-ray observations are available for 4 flares. All simultaneously covered flares, combined with the flare covered in 2003, indicate that the time-lag between the NIR and X-ray flare emission is very small and in agreement with a synchronous evolution. There are no simultaneous flare detections between the NIR/X-ray data and the VLA and SMA data. The excess flux densities detected in the radio and sub-millimeter domain may be linked with the flare activity observed at shorter wavelengths. Conclusions. We find that the flaring state can be explained with a synchrotron self-Compton (SSC) model involving up-scattered submillimeter photons from a compact source component. This model allows for NIR flux density contributions from both the synchrotron and SSC mechanisms. Indications for an exponential cutoff of the NIR/MIR synchrotron spectrum allow for a straightforward explanation of the variable and red spectral indices of NIR flares.