The flare activity of Sagittarius A*

Abstract

Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the SgrA* counterpart associated with the massive 3–4×106M⊙ black hole at the Galactic Center. Aims. The main aim is to investigate the physical processes responsible for the variable emission from SgrA*. 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 minutes. For 4 flares simultaneous infrared/X-ray observations are available. 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 sub-millimeter 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 straight forward explanation of the variable and red spectral indices of NIR flares.