Context. We report new simultaneous near-infrared/sub-millimeter/X-ray observations of the Sgr A* counterpart associated with the massive 3−4 × 10 6 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 × 10 33 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.

Abstract. We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart associated with the massive 3-4 × 10 6 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 × 10 33 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 α NIR/X−ray (S ν ∝ ν −α ) 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.

We present results from the first diffraction-limited images of the Galactic center (GC) at 1.6, 2.1, and 3.8 m with the new adaptive optics (AO) camera NAOS/CONICA at the ESO Very Large Telescope, as well as 3-4 m low-resolution spectroscopy. We have discovered a small (0.13 lt-yr diameter) cluster of compact sources about 0B5 north of IRS 13 with strong IR excesses due to T > 500 K dust. The nature of the sources is unclear. They may be a cluster of highly extincted stars that heat the local environment of the minispiral. We also consider an explanation that involves the presence of young stars at evolutionary stages between young stellar objects and Herbig Ae/Be objects with ages of about 0.1 to 1 million yr. This scenario would imply more recent star formation in the GC than previously suspected. The AO observations also resolve the central IRS 13 complex. In addition to the previously known bright stars E1 and E2, the K-and L 0 -band images for the first time resolve object E3 into two components, E3N and E3c. The latter one is closest to the 7 mm Very Large Array radio continuum source found at the location of the IRS 13 complex. E3c may be associated with a strong stellar wind or a dusty Wolf-Rayet-like star at that location.

Abstract. We present the first L-band spectroscopic observations of a dozen stellar sources in the central 0.5 pc of the GC stellar cluster that are bright in the 2−4 µm wavelength domain. The L-band data were taken with ISAAC at the VLT UT1 (Antu). With the aid of additional K-band spectroscopic data we derive the optical depth spectra of the sources after fitting their continuum emission with a single reddened blackbody continuum. We also derive intrinsic source spectra by correcting the line of sight extinction via the optical depth spectrum of a late type star that is most likely not affected by local dust emission or extinction at the Galactic Center. The good agreement between the two approaches shows that the overall variation of the line-of-sight extinction across the central 0.5 pc is ∆A K ≤ 0.5 mag. The extinction-corrected spectra of the hot He-stars resemble pure Rayleigh-Jeans continuum spectra. The intrinsic spectra of all other sources are in agreement with being the result of the continuum emission and absorption features due to the dust in which they are embedded. We interprete both facts as evidence that a significant amount of the absorption takes place within the central parsec of the Galactic Center and is most likely associated with the individual sources there. We find absorption features at 3.0 µm, 3.4 µm, and 3.48 µm wavelength. Correlations between all three features show that they are very likely to arise in the ISM of the central 0.5 pc. Spatially highly variable hydrogen emission lines seen towards the individual sources give evidence of the complex density and temperature structure of the mini-spiral. The featureless K-band spectra of sources like IRS 21 and IRS 1W are consistent with these sources being massive hot stars embedded in the bow shock created by their motion through the dust and gas of the mini-spiral. The bow shock scenario may be applicable to most of the dust-embedded sources in the central stellar cluster. Spectroscopy of high MIR-excess sources 0.5 north of the IRS 13 complex is largely consistent with them being YSOs. However, a bow-shock nature of these sources cannot be excluded. The L-band spectrum at the location of SgrA* closely resembles that of a hot O-type star, such as S2, which was very close to Sgr A* at the time of our observations.

We present mid-infrared N-and Q-band photometry of the Galactic Center from images obtained with the mid-infrared camera VISIR at the ESO VLT in May 2004. The high resolution and sensitivity possible with VISIR enables us to investigate a total of over 60 point-like sources, an unprecedented number for the Galactic Center at these wavelengths. Combining these data with previous results at shorter wavelengths (Viehmann et al. 2005) enables us to construct SEDs covering the H-to Q-band regions of the spectrum, i.e. 1.6 to 19.5 µm. We find that the SEDs of certain types of Galactic Center sources show characteristic features. We can clearly distinguish between luminous Northern Arm bow-shock sources, lower luminosity bow-shock sources, hot stars, and cool stars. This characterization may help clarify the status of presently unclassified sources.