Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of this kind there is a small but dense cluster of comoving sources (IRS13N) located ∼3 west of SgrA* just 0.5 north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr; or (2) very young, dusty stars with ages younger than 1 Myr. Aims. We present a first K s -band identification and proper motions of the IRS13N members, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sources in the central field. Goal is to constrain the nature of these source. Methods. The L -(3.8 μm) K s -(2.2 μm) and H-band (1.65 μm) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties, and by Gaussian fitting from high-pass filtered and deconvolved images. We also present results of near-infrared (NIR) H-and K s -band ESO-SINFONI integral field spectroscopy of the Galactic center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in K s -and L -band are identical. The HK−sL colors then indicate that the bright L -band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a K s -L -color. Conclusions. Most of the compact L -band excess emission sources have a compact H-or K s -band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are young dusty stars and that star formation at the Galactic center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star.

Context. We report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study the applicability of the adiabatic synchrotron source expansion model and study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. Methods. Our observations were carried out on 18 May 2009 using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the ACIS-I instrument aboard the Chandra X-ray Observatory, the LABOCA bolometer at the Atacama Pathfinder EXperiment (APEX), and the CARMA mm telescope array at Cedar Flat, California. Results. The X-ray flare had an excess 2−8 keV luminosity between 6 and 12×10 33 erg s −1 . The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from an adiabatically expanding synchrotron self-Compton (SSC) component.Modeling of the light curves shows that the sub-mm follows the NIR emission with a delay of about three-quarters of an hour with an expansion velocity of about v exp ∼ 0.009c. We find source component sizes of around one Schwarzschild radius, flux densities of a few Janskys, and spectral indices α of about +1 (S (ν) ∝ ν −α ). At the start of the flare, the spectra of the two main components peak just short of 1 THz. To statistically explain the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range of 300−400 GHz. For the pure synchrotron models, this results in densities of relativistic particles of the order of 10 6.5 cm −3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required.

Context. Recent near-infrared polarization measurements of Sgr A* show that its emission is significantly polarized during flares and consists of a non-or weakly polarized main flare with highly polarized sub-flares. The flare activity suggests a quasi-periodicity of ∼20 min in agreement with previous observations. Aims. By simultaneous fitting of the lightcurve fluctuations and the time-variable polarization angle, we address the question of whether these changes are consistent with a simple hot spot/ring model, in which the interplay of relativistic effects plays the major role, or whether some more complex dependency of the intrinsic emissivity is required. Methods. We discuss the significance of the 20 min peak in the periodogram of a flare from 2003. We consider all general relativistic effects that imprint on the polarization degree and angle and fit the recent polarimetric data, assuming that the synchrotron mechanism is responsible for the intrinsic polarization and considering two different magnetic field configurations. Results. Within the quality of the available data, we think that the model of a single spot in addition to an underlying ring is favoured. In this model the broad near-infrared flares of Sgr A* are due to a sound wave that travels around the MBH once while the sub-flares, superimposed on the broad flare, are due to transiently heated and accelerated electrons which can be modeled as a plasma blob. Within this model it turns out that a strong statement about the spin parameter is difficult to achieve, while the inclination can be constrained to values > ∼ 35• on a 3σ level.

Context. We report on the results of new simulations of near-infrared (NIR) observations of the Sagittarius A* (Sgr A*) counterpart associated with the super-massive black hole at the Galactic Center. Aims. Our goal is to investigate and understand the physical processes behind the variability associated with the NIR flaring emission from Sgr A*. 2007 and 28 may 2008). We used a model of synchrotron emission from relativistic electrons in the inner parts of an accretion disk. The relativistic simulations have been carried out using the Karas-Yaqoob (KY) ray-tracing code. Results. We probe the existence of a correlation between the modulations of the observed flux density light curves and changes in polarimetric data. Furthermore, we confirm that the same correlation is also predicted by the hot spot model. Correlations between intensity and polarimetric parameters of the observed light curves as well as a comparison of predicted and observed light curve features through a pattern recognition algorithm result in the detection of a signature of orbiting matter under the influence of strong gravity. This pattern is detected statistically significant against randomly polarized red noise. Expected results from future observations of VLT interferometry like GRAVITY experiment are also discussed. Conclusions. The observed correlations between flux modulations and changes in linear polarization degree and angle can be a sign that the NIR flares have properties that are not expected from purely random red-noise. We find that the geometric shape of the emission region plays a major role in the predictions of the model. From fully relativistic simulations of a spiral shape emitting region, we conclude that the observed swings of the polarization angle during NIR flares support the idea of compact orbiting spots instead of extended patterns. The effects of gravitational shearing, fast synchrotron cooling of the components and confusion from a variable accretion disk have been taken into account. Simulated centroids of NIR images lead us to the conclusion that a clear observation of the position wander of the center of NIR images with future infrared interferometers will prove the existence of orbiting hot spots in the vicinity of our Galactic super-massive black hole.

Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modeling. Results. The observations reveal strong flare activity in the 0.87 mm (345 GHz) sub-mm domain and in the 3.8 μ/2.2 μm NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5 ± 0.5 h. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of v exp ∼ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of α = 0.8 to 1.8, corresponding to particle spectral indices ∼2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz. Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.