We address a question whether the observed light curves of X-ray flares originating deep in galactic cores can give us independent constraints on the mass of the central supermassive black hole. To this end, we study four brightest flares which have been recorded from Sagittarius A*. They all exhibit an asymmetric shape consistent with a combination of two intrinsically separate peaks which occur at a certain time delay with respect to each other, and are characterized by their mutual flux ratio and the profile of raising/declining parts. Such asymmetric shapes arise naturally in the scenario of a temporary flash from a source orbiting near a supermassive black hole, at a radius of only similar to 10-20 gravitational radii. An interplay of relativistic effects is responsible for the modulation of the observed light curves: Doppler boosting, gravitational redshift, light focusing and light-travel time delays. We find the flare properties to be in agreement with the simulations (our ray-tracing code SIM5LIB). The inferred mass for each of the flares comes out in agreement with previous estimates based on orbits of stars; the latter have been observed at radii and over time-scales two orders of magnitude larger than those typical for the X-ray flares, so the two methods are genuinely different. We test the reliability of the method by applying it to another object, namely, the Seyfert I galaxy RE J1034+396.

We summarize basic observational results on Sagittarius~A* obtained from the radio, infrared and X-ray domain. Infrared observations have revealed that a dusty S-cluster object (DSO/G2) passes by SgrA*, the central super-massive black hole of the Milky Way. It is still expected that this event will give rise to exceptionally intense activity in the entire electromagnetic spectrum. Based on February to September 2014 SINFONI observations. The detection of spatially compact and red-shifted hydrogen recombination line emission allows a us to obtain a new estimate of the orbital parameters of the DSO. We have not detected strong pre-pericenter blue-shifted or post-pericenter red-shifted emission above the noise level at the position of SgrA* or upstream the orbit. The periapse position was reached in May 2014. Our 2004-2012 infrared polarization statistics shows that SgrA* must be a very stable system - both in terms of geometrical orientation of a jet or accretion disk and in terms of the variability spectrum which must be linked to the accretion rate. Hence polarization and variability measurements are the ideal tool to probe for any change in the system as a function of the DSO/G2 fly-by. Due to the 2014 fly-by of the DSO, increased accretion activity of SgrA* may still be upcoming. Future observations of bright flares will improve the derivation of the spin and the inclination of the SMBH from NIR/sub-mm observations.

The Messenger 159 – March 2015 Banafsheh Shahzamanian1,2 Andreas Eckart1,2 Monica Valencia-S.1 Gunther Witzel3 Mohammad Zamaninasab2 Michal Zajaček1,2,4 Nadeen Sabha1 Macarena García-Marín1 Vladimir Karas4 Florian Peissker1 Grischa D. Karssen1 Marzieh Parsa1 Nicolas Grosso5 Enmanuelle Mossoux5 Delphine Porquet5 Behrang Jalali1 Matthew Horrobin1 Rainer Buchholz1 Michal Dovčiak4 Devaky Kunneriath4 Michal Bursa4 Anton Zensus2,1 Rainer Schödel6 Jihane Moultaka7 Christian Straubmeier1

We present a statistical analysis of polarized near-infrared light from Sgr A*, the radio source associated with the supermassive black hole at the center of the Milky Way. The observations were carried out using the adaptive optics instrument NACO at the VLT UT4 in the infrared K-s-band from 2004 to 2012. Several polarized flux excursions were observed during these years. Linear polarization at 2.2 mu m, its statistics, and time variation, can be used constrain the physical conditions of the accretion process onto this supermassive black hole. With an exponent of about 4 for the number density histogram of fluxes above 5 mJy, the distribution of polarized flux density is closely linked to the single state power-law distribution of the total K-s-band flux densities reported earlier. We find typical polarization degrees on the order of 20% +/- 10% and a preferred polarization angle of 13 degrees +/- 15 degrees. Simulations show the uncertainties under a total flux density of similar to 2 mJy are probably dominated by observational effects. At higher flux densities there are intrinsic variations of polarization degree and angle within well constrained ranges. Since the emission is most likely due to optically thin synchrotron radiation, the preferred polarization angle we find is very likely coupled to the intrinsic orientation of the Sgr A* system, i.e. a disk or jet/wind scenario associated with the supermassive black hole. If they are indeed linked to structural features of the source the data imply a rather stable geometry and accretion process for the Sgr A* system.

Based on the monitoring of the Dusty S-cluster Object (DSO/G2) during its closest approach to the Galactic Center supermassive black hole in 2014 and 2015 with ESO VLT/SINFONI, we further explore the model of a young, accreting star to explain observed spectral and morphological features. The stellar scenario is supported by our findings, i.e. ionized-hydrogen emission from the DSO that remains spatially compact before and after the peribothron passage. The detection of DSO/G2 object as a compact single-peak emission-line source is not consistent with the original hypothesis of a core-less cloud that is necessarily tidally stretched, hence producing a double-peak emission line profile around the pericentre passage. This strengthens the evidence that the DSO/G2 source is a dust-enshrouded young star that appears to be in an accretion phase. The infall of material from the circumstellar disc onto the stellar surface can contribute significantly to the emission of Br$\gamma$ line as well as the observed large line width of the order of 10 angstrom.

We analyse and report in detail new near-infrared (1.45 - 2.45 microns) observations of the Dusty S-cluster Object (DSO/G2) during its approach to the black hole at the center of the Galaxy that were carried out with ESO VLT/SINFONI between February and September 2014. Before May 2014 we detect spatially compact Br-gamma and Pa-alpha line emission from the DSO at about 40mas east of SgrA*. The velocity of the source, measured from the red-shifted emission, is 2700+-60 km/s. No blue-shifted emission above the noise level is detected at the position of SgrA* or upstream the presumed orbit. After May we find spatially compact Br-gamma blue-shifted line emission from the DSO at about 30mas west of SgrA* at a velocity of -3320+-60 km/s and no indication for significant red-shifted emission. We do not detect any significant extension of velocity gradient across the source. We find a Br-gamma-line full width at half maximum of 50+-10 Angstroem before and 15+-10 Angstroem after the peribothron transit, i.e. no significant line broadening with respect to last year is observed. Br-gamma line maps show that the bulk of the line emission originates from a region of less than 20mas diameter. This is consistent with a very compact source on an elliptical orbit with a peribothron time passage in 2014.39+-0.14. For the moment, the flaring activity of the black hole in the near-infrared regime has not shown any statistically significant increment. Increased accretion activity of SgrA* may still be upcoming. We discuss details of a source model according to which the DSO is rather a young accreting star than a coreless gas and dust cloud.

A fast moving infrared excess source (G2) which is widely interpreted as a core-less gas and dust cloud approaches Sagittarius A* (SgrA*) on a presumably elliptical orbit. VLT K_s-band and Keck K'-band data result in clear continuum identifications and proper motions of this about 19m Dusty S-cluster Object (DSO). In 2002-2007 it is confused with the star S63, but free of confusion again since 2007. Its near-infrared (NIR) colors and a comparison to other sources in the field speak in favor of the DSO being an IR excess star with photospheric continuum emission at 2 microns than a core-less gas and dust cloud. We also find very compact L'-band emission (<0.1”) contrasted by the reported extended (0.03” up to about 0.2” for the tail) Brgamma emission. The presence of a star will change the expected accretion phenomena, since a stellar Roche lobe may retain a fraction of the material during and after the peri-bothron passage.

The super-massive 4 million solar mass black hole (SMBH) SgrA* shows flare emission from the millimeter to the X-ray domain. The nucleus of the Milky Way has properties (stellar cluster, young stars, molecular gas and an accreting SMBH) that resemble those of currently higher luminous Low Luminosity Active Galactic Nuclei. A detailed analysis of the infrared light curves shows that the flares are probably generated in a single-state process forming a power-law distribution of the flux density. Near-infrared polarimetry shows signatures of strong gravity that are statistically significant against randomly polarized red noise. Details of the emission mechanism are discussed in a synchrotron/self-Compton model. SgrA* also allows to study the interaction of the SMBH with the immediate interstellar and gaseous environment of the central stellar cluster. Through infrared imaging of the central few arcseconds it is possible to study both inflow and outflow phenomena linked to the SgrA* black hole. In this context we also discuss the newly found dusty object that approaches SgrA* and present a comparison between recent Keck and VLT K-band data that clearly supports its detection as a about 19m K'-band continuum source.

The near-infrared (NIR) emission of Sagittarius A* (Sgr A*), the source associated with the supermassive black hole (4.4 x 10(6) M-circle dot) at the center of our galaxy, is polarized and highly variable. Correlations between intensity and polarimetric parameters of the observed light curves compared with the predicted ones for different configurations, allow us to extract information about the geometry of the radiating region. Here we present the theoretical polarimetric light curves expected in the case of optically thin NIR emission from overdense regions close to the marginal stable orbit. Using a numerical code we track the time evolution of detectable polarization properties produced by synchrotron emission of compact sources in the vicinity of the black hole. We show that the different setups lead to very distinctive patterns in the time profiles of polarized flux and the orientation of the polarization vector and as such may be used for determining the geometry of the accretion flow around Sgr A*.