M. Zamaninasab

Polarized NIR and X-ray flares from Sagittarius A

Context. Stellar dynamics indicate the presence of a supermassive 3−4 × 10 6 Mblack hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (SgrA*). Aims. The goal is the investigation and understanding of 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 Observatorys Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). Results. We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moder- ately bright flare event in the X-ray domain with an excess 2−8 keV luminosity of about 8 × 10 33 erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10 min. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. Conclusions. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of syn- chronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.

Simultaneous NIR/sub-mm observation of flare emission from Sagittarius A*

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 model ing. Results. The observations reveal strong flare activity in the 0.87 mm ( 345 GHz) sub-mm domain and in the 3.8µ/2.2µm 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 hours. 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 vexp∼ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of �=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 vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.

Modeling mm- to X-ray flare emission from Sagittarius A*

Context. We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive∼4×10 6 M⊙ black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from SgrA*. Methods. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Lar ge 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. The observations revealed several flare events in all wavele ngth domains. Here we show that the flare emission can be described with a combination of a synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components. The SSC emission at NIR and X-ray wavelengths involves up-scattered sub-millimeter photons from a compact source component. At the start of the flare, spectra of these components peak at fre quencies between several 100 GHz and 2 THz. The adiabatic expansion then accounts for the variable emission observed at sub-mm/mm wavelengths. The derived physical quantities that describe the flare emission give a blob expansion speed of vexp∼ 0.005c, magnetic field of B around 60 G or less and spectral indic es ofα=0.8 to 1.4, corresponding to a particle spectral index p∼2.6 to 3.8. Conclusions. A combined SSC and adiabatic expansion model can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.

The instrumental polarization of the Nasmyth focus polarimetric differential imager NAOS/CONICA (NACO) at the VLT - Implications for time-resolved polarimetric measurements of Sgr A*

Context. We report on the results of calibrating and simulating the instrumental polarization properties of the ESO VLT adaptive optics camera system NAOS/CONICA (NACO) in the Ks-band. Aims. Our goal is to understand the influence of systematic calibration effects on the time-resolved polarimetric observations of the infrared counterpart of the Galactic center super-massive black hole at the position of Sagittarius A* (Sgr A*). Methods. We use the Stokes/Mueller formalism for metallic reflections to describe the instrumental polarization. The model is compared to standard-star observations and time-resolved observations of bright sources in the Galactic center. The differences between calibration methods are simulated and tested for three polarimetric Ks-band light curves of Sgr A*. Results. We find the instrumental polarization to be highly dependent on the pointing position of the telescope and about 4% at maximum. Given the statistical uncertainties in the data acquisition, the systematic effects of the employed calibration method are negligible at high-time resolution, as it is necessary and achieved for in the case of Sgr A*. We report a polarization angle offset of 13.2 ◦ due to a position angle offset of the λ/2-wave plate with respect to the header value that affects the calibration of NACO data taken before autumn 2009. Conclusions. With the new model of the instrumental polarization of NACO it is possible to measure the polarization with an accuracy of 1% in polarization degree. The uncertainty of the polarization angle is ≤5 ◦ for polarization degrees ≥4%. For highly sampled polarimetric time series we find that the improved understanding of the polarization properties gives results that are fully consistent with the previously used method to derive the polarization. The small difference between the derived and the previously employed polarization calibration is well within the statistical uncertainties of the measurements, and for Sgr A* they do not affect the results from our relativistic modeling of the accretion process.

Comet-shaped sources at the Galactic center - Evidence of a wind from the central 0.2 pc

Context. In 2007 we reported two comet-shaped sources in the vicinity of Sgr A* (0.8 �� and 3.4 �� projected distance), named X7 and X3. The symmetry axes of the two sources are aligned to within 5 ◦ in the plane of the sky, and the tips of their bow shocks point towards Sgr A*. Our measurements show that the proper motion vectors of both features are pointing in directions more than 45 ◦ away from the line that connects them with Sgr A*. This misalignment of the bow-shock symmetry axes and their proper motion vectors, combined with the high proper motion velocities of several 100 km s −1 , suggest that the bow shocks must be produced by an interaction with some external fast wind, possibly coming from Sgr A*, or from stars in its vicinity. Aims. We have developed a bow-shock model to fit the observed morphology and constrain the source of the external wind. Methods. The result of our modeling gives the best solution for bow-shock standoff distances for the two features, which allows us to estimate the velocity of the external wind, making certain that all likely stellar types of the bow-shock stars are considered. Results. We show that neither of the two bow shocks (one of which is clearly associated with a stellar source) can be produced by the influence of a stellar wind of a single mass-losing star in the central parsec. Instead, an outflow carrying a momentum comparable to the one contributed by the ensemble of the massive young stars can drive shock velocities capable of producing the observed comet-shaped features. We argue that a collimated outflow arising perpendicular to the plane of the clockwise rotating stars (CWS) can easily account for the two features and the mini-cavity. However, the collective wind from the CWS has a scale of >10 �� .T he presence of a strong, mass-loaded outbound wind at projected distances from Sgr A* of <1 �� in fact agrees with models that predict a

Near infrared flares of Sagittarius A* - Importance of near infrared polarimetry

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 * . Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope and CIAO NIR camera on the Subaru telescope (13 june 2004; 30 july 2005; 1 june 2006; 15 may 2007; 17 may 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.

The extreme luminosity states of Sagittarius A

We discuss mm-wavelength radio, 2.2–11.8 μm NIR and 2–10 keV X-ray light curves of the super massive black hole (SMBH) counterpart of Sagittarius A* (SgrA*) near its lowest and highest observed luminosity states. We investigate the structure and brightness of the central S-star cluster harboring the SMBH to obtain reliable flux density estimates of SgrA* during its low luminosity phases. We then discuss the physical processes responsible for the brightest flare as well as the faintest flare or quiescent emission in the NIR and X-ray domain. To investigate the low state of SgrA* we use three independent methods to remove or strongly suppress the flux density contributions of stars in the central 2 �� diameter region around SgrA*. The three methods are: a) low-pass filtering the image; b) iterative identification and removal of individual stars; c) automatic point spread function (PSF) subtraction. For the lowest observed flux density state all 3 image reduction methods result in the detection of faint extended emission with a diameter of 0.5 �� –1.0 �� and centered on the position of SgrA*. We analyzed two datasets that cover the lowest luminosity states of SgrA* we observed to date. In one case we detect a faint K-band (2.2 μm) source of ∼4 mJy brightness (de-reddened with AK = 2.8) which we identify as SgrA* in its low state. In the other case no source brighter or equal to a de-reddened K-band flux density of ∼ 2m Jy was detected at that position. As physical emission mechanisms for SgrA* we discuss bremsstrahlung, thermal emission of a hypothetical optically thick disk, synchrotron and synchrotron self-Compton (SSC) emission, and in the case of a bright flare the associated radio response due to adiabatic expansion of the synchrotron radiation emitting source component. The luminosity during the low state can be interpreted as synchrotron emission from a continuous or even spotted accretion disk. For the high luminosity state SSC emission from THz peaked source components can fully account for the flux density variations observed in the NIR and X-ray domain. We conclude that at near-infrared wavelengths the SSC mechanism is responsible for all emission from the lowest to the brightest flare from SgrA*. For the bright flare event of 4 April 2007 that was covered from the radio to the X-ray domain, the SSC model combined with adiabatic expansion can explain the related peak luminosities and different widths of the flare profiles obtained in the NIR and X-ray regime as well as the non detection in the radio domain.

The near-infrared spectral index of Sagittarius A* derived from Ks- and H-band flare statistics

Context. The near-infrared (NIR) counterpart of Sagittarius A* (SgrA*) at the position of the 4 x 10(6) M(circle dot) supermassive black hole at the center of the Milky Way has strongly varying flux densities. The broad-band near-infrared spectral index is an essential parameter to determine the underlying emission mechanism for the observed flare emission. Aims. We present a method to derive the NIR spectral index of SgrA* between the H- and Ks-band from the statistics of the observed flare emission. Our spectral index derivation is therefore based on an unprecedentedly large timebase of about seven years of monitoring the infrared counterpart of SgrA*. Methods. We examined NIR light curves of SgrA* in the H- and Ks-band and established flare number distributions as a function of peak flare flux. We assume that in both bands the same optically thin dominant emission mechanism is at work and produces similar number distributions of flares. We cross-correlated these histograms and determined a statistical expectation value of the H-Ks-band spectral index during the bright phases of SgrA*. Results. With this new method, we can independently confirm that the expectation value of the spectral index for brighter flares is consistent with alpha = -0.7 (with the flux density (F(nu) proportional to nu(+alpha))) which is expected for pure synchrotron radiation. We find a tendency for weaker flares to exhibit a steeper spectrum. Conclusions. We conclude that the distribution of spectral indices as a function of Ks-band flux density can successfully be described by an exponential cutoff proportional to expleft perpendicular-(nu/nu(0))(0.5)right perpendicular because of synchrotron losses, with nu(0) being a characteristic cutoff frequency. Varying nu(0) between the NIR and sub-mm domain and assuming a sub-mm flux density variation of about one Jansky and optically thin spectral indices of alpha = -0.7 +/- 0.3 explains the observed spectral properties of SgrA* in the NIR.

Near-infrared polarimetry as a tool for testing properties of accreting supermassive black holes

Several massive black holes exhibit flux variability on time-scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray polarimetry to constrain physical properties of such black hole systems, namely, their spin and inclination. We have focused on a model where an orbiting hotspot is embedded in an accretion disc. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hotspot models that samples a wide range of parameter space. We found that the hotspot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamic simulations), then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin–inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) are presented. It is shown that even with the currently available data, the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool.

EXTENDED SUBMILLIMETER EMISSION OF THE GALACTIC CENTER AND NEAR-INFRARED/SUBMILLIMETER VARIABILITY OF ITS SUPERMASSIVE BLACK HOLE*

The innermost tens of parsecs of our Galaxy are characterized by the presence of molecular cloud complexes surrounding Sgr?A*, the radiative counterpart of the supermassive black hole (~4 ? 106 M ?) at the Galactic center. We seek to distinguish the different physical mechanisms that dominate the molecular clouds at the Galactic center, with special emphasis on the circumnuclear disk (CND). We also want to study the energy flow and model the variable emission of Sgr?A*. Our study is based on NIR and submillimeter (sub-mm) observations. Using sub-mm maps, we describe the complex morphology of the molecular clouds and the circumnuclear disk, along with their masses (of order 105-106 M ?), and derive also the temperature and spectral index maps of the regions under study. We conclude that the average temperature of the dust is 14 ? 4?K. The spectral index map shows that the 20 and 50?km?s?1 clouds are dominated by dust emission. Comparatively, in the CND and its surroundings the spectral indices decrease toward Sgr?A* and range between about 1 and ?0.6. These values are mostly explained with a combination of dust, synchrotron, and free-free emission in different ratios. The presence of non-thermal emission also accounts for the apparent low temperatures derived in these areas, indicating their unreliability. The Sgr?A* light curves show significant flux density excursions in both the NIR and sub-mm domains. We have defined a classification system to account for the NIR variability of Sgr?A*. Also, we have modeled on the NIR/sub-mm events. From our modeling results we can infer a sub-mm emission delay with respect to the NIR; we argue that the delay is due to the adiabatic expansion of the synchrotron source components.