N. Sabha

SOURCE-INTRINSIC NEAR-INFRARED PROPERTIES OF SGR A*: TOTAL INTENSITY MEASUREMENTS

We present a comprehensive data description for K s-band measurements of Sgr A*. We characterize the statistical properties of the variability of Sgr A* in the near-infrared, which we find to be consistent with a single-state process forming a power-law distribution of the flux density. We discover a linear rms-flux relation for the flux density range up to 12?mJy on a timescale of 24?minutes. This and the power-law flux density distribution implies a phenomenological, formally nonlinear statistical variability model with which we can simulate the observed variability and extrapolate its behavior to higher flux levels and longer timescales. We present reasons why data with our cadence cannot be used to decide on the question whether the power spectral density of the underlying random process shows more structure at timescales between 25?minutes and 100?minutes compared to what is expected from a red-noise random process.

Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic Center

Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature a nd origin is unclear. In addition to several isolated objects o f 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 member s, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sour ces in the central field. Goal is to constrain the nature of the se source. Methods. The L ′ - (3.8� m) Ks- (2.2� m) and H-band (1.65� m) 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 functi on of time, weighted by positional uncertainties, and by Gaussian fitti ng from high-pass filtered and deconvolved images. We also pr esent results of near-infrared (NIR) H- and Ks-band ESO-SINFONI integral field spectroscopy of the Galact ic Center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in Ks- 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 Ks-L ′ -color. Conclusions. Most of the compact L ′ -band excess emission sources have a compact H- or Ks-band counterpart and therefore are likely stars with dust shells or disks. Our new results and or bital analysis from our previous work favor the hypothesis t hat 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.

Millimeter to X-ray flares from Sagittarius A*

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 vexp ∼ 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 vexp 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.

Monitoring the Dusty S-cluster Object (DSO/G2) on its Orbit toward the Galactic Center Black Hole

We analyze and report in detail new near-infrared (1.45-2.45 ?m) 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 the ESO Very Large Telescope/SINFONI between 2014 February and September. Before 2014 May we detect spatially compact Br? and Pa? line emission from the DSO at about 40 mas east of Sgr A*. The velocity of the source, measured from the redshifted emission, is 2700???60 km s?1. No blueshifted emission above the noise level is detected at the position of Sgr A* or upstream of the presumed orbit. After May we find spatially compact Br? blueshifted line emission from the DSO at about 30 mas west of Sgr A* at a velocity of ?3320???60 km s?1 and no indication for significant redshifted emission. We do not detect any significant extension of the velocity gradient across the source. We find a Br? line FWHM of 50???10 ? before and 15???10 ? after the peribothron transit, i.e., no significant line broadening with respect to last year is observed. Br? line maps show that the bulk of the line emission originates from a region of less than 20 mas 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 Sgr A* may still be upcoming. We discuss details of a source model according to which the DSO is a young accreting star rather than a coreless gas and dust cloud.

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.

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.

Polarized light from Sagittarius A* in the near-infrared Ks-band

We present a statistical analysis of polarized near-infrared (NIR) light from Sgr A*, the radio source associated with the supermassive black hole at the center of the Milky Way. The observations have been carried out using the adaptive optics instrument NACO at the VLT UT4 in the infrared

The S-star cluster at the center of the Milky Way - On the nature of diffuse NIR emission in the inner tenth of a parsec

K_\mathrm{s}

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

-band from 2004 to 2012. Several polarized flux excursions were observed during these years. Linear polarization at 2.2