Search

We present the analysis of $\sim 100$pc-scale compact radio continuum sources detected in 63 local (Ultra) Luminous Infrared Galaxies (U/LIRGs; $L_{\rm IR} \ge 10^{11} L_\odot$), using FWHM $\lesssim 0''.1 - 0''.2$ resolution 15 and 33 GHz observations with the Karl G. Jansky Very Large Array. We identify a total of 133 compact radio sources with effective radii of 8 - 170pc, which are classified into four main categories -- "AGN" (AGN), "AGN/SBnuc" (AGN-starburst composite nucleus), "SBnuc" (starburst nucleus) and "SF" (star-forming clumps) -- based on ancillary datasets and the literature. We find that "AGN" and "AGN/SBnuc" more frequently occur in late-stage mergers and have up to 3 dex higher 33 GHz luminosities and surface densities compared with "SBnuc" and "SF", which may be attributed to extreme nuclear starburst and/or AGN activity in the former. Star formation rates (SFRs) and surface densities ($\Sigma_{\rm SFR}$) are measured for "SF" and "SBnuc" using both the total 33 GHz continuum emission (SFR $\sim 0.14 - 13$ M$_\odot$ yr$^{-1}$, $\Sigma_{\rm SFR} \sim 13 - 1600$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$) and the thermal free-free emission from HII regions (median SFR$_{\rm th} \sim 0.4$ M$_\odot$ yr$^{-1}$, $\Sigma_{\rm SFR_{th}} \sim 44$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$). These values are 1 - 2 dex higher than those measured for similar-sized clumps in nearby normal (non-U/LIRGs). The latter also have much flatter median 15 - 33 GHz spectral index ($\sim -0.08$) compared with "SBnuc" and "SF" ($\sim -0.46$), which may reflect higher non-thermal contribution from supernovae and/or ISM densities in local U/LIRGs that directly result from and/or lead to their extreme star-forming activities on 100\,pc scales., Comment: Accepted for publication in ApJ

Nuclear rings are excellent laboratories for studying intense star formation. We present results from a study of nuclear star-forming rings in five nearby normal galaxies from the Star Formation in Radio Survey (SFRS) and four local LIRGs from the Great Observatories All-sky LIRG Survey (GOALS) at sub-kpc resolutions using VLA high-frequency radio continuum observations. We find that nuclear ring star formation (NRSF) contributes 49 - 60\% of the total star formation of the LIRGs, compared to 7 - 40\% for the normal galaxies. We characterize a total of 58 individual star-forming regions in these rings, and find that with measured sizes of 10 - 200 pc, NRSF regions in the LIRGs have SFR and $\Sigma_\mathrm{SFR}$ up to 1.7 M$_\odot$yr$^{-1}$ and 402 M$_\odot$yr$^{-1}$kpc$^{-2}$, respectively, which are about 10 times higher than NRSF regions in the normal galaxies with similar sizes, and comparable to lensed high-$z$ star-forming regions. At $\sim 100 - 300$ pc scales, we estimate low contributions ($< 50\%$) of thermal free-free emission to total radio continuum emission at 33 GHz in the NRSF regions in the LIRGs, but large variations possibly exist at smaller physical scales. Finally, using archival sub-kpc resolution CO (J=1-0) data of nuclear rings in the normal galaxies and NGC 7469 (LIRG), we find a large scatter in gas depletion times at similar molecular gas surface densities, which tentatively points to a multi-modal star formation relation on sub-kpc scales., Comment: Accepted for publication in ApJ

We study the environment of Sgr A* using spectral and continuum observations with the ALMA and VLA. Our analysis of sub-arcsecond H30alpha, H39alpha, H52alpha and H56alpha line emission towards Sgr A* confirm the recently published broad peak ~500 km/s~spectrum toward Sgr~A*. We also detect emission at more extreme radial velocities peaking near -2500 and 4000 km/s, within 0.2''. We then present broad band radio continuum images at multiple frequencies on scales from arcseconds to arcminutes. A number of elongated continuum structures lie parallel to the Galactic plane, extending from ~0.4'' to 10'. We note a nonthermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the mini-spiral is depressed by optical depth effects. The position angle of this elongated structure and the sense of motion of ionized features with respect to Sgr A* suggest a symmetric, collimated jet emerging from Sgr A* with an opening angle of ~30deg and a position angle of ~60deg punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with estimated mass flow rate ~1.4x10^{-5} solar mass/yr emerges perpendicular to the equatorial plane of the accretion flow near the event horizon of Sgr A* and runs along the Galactic plane. To explain a number of east-west features near Sgr A*, we also consider the possibility of an outflow component with a wider-angle launched from the accretion flow at larger radii., Comment: 26 pages, 14 figures, MNRAS, in press

We present results of 3 mm observations of SiO maser sources in the Galactic Centre (GC) from observations with the Australia Telescope Compact Array between $2010-2014$, along the transitions of the SiO molecule at $v = 1, J = 2-1$ at 86.243 GHz and $v = 2, J = 2-1$ at 85.640 GHz. We also present the results of the 3 mm observations with Atacama Large Millimeter/Submillimeter Array (ALMA). We detected 5 maser sources from the ATCA data, IRS 7, IRS 9, IRS 10EE, IRS 12N, and IRS 28; and 20 sources from the ALMA data including 4 new sources. These sources are predominantly late-type giants or emission line stars with strong circumstellar maser emission. We analyse these sources and calculate their proper motions. We also study the variability of the maser emission. IRS 7, IRS 12N and IRS 28 exhibit long period variability of the order of $1 - 2$ years, while other sources show steady increase or decrease in flux density and irregular variability over observation timescales. This behaviour is consistent with the previous observations., Comment: 22 pages, 5 figures, to be published in the proceedings of the XIII workshop on "Multifrequency Behaviour of High Energy Cosmic Sources - XIII - MULTIF2019", held on 3-8 June, 2019, in Palermo, Italy

Bright Active Galactic Nuclei are powered by accretion of mass onto the super massive black holes at the centers of the host galaxies. For fainter objects star formation may significantly contribute to the luminosity. We summarize experimental indicators of the accretion processes in Active Galactic Nuclei (AGN), i.e., observable activity indicators that allow us to conclude on the nature of accretion. The Galactic Center is the closest galactic nucleus that can be studied with unprecedented angular resolution and sensitivity. Therefore, here we also include the presentation of recent observational results on Sagittarius A* and the conditions for star formation in the central stellar cluster. We cover results across the electromagnetic spectrum and find that the Sagittarius A* (SgrA*) system is well ordered with respect to its geometrical orientation and its emission processes of which we assume to reflect the accretion process onto the super massive black hole., Comment: 16 pages, 4 figures, conference proceeding: Accretion Processes in Cosmic Sources - APCS2016 - 5-10 September 2016, Saint Petersburg, Russia

We report the discovery of 11 bipolar outflows within a projected distance of 1pc from Sgr A* based on deep ALMA observations of $^{13}$CO, H30$\alpha$ and SiO (5-4) lines with sub-arcsecond and $\sim1.3$ km/s, resolutions. These unambiguous signatures of young protostars manifest as approaching and receding lobes of dense gas swept up by the jets created during the formation and early evolution of stars. The lobe masses and momentum transfer rates are consistent with young protostellar outflows found throughout the disk of the Galaxy. The mean dynamical age of the outflow population is estimated to be $6.5^{+8.1}_{-3.6}\times10^3$ years. The rate of star formation is $\sim5\times10^{-4}$\msol\,yr$^{-1}$ assuming a mean stellar mass of $\sim0.3$ \msol. This discovery provides evidence that star formation is taking place within clouds surprisingly close to Sgr A*, perhaps due to events that compress the host cloud, creating condensations with sufficient self-gravity to resist tidal disruption by Sgr A*. Low-mass star formation over the past few billion years at this level would contribute significantly to the stellar mass budget in the central few pc of the Galaxy. The presence of many dense clumps of molecular material within 1pc of Sgr A* suggests that star formation could take place in the immediate vicinity of supermassive black holes in the nuclei of external galaxies, Comment: 17 pages, 4 figures

The Galactic Center is the closest galactic nucleus that can be studied with unprecedented angular resolution and sensitivity. We summarize recent basic observational results on Sagittarius A* and the conditions for star formation in the central stellar cluster. We cover results from the radio, infrared, and X-ray domain and include results from simulation as well. From (sub-)mm and near-infrared variability and near-infrared polarization data we find that the SgrA* system (supermassive black hole spin, a potential temporary accretion disk and/or outflow) is well ordered in its geometrical orientation and in its emission process that we assume to reflect the accretion process onto the supermassive black hole (SMBH)., Comment: 11 pages, 4 figures, 1 table; published in PoS-SISSA Proceedings of the: Frontier Research in Astrophysics - II, 23-28 May 2016, Mondello (Palermo), Italy

We present radio and mm continuum observations of the Galactic center taken with the VLA and ALMA at 44 and 226 GHz, respectively. We detect radio and mm emission from IRS 3, lying ~4.5" NW of Sgr A*, with a spectrum that is consistent with the photospheric emission from an AGB star at the Galactic center. Millimeter images reveal that the envelope of IRS 3, the brightest and most extended 3.8$\mu$m Galactic center stellar source, consists of two semi-circular dust shells facing the direction of Sgr A*. The outer circumstellar shell at the distance of 1.6$\times10^4$ AU, appears to break up into "fingers" of dust directed toward Sgr A*. These features coincide with molecular CS (5-4) emission and a near-IR extinction cloud distributed between IRS 3 and Sgr A*. The NE-SW asymmetric shape of the IRS 3 shells seen at 3.8 micron and radio are interpreted as structures that are tidally distorted by Sgr A*. Using the kinematics of CS emission and the proper motion of IRS 3, the tidally distorted outflowing material from the envelope after 5000 years constrains the distance of IRS 3 to $\sim$0.7 pc in front of or $\sim$0.5 pc behind Sgr A*. This suggests that the mass loss by stars near Sgr A* can supply a reservoir of molecular material near Sgr A*. We also present dark features in radio continuum images coincident with the envelope of IRS 3. These dusty stars provide examples in which high resolution radio continuum images can identify dust enshrouded stellar sources embedded an ionized medium., Comment: 16 pages, 5 figures, ApJ (in press)

Using the VLA, we recently detected a large number of protoplanetary disk (proplyd) candidates lying within a couple of light years of the massive black hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the young massive stars located near Sgr A*. Similar to Orion proplyds, the strong UV radiation from the cluster of massive stars at the Galactic center is expected to photoevaporate and photoionize the circumstellar disks around young, low mass stars, thus allowing detection of the ionized outflows from the photoionized layer surrounding cool and dense gaseous disks. To confirm this picture, ALMA observations detect millimeter emission at 226 GHz from five proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We present the derived disk masses for four sources as a function of the assumed dust temperature. The mass of protoplanetary disks from cool dust emission ranges between 0.03 -- 0.05 solar mass. These estimates are consistent with the disk masses found in star forming sites in the Galaxy. These measurements show the presence of on-going star formation with the implication that gas clouds can survive near Sgr A* and the relative importance of high vs low-mass star formation in the strong tidal and radiation fields of the Galactic center., Comment: 13 pages, 3 figures, MNRAS (in press)

We study the conditions for the onset of Thermal Instability in the innermost regions of compact galactic nuclei, where the properties of the interstellar environment are governed by the interplay of quasi-spherical accretion onto a supermassive black hole (SMBH) and the heating/cooling processes of gas in a dense nuclear star cluster. Stellar winds are the source of material for radiatively inefficient (quasi-spherical, non-magnetised) inflow/outflow onto the central SMBH, where a stagnation point develops within the Bondi type accretion. We study the local thermal equilibrium to determine the parameter space which allows cold and hot phases in mutual contact to co-exist. We include the effects of mechanical heating by stellar winds and radiative cooling/heating by the ambient field of the dense star cluster. We consider two examples: the Nuclear Star Cluster (NSC) in the Milky Way central region (including the gaseous Mini-spiral of Sgr~A*), and the Ultra-Compact Dwarf galaxy M60-UCD1. We find that the two systems behave in different ways because they are placed in different areas of parameter space in the instability diagram: gas temperature vs. dynamical ionization parameter. In the case of Sgr~A*, stellar heating prevents the spontaneous formation of cold clouds. The plasma from stellar winds joins the hot X-ray emitting phase and forms an outflow. In M60-UCD1 our model predicts spontaneous formation of cold clouds in the inner part of the galaxy. These cold clouds may survive since the cooling timescale is shorter than the inflow/outflow timescale., Comment: 14 pages, 14 figures, accepted for publication in MNRAS

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow. We performed the monitoring observations of the Galactic Centre to study the flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 to 1.0 Jy, which last for 1.5 $-$ 3 hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed $v_{\mathrm{exp}}$, source sizes, spectral indices, and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock., Comment: 15 pages, 24 figures

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas., Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&A

The super-massive 4 million solar mass black hole Sagittarius~A* (SgrA*) shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future. Here the interpretation of recent data and ongoing observations are discussed., Comment: 30 pages - 7 figures - accepted for publication by Springer's "Fundamental Theories of Physics" series; summarizing GC contributions of 2 conferences: 'Equations of Motion in Relativistic Gravity' at the Physikzentrum Bad Honnef, Bad Honnef, Germany, (Feb. 17-23, 2013) and the COST MP0905 'The Galactic Center Black Hole Laboratory' Granada, Spain (Nov. 19 - 22, 2013)

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 $K_\mathrm{s}$-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 $K_\mathrm{s}$-band flux densities reported earlier. We find typical polarization degrees of the order of 20\%$\pm$10\% and a preferred polarization angle of 13$^o$$\pm$15$^o$. Simulations show the uncertainties under a total flux density of $\sim 2\,{\rm mJy}$ are probably dominated by observational effects. At higher flux densities there are intrinsic variations of polarization degree and angle within rather well constrained ranges. Since the emission is most likely due to optically thin synchrotron radiation, this 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 super massive 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., Comment: 19 pages, 18 figures, 4 tables, submitted to Astronomy & Astrophysics

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., Comment: 32 pages - 3 tables - 17 figure - accepted by ApJ

The Galactic center (GC) holds the closest-to-Earth supermassive black hole (SMBH), which makes it the best laboratory to study the close environment of extremely massive compact objects. Polarimetry is sensitive to geometry of the source, which makes it a particularly suitable technique to probe the medium surrounding the GC SMBH. The detection of hard X-ray spectra and prominent iron K$\alpha$ fluorescence features coincident with localized gas clouds (e.g. Sgr~B2, Sgr~C) is known for nearly twenty years now and is commonly associated with a past outburst of the SMBH whose radiation is reprocessing onto the so-called "reflection nebulae". Since scattering leads to polarization, the re-emitted signal from the giant molecular clouds in the first 100~pc of the GC is expected to be polarized. X-ray polarization measurement is thus particularly adapted to probe the origin of the diffuse X-ray emission from the GC reflection nebulae and reveal the past activity of the central SMBH. In this research note, we summarize the results from past and current polarimetric simulations in order to show how a future X-ray polarimeter equipped with imaging detectors could improve our understanding of high-energy astrophysics., Comment: 4 pages, 2 figures, to be published in the Proceedings of the annual meeting of the French Astronomical Society

Despite past panchromatic observations of the innermost part of the Milky Way, the overall structure of the Galactic Centre (GC) remains enigmatic in terms of geometry. In this paper, we aim to show how polarimetry can probe the three-dimensional position of the molecular material in the central ~100 pc of the GC. We investigate a model where the central supermassive black hole Sgr A* is radiatively coupled to a fragmented circumnuclear disc (CND), an elliptical twisted ring representative of the central molecular zone (CMZ), and the two main, bright molecular clouds Sgr B2 and Sgr C. 8 -- 35 keV integrated polarization mapping reveals that Sgr B2 and Sgr C, situated at the two sides of the CMZ, present the highest polarization degrees (66.5 and 47.8 per cent respectively), both associated with a polarization position angle psi = 90deg (normal to the scattering plane). The CND shows a lower polarization degree, 1.0 per cent with psi = -20.5deg, tracing the inclination of the CND with respect to the Galactic plane. The CMZ polarization is spatially variable. We also consider a range of spatial locations for Sgr A* and the reprocessing media, and investigate how the modeled three-dimensional geometry influences the resulting GC polarization. The two reflection nebulae are found to always produce high polarization degrees (>> 10 per cent). We show that a 500 ks observation with a broadband polarimeter could constrain the location and the morphology of the scattering material with respect to the emitting source, revealing the past activity of Sgr A*., Comment: 8 pages, 4 Figures, 4 Tables, accepted for publication in MNRAS

(abridged) In this paper we aim at determining the shape, size, and luminosity/mass of the Milky Way Nuclear Star Cluster (MWNSC). We use Spitzer/IRAC images at 3.6 and 4.5 micrometer, where interstellar extinction is at a minimum but the overall emission is still dominated by stars. We correct the 4.5 micrometer image for PAH emission with the help of the IRAC 8.0 micrometer map and for extinction with the help of a [3.6-4.5] colour map. We present an extinction map for the central ~300x200 pc^2 of the Milky Way, as well as a PAH-emission and extinction corrected image of the stellar emission, with a resolution of about 0.2 pc. We find that the MWNSC appears in projection intrinsically point-symmetric, that it is significantly flattened, with its major axis aligned along the Galactic Plane, and that it is centred on the black hole, Sagittarius A*. Its density follows the well known approximate rho~r^{-2}-law at distances of a few parsecs from Sagittarius A*, but may become as steep as rho~r^{-3} at projected radii around 5 pc. We derive a half light radius of 4.2+-0.4 pc, a total luminosity of L_MWNSC=(4.1+-0.4)x10^{7} L_Sun, and a mass of M_{MWNSC}=(2.5+-0.4)x10^{7} M_Sun. The overall properties of the MWNSC agree well with the ones of its extragalactic counterparts, which underlines its role as a template for these objects. Its flattening agrees well with its previously established rotation parallel to Galactic rotation and suggests that it has formed by accretion of material that fell in preferentially along the Galactic Plane. Our findings support the in situ growth scenario for nuclear clusters and emphasize the need to increase the complexity of theoretical models for their formation and for the interaction between their stars and the central black hole in order to include rotation, axisymmetry, and growth in recurrent episodes., Comment: Final, accepted manuscript

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., Comment: 4 pages, 2 figures, Proc. of a Conf. on IAU Symposium No.303: The GC: Feeding and Feedback in a Normal Galactic Nucleus 2013 September 30 - October 4, Santa Fe New Mexico (USA)

We present a comprehensive data description for Ks-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 non-linear 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 min and 100 min compared to what is expected from a red noise random process., Comment: Accepted to ApJS, August 27, 2012

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 we find a small but dense cluster of co-moving sources (IRS13N) about 3" west of SgrA* just 0.5" north of the bright IRS13E cluster of WR and O-type stars. Based on their color and brightness, there are two main possibilities: (1) they may be dust embedded stars older than few Myr, or (2) extremely young, dusty stars with ages less than 1Myr. We present fist H- and Ks-band identifications or proper motions of the IRS13N members, the high velocity dusty S-cluster object (DSO), and other infrared excess sources in the central field. We also present results of NIR H- and Ks-band ESO-SINFONI integral field spectroscopy of ISR13N. We show that within the uncertainties, the proper motions of the IRS13N sources in Ks- and L'-band are identical. This indicates that the bright L'-band IRS13N sources are indeed dust enshrouded stars rather than core-less dust clouds. The proper motions show that the IRS13N sources are not strongly gravitationally bound to each other implying that they have been formed recently. We also present a first H- and Ks-band identification as well as proper motions and HKsL'-colors of a fast moving DSO which was recently found in the cluster of high speed S-stars that surround the super-massive black hole Sagittarius A* (SgrA*). 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 orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are very young dusty stars and that star formation at the GC is a continuously ongoing process., Comment: 20 pages, 18 figures, 4 tables plus appendix with 16 figures and 3 tables accepted by A&A

Based on Bremer et al. (2011) and Eckart et al. (2012) we report on 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 black hole at the Galactic Center. We study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. To explain the statistics of 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. The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from adiabatically expanding synchrotron self-Compton (SSC) components. The model 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 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. We discuss the results in the framework of possible deviations from equilibrium between particle and magnetic field energy. We also summarize alternative models to explain the broad-band variability of SgrA*., Comment: 14 pages, 3 figures, Conf. Proc. The Central Kiloparsec in Galactic Nuclei; Astronomy at High Angular Resolution 2011 (AHAR2011), held 29.8.2011-2.9.2011 at the DPG Physikzentrum Bad Honnef (Cologne/Bonn) Germany

The mini-spiral is a feature of the interstellar medium in the central ~2 pc of the Galactic center. It is composed of several streamers of dust and ionised and atomic gas with temperatures between a few 100 K to 10^4 K. There is evidence that these streamers are related to the so-called circumnuclear disk of molecular gas and are ionized by photons from massive, hot stars in the central parsec. We attempt to constrain the emission mechanisms and physical properties of the ionized gas and dust of the mini-spiral region with the help of our multiwavelength data sets. Our observations were carried out at 1.3 mm and 3 mm with the mm interferometric array CARMA in California in March and April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum. We find electron densities within the range 0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm observations of the extended emission of the mini-spiral, which we interpret as evidence that there are a range of contributions to the thermal free-free emission by the ionized gas emission and by dust at 1.3 mm., Comment: 9 pages, 11 figures, accepted to A&A

The compact radio source Sgr\,A*, associated with the super massive black hole at the center of the Galaxy, has been studied with VLBA observations at 3 frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The total VLBI flux density of Sgr\,A* varies from day to day. The variability is correlated at the 3 observing frequencies with higher variability amplitudes appearing at the higher frequencies. For the modulation indices, we find 8.4\,% at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron self-absorption with a turnover frequency at or above 86\,GHz. The radio spectral index correlates with the flux density, which is harder (more inverted spectrum) when the source is brighter. The average source size does not appear to be variable over the 10-day observing interval. However, we see a tendency for the sizes of the minor axis to increase with increasing total flux, whereas the major axis remains constant. Towards higher frequencies, the position angle of the elliptical Gaussian increases, indicative of intrinsic structure, which begins to dominate the scatter broadening. At cm-wavelength, the source size varies with wavelength as $\lambda^{2.12\pm0.12}$, which is interpreted as the result of interstellar scatter broadening. After removal of this scatter broadening, the intrinsic source size varies as $\lambda^{1.4 ... 1.5}$. The VLBI closure phases at 22, 43, and 86\,GHz are zero within a few degrees, indicating a symmetric or point-like source structure. In the context of an expanding plasmon model, we obtain an upper limit of the expansion velocity of about 0.1\,c from the non-variable VLBI structure. This agrees with the velocity range derived from the radiation transport modeling of the flares from the radio to NIR wavelengths.}, Comment: 14pages, 14 Figures, Accepted for publication in A&A

We discuss mm-wavelength radio, 2.2-11.8um 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. 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., Comment: 18 pages, 13 figures, accepted by A&A

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. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's 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. 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., Comment: 26 pages, 38 figures, accepted for publication by A&A

We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive black hole at the Galactic Center. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the ESO NACO adaptive optics instrument and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the SMA and the VLA. The observations revealed several flare events in all wavelength domains. Here we show that a combined synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components 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 physical quantities that describe the flare emission give a blob expansion speed of v{exp}=0.005c, magnetic field of < 60G and spectral indices of 0.8 to 1.4. The derived model parameters suggest that the adiabatic expansion takes place in source components that 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*., Comment: 13 pages, 10 figures, A&A in press