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We study the environment of Sgr A* using spectral and continuum observations with the Atacama Large Millimeter/submillimeter Array and Very Large Array. Our analysis of subarcsecond H30 α, H39 α, H52 α , and H56 α line emission towards Sgr A* confirms the recently published broad-peak ∼500 km s−1 spectrum towards Sgr A*. We also detect emission at more extreme radial velocities peaking near −2500 and 4000 km s−1 within 0.2 arcsec. 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 arcsec to ∼10 arcmin. We note a non-thermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the minispiral 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 ∼30○ and a position angle of ∼60○ punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with an estimated mass flow rate of ∼1.4 × 10−5 M⊙ yr−1 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. [ABSTRACT FROM AUTHOR]

We present 44 and 226 GHz observations of the Galactic Centre within 20 arcsec of Sgr A*. Millimetre continuum emission at 226 GHz is detected from eight stars that have previously been identified at near-IR and radio wavelengths.We also detect a 5.8 mJy source at 226 GHz coincident with the magnetar SGR J1745-29 located 2.39 arcsec SE of Sgr A* and identify a new 2.5 arcsec × 1.5 arcsec halo of mm emission centred on Sgr A*. The X-ray emission from this halo has been detected previously and is interpreted in terms of a radiatively inefficient accretion flow. The mm halo surrounds an EW linear feature that appears to arise from Sgr A* and coincides with the diffuse X-ray emission and a minimum in the near-IR extinction. We argue that the millimetre emission is produced by synchrotron emission from relativistic electrons in equipartition with an ~1.5 mG magnetic field. The origin of this is unclear but its coexistence with hot gas supports scenarios in which the gas is produced by the interaction of winds either from the fast moving S-stars, the photoevaporation of low-mass YSO discs or by a jet-driven outflow from Sgr A*. The spatial anti-correlation of the X-ray, radio and mm emission from the halo and the low near-IR extinction provides a compelling evidence of an outflow sweeping up the interstellar material, creating a dust cavity within 2 arcsec of Sgr A*. Finally, the radio and mm counterparts to eight near-IR identified stars within ~10 arcsec of SgrA* provide accurate astrometry to determine the positional shift between the peak emission at 44 and 226 GHz. [ABSTRACT FROM AUTHOR]

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. Between 2010 and 2014, we performed monitoring observations of the Galactic Centre to study the flux-density variations at 3 mm using the Australia Telescope Compact Array (ATCA). We obtain 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 and 1.0 Jy, which last for 1.5-3 h. 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 vexp, 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 vexp. 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. [ABSTRACT FROM AUTHOR]

Context. 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 100K to 104 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. Aims. 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. Methods. Our observations were carried out at 1.3mm and 3mm 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 Brγ with VLT NACO in August 2009. Results. We present high resolution maps of the mini-spiral, and obtain a spectral index of 0.5 ± 0.25 for Sgr A", indicating an inverted synchrotron spectrum. We find electron densities within the range 0.8-1.5 × 104 cm-3 for the mini-spiral from the radio continuum maps, along with a dust mass contribution of ∼0.25 M☉ from the MIR dust continuum, and extinctions ranging from 1.8-3 at 2.16 μm in the Brγ line. Conclusions. We observe a mixture of negative and positive spectral indices in our 1.3mm and 3mm 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. [ABSTRACT FROM AUTHOR]

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 × 106 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 12×1033 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 (v) ∝ v-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 106.5 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. [ABSTRACT FROM AUTHOR]