ABSTRACT We present radio images within 30″ of Sgr A* based on recent VLA observations at 34 GHz with 7.8 Jy sensitivity and resolution of mas. We report 44 partially resolved compact sources ...clustered in two regions in the E arm of ionized gas that orbits Sgr A*. These sources have size scales ranging between ∼50 and 200 mas (400-1600 AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the bow-shock sources previously identified in the near-IR but associated with massive stars, these 34 GHz sources do not appear to have near-IR counterparts at 3.8 m. We interpret these sources as a candidate population of photoevaporative protoplanetary disks (proplyds) that are associated with newly formed low mass stars with mass loss rates yr−1 and are located at the edge of a molecular cloud outlined by ionized gas. The disks are externally illuminated by strong Lyman continuum radiation from the ∼100 OB and WR massive stars distributed within 10″ of Sgr A*. The presence of proplyds implies current in situ star formation activity near Sgr A* and opens a window for the first time to study low mass star, planetary, and brown dwarf formations near a supermassive black hole.
Sgr A*, the massive black hole at the center of the Galaxy, varies in radio through X-ray emission on hourly timescales. The flare activity is thought to arise from the innermost region of an ...accretion flow onto Sgr A*. We present simultaneous light curves of Sgr A* in radio, submillimeter and X- rays that show a possible time delay of image minutes between X-ray and 850 mum suggesting that the submillimeter flare emission is optically thick. At radio wavelengths, we detect time lags of image, and image minutes between the flare peaks observed at 13 and 7 mm (22 and 43 GHz) in three different epochs using the VLA. Linear polarization of image and image % is detected at 7 and 13 mm, respectively, when averaged over the entire observation on 2006 July 17. A simple model of a bubble of synchrotron- emitting electrons cooling via adiabatic expansion can explain the time delay between various wavelengths, the asymmetric shape of the light curves, and the observed polarization of the flare emission at 43 and 22 GHz. The derived physical quantities that characterize the emission give an expansion speed of image c, magnetic field of image G, and particle spectral index image. These parameters suggest that the associated plasma cannot escape from Sgr A* unless it has a large bulk motion.
In this paper, we examine properties of the variable source Sgr A* in the near-infrared (NIR) using a very extensive Ks-band data set from NACO/VLT observations taken in 2004-2009. We investigate the ...variability of Sgr A* with two different photometric methods and analyze its flux distribution. We find that Sgr A* is continuously emitting and continuously variable in the near-infrared, with some variability occurring on timescales as long as weeks. The flux distribution can be described by a lognormal distribution at low intrinsic fluxes (5 mJy, dereddened with AKs = 2.5). The lognormal distribution has a median flux of 1.1 mJy, but above 5 mJy the flux distribution is significantly flatter (high flux events are more common) than expected for the extrapolation of the lognormal distribution to high fluxes. We make a general identification of the low-level emission above 5 mJy as flaring emission and of the low-level emission as the quiescent state. We also report here the brightest Ks-band flare ever observed (from 2008 August 5) which reached an intrinsic Ks-band flux of 27.5 mJy (mKs = 13.5). This flare was a factor 27 increase over the median flux of Sgr A*, close to double the brightness of the star S2, and 40% brighter than the next brightest flare ever observed from Sgr A*.
Abstract
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 Sgr A* provide accurate astrometry to determine the positional shift between the peak emission at 44 and 226 GHz.
We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and ...at 6 and 10 GHz with the
Karl G. Jansky
Very Large Array. The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary, and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function.
We show radio continuum images of several molecular complexes in the inner Galaxy and report the presence of dark features that coincide with dense molecular clouds. Unlike infrared dark clouds, ...these features which we call "radio dark clouds" are produced by a deficiency in radio continuum emission from molecular clouds that are embedded in a bath of UV radiation field or synchrotron emitting cosmic-ray particles. The contribution of the continuum emission along different path lengths results in dark features that trace embedded molecular clouds. The new technique of identifying cold clouds can place constraints on the depth and the magnetic field of molecular clouds when compared to those of the surrounding hot plasma radiating at radio wavelengths. The study of five molecular complexes in the inner Galaxy, Sgr A, Sgr B2, radio Arc, the Snake filament, and G359.75-0.13 demonstrates an anti-correlation between the distributions of radio continuum and molecular line and dust emission. Radio dark clouds are identified in Green Bank Telescope maps and Very Large Array images taken with uniform sampling of uv coverage. The level at which the continuum flux is suppressed in these sources suggests that the depth of the molecular cloud is similar to the size of the continuum emission within a factor of two. These examples suggest that high-resolution, high-dynamic-range continuum images can be powerful probes of interacting molecular clouds with massive stars and supernova remnants in regions where the kinematic distance estimates are ambiguous as well as in the nuclei of active galaxies.
ABSTRACT We present high-resolution multiwavelength radio continuum images of the region within 150″ of Sgr A*, revealing a number of new extended features and stellar sources in this region. First, ...we detect a continuous 2″ east-west ridge of radio emission, linking Sgr A* and a cluster of stars associated with IRS 13 N and IRS 13E. The ridge suggests that an outflow of east-west blob-like structures is emerging from Sgr A*. In particular, we find arc-like radio structures within the ridge with morphologies suggestive of photoevaporative protoplanetary disks. We use infrared Ks and L′ fluxes to show that the emission has similar characteristics to those of a protoplanetary disk irradiated by the intense radiation field at the Galactic center. This suggests that star formation has taken place within the S-cluster 2″ from Sgr A*. We suggest that the diffuse X-ray emission associated with Sgr A* is due to an expanding hot wind produced by the mass loss from B-type main sequence stars, and/or the disks of photoevaporation of low mass young stellar objects (YSOs) at a rate of ∼10−6 yr−1. The proposed model naturally reduces the inferred accretion rate and is an alternative to the inflow-outflow style models to explain the underluminous nature of Sgr A*. Second, on a scale of 5″ from Sgr A*, we detect new cometary radio and infrared sources at a position angle PA ∼ 50° which is similar to that of two other cometary sources X3 and X7, all of which face Sgr A*. In addition, we detect a striking tower of radio emission at a PA ∼ 50°-60° along the major axis of the Sgr A East supernova remnant shell on a scale of 150″ from Sgr A*. We suggest that the cometary sources and the tower feature are tracing interaction sites of a mildly relativistic jet from Sgr A* with the atmosphere of stars and the nonthermal Sgr A East shell at a PA ∼ 50°-60° with , and opening angle 10°. Lastly, we suggest that the east-west ridge of radio emission traces an outflow that is potentially associated with past flaring activity from Sgr A*. The position angle of the outflow driven by flaring activity is close to −90°.
Variable emission from Sgr A*, the luminous counterpart to the super-massive black hole at the center of our Galaxy, arises from the innermost portions of the accretion flow. Better characterization ...of the variability is important for constraining models of the low-luminosity accretion mode powering Sgr A*, and could further our ability to use variable emission as a probe of the strong gravitational potential in the vicinity of the 4 x 10 super(6)M sub(middot in circle) black hole. We use the Herschel Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr A* at wavelengths that are difficult or impossible to observe from the ground. We find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal structure that is highly correlated across these wavelengths. While the variations correspond to <1% changes in the total intensity in the Herschel beam containing Sgr A*, comparison to independent, simultaneous observations at 0.85 mm strongly supports the reality of the variations. The lowest point in the light curves, ~0.5 Jy below the time-averaged flux density, places a lower bound on the emission of Sgr A* at 0.25 mm, the first such constraint on the THz portion of the spectral energy distribution. The variability on few hour timescales in the SPIRE light curves is similar to that seen in historical 1.3 mm data, where the longest time series is available, but the distribution of variations in the sub-mm do not show a tail of large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from XMM-Newton shows no significant variation within our observing period, which may explain the lack of very large submillimeter variations in our data if X-ray and submillimeter flares are correlated.
Context. The supermassive black hole named Sgr A* is located at the dynamical center of the Milky Way. This closest supermassive black hole is known to have a luminosity several orders of magnitude ...lower than the Eddington luminosity. Flares coming from the Sgr A* environment can be observed in infrared, X-ray, and submillimeter wavelengths, but their origins are still debated. Interestingly, the close passage of the Dusty S-cluster Object (DSO)/G2 near Sgr A* may increase the black hole flaring activity and could therefore help us to better constrain the radiation mechanisms from Sgr A*. Aims. Our aim is to study the X-ray, infrared, and radio flaring activity of Sgr A* close to the time of the DSO/G2 pericenter passage in order to constrain the physical properties and origin of the flares. Methods. Simultaneous observations were made with XMM-Newton and WFC3 onboard HST during the period Feb.–Apr. 2014, in addition to coordinated observations with SINFONI at ESO’s VLT, VLA in its A-configuration, and CARMA. Results. We detected two X-ray flares on 2014 Mar. 10 and Apr. 2 with XMM-Newton, three near-infrared (NIR) flares with HST on 2014 Mar. 10 and Apr. 2, and two NIR flares on 2014 Apr. 3 and 4 with VLT. The X-ray flare on 2014 Mar. 10 is characterized by a long rise (~7700 s) and a rapid decay (~844 s). Its total duration is one of the longest detected so far in X-rays. Its NIR counterpart peaked well before (4320 s) the X-ray maximum, implying a dramatic change in the X-ray-to-NIR flux ratio during this event. This NIR/X-ray flare is interpreted as either a single flare where variation in the X-ray-to-NIR flux ratio is explained by the adiabatic compression of a plasmon, or two distinct flaring components separated by 1.2 h with simultaneous peaks in X-rays and NIR. We identified an increase in the rising radio flux density at 13.37 GHz on 2014 Mar. 10 with the VLA that could be the delayed radio emission from a NIR/X-ray flare that occurred before the start of our observation. The X-ray flare on 2014 Apr. 2 occurred for HST during the occultation of Sgr A* by the Earth, therefore we only observed the start of its NIR counterpart. With NIR synchrotron emission from accelerated electrons and assuming X-rays from synchrotron self-Compton emission, the region of this NIR/X-ray flare has a size of 0.03−7 times the Schwarzschild radius and an electron density of 108.5–1010.2 cm-3, assuming a synchrotron spectral index of 0.3−1.5. When Sgr A* reappeared to the HST view, we observed the decay phase of a distinct bright NIR flare with no detectable counterpart in X-rays. On 2014 Apr. 3, two 3.2-mm flares were observed with CARMA, where the first may be the delayed (4.4 h) emission of a NIR flare observed with VLT. Conclusions. We observed a total of seven NIR flares, with three having a detected X-ray counterpart. The physical parameters of the flaring region are less constrained for the NIR flare without a detected X-ray counterpart, but none of the possible radiative processes (synchrotron, synchrotron self-Compton, or inverse Compton) can be ruled out for the production of the X-ray flares. The three X-ray flares were observed during the XMM-Newton total effective exposure of ~256 ks. This flaring rate is statistically consistent with those observed during the 2012 Chandra XVP campaign, implying that no increase in the flaring activity was triggered close to the pericenter passage of the DSO/G2. Moreover, higher flaring rates had already been observed with Chandra and XMM-Newton without any increase in the quiescent level, showing that there is no direct link between an increase in the flaring rate in X-rays and the change in the accretion rate.
We describe the results of a radio continuum survey of the central image with the 100 m Green Bank Telescope (GBT) at wavelengths of 3.5, 6, 20, and 90 cm. The 3.5 and 6 cm surveys are the most ...sensitive and highest resolution single-dish surveys made of the central degrees of our Galaxy. We present catalogs of compact and extended sources in the central 4 degree of our Galaxy, including detailed spectral index studies of all sources. The analysis covers star-forming regions such as Sgr B and Sgr C, where we find evidence of a mixture of thermal and nonthermal emission. The analysis quantifies the relative contribution of thermal and nonthermal processes to the radio continuum flux density toward the Galactic center (GC) region. In the central image of the GC, the thermal and nonthermal flux fractions for all compact and diffuse sources are 28%/72% at 3.5 cm and 19%/81% at 6 cm. The total flux densities from these sources are image and image Jy at 3.5 and 6 cm, respectively, excluding the contribution of Galactic synchrotron emission.