ABSTRACT Sgr B2 is a well-known star-forming molecular cloud complex in the Galactic center region showing evidence of high energy activity as traced by the K neutral Fe i line at 6.4 keV, as well as ...GeV and TeV γ-ray emission. Here, we present Very Large Array and GMRT observations with respective resolutions of and and report the detection of an OH(1720 MHz) maser, with no accompanying OH 1665, 1667, and 1612 MHz maser emission. The maser coincides with a 150 MHz nonthermal radio source in Sgr B2(M). This rare class of OH(1720 MHz) masers or the so-called supernova remnant (SNR) masers, with no main line transitions, trace shocked gas and signal the interaction of an expanding SNR with a molecular cloud. We interpret the 150 MHz radio source as either the site of a SNR-molecular gas interaction or a wind-wind collision in a massive binary system. The interaction of the molecular cloud and the nonthermal source enhances the cosmic-ray ionization rate, allows the diffusion of cosmic rays into the cloud, and produces the variable 6.4 keV line, GeV, and TeV γ-ray emission from Sgr B2(M). The cosmic-ray electron interaction with the gas in the Galactic center can not only explain the measured high values of cosmic-ray ionization and heating rates but also contribute to nonthermal bremsstrahlung continuum emission, all of which are consistent with observations.
Abstract
Using the Very Large Array (VLA), we recently detected a large number of protoplanetary disc (proplyd) candidates lying within a couple of light years of the massive black hole Sgr A*. The ...bow-shock appearance of proplyd candidates points towards 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 centre is expected to photoevaporate and photoionize the circumstellar discs around young, low-mass stars, thus allowing detection of the ionized outflows from the photoionized layer surrounding cool and dense gaseous discs. 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 disc masses for four sources as a function of the assumed dust temperature. The mass of protoplanetary discs from cool dust emission ranges between 0.03 and 0.05 M⊙. These estimates are consistent with the disc masses found in star-forming sites in the Galaxy. These measurements show the presence of ongoing star formation with the implication that gas clouds can survive near Sgr A* and the relative importance of high- versus low-mass star formation in the strong tidal and radiation fields of the Galactic centre.
ABSTRACT We present radio and infrared observations indicating ongoing star formation activity inside the ∼2-5 pc circumnuclear ring at the Galactic center. Collectively these measurements suggest a ...continued disk-based mode of ongoing star formation has taken place near Sgr A* over the last few million years. First, Very Large Array observations with spatial resolution 2 17 × 0 81 reveal 13 water masers, several of which have multiple velocity components. The presence of interstellar water masers suggests gas densities that are sufficient for self-gravity to overcome the tidal shear of the black hole. Second, spectral energy distribution modeling of stellar sources indicates massive young stellar object (YSO) candidates interior to the molecular ring, supporting in situ star formation near Sgr A* and appear to show a distribution similar to that of the counter-rotating disks of ∼100 OB stars orbiting Sgr A*. Some YSO candidates (e.g., IRS 5) have bow shock structures, suggesting that they have gaseous disks that are phototoevaporated and photoionized by the strong radiation field. Third, we detect clumps of SiO (2-1) and (5-4) line emission in the ring based on Combined Array for Research in Millimeter-wave Astronomy and Sub-Millimeter Array observations. The FWHM and luminosity of the SiO emission is consistent with shocked protostellar outflows. Fourth, two linear ionized features with an extent of ∼0.8 pc show blue and redshifted velocities between +50 and −40 km s−1, suggesting protostellar jet driven outflows with mass-loss rates of yr−1. Finally, we present the imprint of radio dark clouds at 44 GHz, representing a reservoir of molecular gas that feeds star formation activity close to Sgr A*.
We have used the capabilities of the Atacama Large Millimeter/Submillimeter Array to probe the ionized gas toward the Galactic center with the H30 mm hydrogen recombination line within 30″ of the ...Galaxy's dynamical center. The observations are made with spatial and spectral resolutions of 0 46 × 0 35 and 3 km s−1, respectively. Multiple compact and extended sources are detected in the minicavity region with extreme negative radial velocities ranging from −480 to −300 km s−1 2″-3″ (0.08-0.12 pc) from Sgr A*. These are the highest radial velocities of ionized gas detected beyond the inner 1″ of Sgr A*. This new component is kinematically isolated from the orbiting ionized gas. The detected gas has a velocity gradient ranging from −50 to −200 km s−1 arcsec−1 located to the southwest of Sgr A* at a position angle of ∼−160°. Previous proper-motion measurements in the immediate vicinity of these extreme high-velocity ionized components have been made, and they show transverse velocities that range from 313 to 865 km s−1. If we assume that they are associated with one another, then the total velocity implies these components are gravitationally unbound. In addition, we report the kinematics of cometary radio and infrared sources. These sources are diagonally distributed with a position angle of ∼50° within 14″ of Sgr A*. We interpret the highly blueshifted features to the southwest where the minicavity is located, and the redshifted cometary sources to the northeast in terms of the interaction of a collimated outflow with an opening angle of ∼30°. An expected mass outflow rate of 2 × 10−7 or 4 × 10−5 M yr−1 is estimated by a relativistic jet-driven outflow or collimated stellar winds, respectively.
Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. ...However, the recent observations of occasional rapid X-ray flares from Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares. Methods. In Spring 2007, we observed Sgr A* with XMM-Newton with a total exposure of similar to 230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of Sgr A* for comparison. Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration ( similar to 3 ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_{\rm H} = 12.3_\times 10 cm super(-2) and Gamma = 2.3 pm 0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower (N_{\rm H} = 8.8_ times 10 cm super(-2) and Gamma = 1.7_), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward Sgr A* (A_{\rm V} similar to 25 mag), i.e., 4.5 times 10 super(22) cm super(-2). However, our fitting of the Sgr A* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra.
We present results of interferometric polarization observations of the recently discovered magnetar J1745−2900 in the vicinity of the Galactic Centre. The observations were made with the Karl G. ...Jansky Very Large Array (VLA) on 2014 February 21 in the range 40–48 GHz. The full polarization mode and A configuration of the array were used. The average total and linearly polarized flux density of the pulsar amounts to 2.3 ± 0.31 and 1.5 ± 0.2 mJy beam−1, respectively. Analysis shows a rotation measure (RM) of (−67 ± 3) × 103 rad m−2, which is in good agreement with previous measurements at longer wavelengths. These high-frequency observations are sensitive to RM values of up to ∼2 × 107 rad m−2. However, application of the Faraday RM synthesis technique did not reveal other significant RM components in the pulsar emission. This supports an external nature of a single thin Faraday-rotating screen which should be located close to the Galactic Centre. The Faraday-corrected intrinsic electric vector position angle is 16 ± 9 deg east of north, and coincides with the position angle of the pulsar's transverse velocity. All measurements of the pulsar's RM value to date, including the one presented here, well agree within errors, which points towards a steady nature of the Faraday-rotating medium.
We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2°X 08 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor ...of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 X 1032 erg s-1 (0.5-8.0 keV; 90% confidence) over an area of 1 deg2, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A*. The positions of 60% of our sources are accurate to <1 '' (95% confidence), and 20% have positions accurate to <05. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7 - 285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8s evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l 01.
A ring of dense molecular gas extending 2-7 pc orbits the supermassive black hole Sgr A* at the center of our Galaxy. Using the Green Bank Telescope, we detected water maser lines and both narrow ...(0.35 km simage) and broad (30-50 km simage) methanol emission from the molecular ring. Two of the strongest methanol lines at 44 GHz are confirmed as masers by interferometric observations. These Class I methanol masers are collisionally excited and are signatures of early phases of massive star formation in the disk of the Galaxy, suggesting that star formation in the molecular ring is in its early phase. Close inspection of the kinematics of the associated molecular clumps in the HCN line reveals broad redshifted wings indicative of disturbance by protostellar outflows from young (a few x 10 super(4) yr), massive stars embedded in the clumps. The thermal methanol profile has a similar shape, with a narrow maser line superimposed on a broad, redshifted wing. Additional evidence for the presence of young massive protostars is provided by shocked molecular hydrogen and a number of striking ionized and molecular linear filaments in the vicinity of methanol sources suggestive of 0.5 pc scale protostellar jets. Given that the circumnuclear molecular ring is kinematically unsettled and thus is likely be the result of a recent capture, the presence of both methanol emission and broad, redshifted HCN emission suggests that star formation in the circumnuclear ring is in its infancy.
We present radio and millimeter continuum observations of the Galactic center taken with the Very Large Array (VLA) and ALMA at 44 and 226 GHz, respectively. We detect radio and millimeter 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 m Galactic center stellar source, consists of two semicircular dust shells facing the direction of Sgr A*. The outer circumstellar shell, at a distance of 1.6 × 104 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 shapes of the IRS 3 shells seen at 3.8 m 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 yr constrains the distance of IRS 3 to ∼0.7 pc in front of or ∼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 in an ionized medium.
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