* Astronomy

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RE: Galactic Center Cloud G2

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Title: No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2
Author: J.-H. Park (SNU Seoul), S. Trippe (SNU Seoul), T. P. Krichbaum (MPIfR Bonn), J.-Y. Kim (SNU Seoul), M. Kino (KASI Daejeon), A. Bertarini (MPIfR Bonn, U Bonn), M. Bremer (IRAM Grenoble), P. de Vicente (Yebes Obs.)

The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for possible transient asymmetric structure -- such as jets or winds from hot accretion flows -- around Sgr A* caused by accretion of material from G2. The interferometric closure phases remained zero within errors during the observation time. We thus conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ~2.5 mas along the major axis, ~0.4 mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are suspected to convert magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, one finds an effective accretion rate eta*Mdot < 1.5*10^9 kg/s ~ 7.7*10^-9 Mearth/yr for material from G2. Exploiting the kinetic jet power--accretion power relation of radio galaxies, one finds that the rate of accretion of matter that ends up in jets is limited to Mdot < 10^17 kg/s ~ 0.5 Mearth/yr, less than about 20% of the mass of G2. Accordingly, G2 appears to be largely stable against loss of angular momentum and subsequent (partial) accretion at least on time scales < 1 year.

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Best View Yet of Dusty Cloud Passing Galactic Centre Black Hole

The best observations so far of the dusty gas cloud G2 confirm that it made its closest approach to the supermassive black hole at the centre of the Milky Way in May 2014 and has survived the experience. The new result from ESO's Very Large Telescope shows that the object appears not to have been significantly stretched and that it is very compact. It is most likely to be a young star with a massive core that is still accreting material. The black hole itself has not yet shown any increase in activity.
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Mystery over monster cosmic cloud

Observations of a cosmic confrontation between a huge gas cloud and the black hole at the centre of our galaxy have sparked debate among astronomers.
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Title: No Microwave Flare of Sagittarius A* around the G2 Periastron Passing
Author: Masato Tsuboi, Yoshiharu Asaki, Osamu Kameya, Yoshinori Yonekura, Yusuke Miyamoto, Hiroyuki Kaneko, Masumichi Seta, Naomasa Nakai, Hiroshi Takaba, Ken-ichi Wakamatsu, Makoto Miyoshi, Yoshihiro Fukuzaki, Kenta Uehara, Mamoru Sekido

In order to explore any change caused by the G2 cloud approaching, we have monitored the flux density of Sgr A* at 22 GHz from Feb. 2013 to Aug. 2014 with a sub-array of Japanese VLBI Network . The observation period included the expected periastron dates. The number of observation epochs was 283 days. We have observed no significant microwave enhancement of Sgr A* in the whole observation period. The average flux density in the period is S=1.23±0.33 Jy. The average is consistent with the usually observed flux density range of Sgr A* at 22 GHz.

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Title: Detection of Galactic Center source G2 at 3.8 µm during periapse passage
Author: Gunther Witzel, Andrea Ghez, Mark Morris, Breann Sitarski, Anna Boehle, Smadar. Naoz, Randall Campbell, Eric Becklin, Gabriela Canalizo, Samantha Chappell, Tuan Do, Jessica Lu, Keith Matthews, Leo Meyer, Alan Stockton, Peter Wizinowich, Sylvana Yelda

We report new observations of Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears closest approach (periapse) with the Galaxy's central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K' [2.1 µm] and L' [3.8 µm] broadband filters. Several key results emerge from these observations: 1) G2 has survived its closest approach to the central black hole as a compact, unresolved source at L'; 2) G2's L' brightness measurements are consistent with those over the last decade; 3) G2's motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of 30 solar luminocity and is surrounded by a large (2 AU) optically thick dust shell. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster). In memoriam of Gerry Neugebauer (1932-2014).

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Title: A Stellar Wind Origin for the G2 Cloud: Three-Dimensional Numerical Simulations
Author: Fabio De Colle, A.C. Raga, Flavio F. Contreras-Torres, Juan C. Toledo-Roy

We present 3D, adaptive mesh refinement simulations of G2, a cloud of gas moving in a highly eccentric orbit towards the galactic center. We assume that G2 originates from a stellar wind interacting with the environment of the Sgr A* black hole. The stellar wind forms a cometary bubble which becomes increasingly elongated as the star approaches periastron. A few months after periastron passage, streams of material begin to accrete on the central black hole with accretion rates M ~10^-8 solar masses yr-1. Predicted Br emission maps and position-velocity diagrams show an elongated emission resembling recent observations of G2. A large increase in luminosity is predicted by the emission coming from the shocked wind region during periastron passage. The observations, showing a constant Brluminosity, remain puzzling, and are explained here assuming that the emission is dominated by the free-wind region. The observed Brgammaluminosity (~8 x 10^30 erg s-1) is reproduced by a model with a vw=50 km s-1 wind velocity and a 10^-7 solar masses yr-1 mass loss rate if the emission comes from the shocked wind. A faster and less dense wind reproduces the Brgamma luminosity if the emission comes from the inner, free wind region. The extended cometary wind bubble, largely destroyed by the tidal interaction with the black hole, reforms a few years after periastron passage. As a result, the Br emission is more compact after periastron passage.

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ATel 6110: Detection of Galactic Center Source G2 at 3.8 micron during Periapse Passage Around the Central Black Hole

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Title: The Keplerian orbit of G2
Author: L. Meyer, A. M. Ghez, G. Witzel, T. Do, K. Phifer, B. N. Sitarski, M. R. Morris, A. Boehle, S. Yelda, J. R. Lu, E. Becklin

We give an update of the observations and analysis of G2 - the gaseous red emission-line object that is on a very eccentric orbit around the Galaxy's central black hole and predicted to come within 2400 Rs in early 2014. During 2013, the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes were used to obtain three epochs of spectroscopy and imaging at the highest spatial resolution currently possible in the near-IR. The updated orbital solution derived from radial velocities in addition to Br-Gamma line astrometry is consistent with our earlier estimates. Strikingly, even ~6 months before pericenter passage there is no perceptible deviation from a Keplerian orbit. We furthermore show that a proposed "tail" of G2 is likely not associated with it but is rather an independent gas structure. We also show that G2 does not seem to be unique, since several red emission-line objects can be found in the central arcsecond. Taken together, it seems more likely that G2 is ultimately stellar in nature, although there is clearly gas associated with it.

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Ripped Apart by a Black Hole

New observations from ESO's Very Large Telescope show for the first time a gas cloud being ripped apart by the supermassive black hole at the centre of the galaxy. The cloud is now so stretched that its front part has passed the closest point and is travelling away from the black hole at more than 10 million km/h, whilst the tail is still falling towards it.
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