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Galactic Center Cloud G2
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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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Title: Colliding with G2 near the Galactic Centre: a geometrical approach
Authors: R. de la Fuente Marcos, C. de la Fuente Marcos

The object G2 will pass within nearly 100 au from Sgr A* in 2014. Due to its very short periapse, the study of the dynamical evolution of this object in the short-term future may offer some insight into the region surrounding the supermassive black hole at the centre of the Galaxy. With this scenario in mind, it has recently been proposed by Bartos et al. (arXiv:1302.3220) that, prior to its perinigricon, G2 will likely experience multiple encounters with members of the black hole and neutron star populations believed to orbit near the Galactic Centre. Here, we further explore this possibility and study the general case for collisions with the G2 object using the latest orbital solutions provided by Phifer et al. (arXiv:1304.5280) and Gillessen et al., (arXiv:1306.1374) and a Monte Carlo approach to estimate the minimum orbit intersection distance (MOID) with G2 as a function of the orbital parameters of the incoming body. Our results indicate that encounters at distances closer than 100 au started to become statistically significant only during the last few years or so. MOIDs under 100 au are statistically more probable for the most dynamically cold orbits. If there is a population of objects moving in low-inclination, low-eccentricity orbits around the central black hole, the highest probability for a close encounter with G2 is found to be in the period 2014 January-March but enhanced activity due to encounters may start as early as 2013 July-August.

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Title: Pericenter passage of the gas cloud G2 in the Galactic Center
Authors: Stefan Gillessen, Reinhard Genzel, Tobias K Fritz, Frank Eisenhauer, Oliver Pfuhl, Thomas Ott, Marc Schartmann, Alessandro Ballone, Andreas Burkert

We have further followed the evolution of the orbital and physical properties of G2, the object currently falling toward the massive black hole in the Galactic Center on a near-radial orbit. New, very sensitive data were taken in April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to be stretched ever further along the orbit in position-velocity space. A fraction of its emission appears to be already emerging on the blue-shifted side of the orbit, past pericenter approach. Ionised gas in the head is now stretched over more than 15,000 Schwarzschild radii RS around the pericenter of the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter passage of G2 will be a process stretching over a period of at least one year. The Brackett-{\gamma} luminosity of the head has been constant over the past 9 years, to within ± 25%, as have the line ratios Brackett-{\gamma} / Paschen-{\alpha} and Brackett-{\gamma} / Helium-I. We do not see any significant evidence for deviations of G2's dynamical evolution, due to hydrodynamical interactions with the hot gas around the black hole, from a ballistic orbit of an initially compact cloud with moderate velocity dispersion. The constant luminosity and the increasingly stretched appearance of the head of G2 in the position-velocity plane, without a central peak, is not consistent with several proposed models with continuous gas release from an initially bound zone around a faint star on the same orbit as G2.

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Black hole bonanza possible as immense gas cloud passes

A vast and hidden field of small black holes predicted to be near the centre of our galaxy could be revealed as a giant gas cloud passes by.
The G2 cloud is as large as our Solar System, and bound for a "supermassive" black hole at the Milky Way's core.
On the way, it should encounter many black holes just tens of km across.

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Title: Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?
Authors: K. Phifer, T. Do, L. Meyer, A. M. Ghez, G. Witzel, S. Yelda, A. Boehle, J. R. Lu, M. R. Morris, E. E. Becklin, K. Matthews

We present new observations and analysis of G2 - the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ~ 3600) centered on the Hydrogen Br-gamma line as well as K' (2.1 micrometer) and L' (3.8 micrometer) imaging. Analysis of these observations shows the Br-gamma line emission has a positional offset from the L' continuum. This offset is likely due to background source confusion at L'. We therefore present the first orbital solution derived from Br-gamma line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 ± 0.14), closer periastron (130 AU, 1900Rs), and higher eccentricity (0.9814 ± 0.0060) compared to a solution using L' astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness "tail". It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Br-gamma line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-gamma data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.

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Title: Location of the bow shock ahead of cloud G2 at the Galactic Center
Authors: A. Sadowski, R. Narayan, L. Sironi, F. Ozel

We perform detailed magnetohydrodynamic simulations of interaction between the gas cloud G2 with the accretion flow around the Galactic Center black hole Sgr A*. We take as our initial conditions a steady-state, converged solution of the accretion flow we obtained using the general-relativistic magnetohydrodynamic code HARM. Using the observed parameters for the cloud's orbit, we compute the interaction of the cloud with the ambient gas and identify the shock structure that forms ahead of the cloud. We show that for many configurations, the cloud front crossed the orbit pericenter around January 2013 and that the first signatures of the shock, i.e., synchrotron radio emission from electrons accelerated at the bow shock, should already be visible. We argue that the radio emission is likely to reach peak values in February/March 2013.

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