* Astronomy

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RE: Sagittarius A*

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NASA'S NuSTAR Reveals Flare From Milky Way's Black Hole

NASA's newest set of X-ray eyes in the sky, the Nuclear Spectroscopic Telescope Array (NuSTAR), has caught its first look at the giant black hole parked at the center of our galaxy. The observations show the typically mild-mannered black hole during the middle of a flare-up.
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Title: Can we see pulsars around Sgr A*? - The latest searches with the Effelsberg telescope
Authors: R. P. Eatough, M. Kramer, B. Klein, R. Karuppusamy, D. J. Champion, P. C. C. Freire, N. Wex, K. Liu

Radio pulsars in relativistic binary systems are unique tools to study the curved space-time around massive compact objects. The discovery of a pulsar closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr A*, would provide a superb test-bed for gravitational physics. To date, the absence of any radio pulsar discoveries within a few arc minutes of Sgr A* has been explained by one principal factor: extreme scattering of radio waves caused by inhomogeneities in the ionised component of the interstellar medium in the central 100 pc around Sgr A*. Scattering, which causes temporal broadening of pulses, can only be mitigated by observing at higher frequencies. Here we describe recent searches of the Galactic centre region performed at a frequency of 18.95 GHz with the Effelsberg radio telescope.

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Title: An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
Authors: F. Yusef-Zadeh, M. Wardle, K. Dodds-Eden, C. O. Heinke, S. Gillessen, R. Genzel, H. Bushouse, N. Grosso, D. Porquet

The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a quiescent component dominates the emission at radio and sub-mm wavelengths. The spectral energy distribution of the quiescent emission from Sgr A* peaks at sub-mm wavelengths and is modelled as synchrotron radiation from a thermal population of electrons in the accretion flow, with electron temperatures ranging up to ~ 5-20\,MeV. Here we investigate the mechanism by which X-ray flare emission is produced through the interaction of the quiescent and flaring components of Sgr A*. The X-ray flare emission has been interpreted as inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present results of simultaneous X-ray and near-IR observations and show evidence that X-ray peak flare emission lags behind near-IR flare emission with a time delay ranging from a few to tens of minutes. Our Inverse Compton scattering modelling places constraints on the electron density and temperature distributions of the accretion flow and on the locations where flares are produced. In the context of this model, the strong X-ray counterparts to near-IR flares arising from the inner disk should show no significant time delay, whereas near-IR flares in the outer disk should show a broadened and delayed X-ray flare.

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NASA's Chandra Finds Milky Way's Black Hole May be Grazing on Asteroids

A new study provides a possible explanation for the mysterious flares. The suggestion is that there is a cloud around Sgr A* containing hundreds of trillions of asteroids and comets, which have been stripped from their parent stars. The panel on the left is an image containing nearly a million seconds of Chandra observing of the region around the black hole, with red representing low-energy X-rays, green as medium-energy X-rays, and blue being the highest.
An asteroid that undergoes a close encounter with another object, such as a star or planet, can be thrown into an orbit headed towards Sgr A*, as seen in a series of artist's illustrations beginning with the top-right panel. If the asteroid passes within about 100 million miles of the black hole, roughly the distance between the Earth and the Sun, it would be torn into pieces by the tidal forces from the black hole (middle-right panel).
These fragments would then be vaporised by friction as they pass through the hot, thin gas flowing onto Sgr A*, similar to a meteor heating up and glowing as it falls through Earth's atmosphere. A flare is produced (bottom-right panel) and eventually the remains of the asteroid are swallowed by the black hole.
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Title: Constraining the Flaring Region of Sagittarius A* By 1.3mm VLBI Measurements
Authors: Lei Huang, Zhi-Qiang Shen, Feng Gao

We use a model of an accretion flow coupled with an emergent flare to interpret the latest 1.3mm VLBI measurements for Sagittarius A*. The visibility data constrained the distances from the flare centre to the black hole centre as d_{EW}\lesssim20{R_g} and d_{NS}\lesssim80{R_g} in the East-West and North-South directions, respectively. If interpreted by the hot-spot model, the flare was preferred to pass in front of the black hole at a radius much larger than d_{EW}. If interpreted by the episodic jet launched from a nearly edge-on hot accretion flow, the flare was preferred to be ejected with \theta_{j}\gtrsim40° off the black hole rotating axis. This method can be generalised to help us understand future sub-millimetre VLBI observations, and study the millimetre/sub-millimetre variabilities in the vicinity of the Galactic Centre supermassive black hole.

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A gas cloud falling towards the supermassive black hole at the centre of the Milky Way

This video was created from a series of images of the centre of the Milky Way that were taken with ESOs Very Large Telescope between 2002 and 2011. They show not only the motions of the stars orbiting the central supermassive black hole, but also reveal a glowing blob of gas (in the circle) that is falling rapidly towards the black hole.
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Title: Disruption of a Proto-Planetary Disk by the Black Hole at the Milky Way Centre
Authors: Ruth A. Murray-Clay, Abraham Loeb (CfA)

Recently, Gillessen et al. discovered an ionised cloud of gas plunging toward the supermassive black hole, SgrA*, at the centre of the Milky Way. The cloud is being tidally disrupted along its path to closest approach at ~3100 Schwarzschild radii from the black hole. Here, we show that this cloud of gas naturally originates from a proto-planetary disk surrounding a low-mass star, which was scattered a century ago from the observed ring of young stars orbiting Sgr A*. As the young star approaches the black hole, its disk experiences both photo-evaporation and tidal disruption, producing a cloud with the observed properties. Our model implies that planets form in the Galactic centre, and that tidal debris from proto-planetary disks can flag low mass stars which are otherwise too faint to be detected.

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Cloud suicide will wake black hole sleeping giant

The sleeping giant at the centre of the Milky Way is about to wake up. A suicidal gas cloud is heading towards the galaxy's supermassive black hole, which will probably swallow the cloud, generating enormous flares of radiation that could help explain why the black hole is normally so placid.
The doomed cloud was a surprise to astronomers.
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Supermassive black hole will 'eat' gas cloud

Researchers have spotted a giant gas cloud spiralling into the supermassive black hole at our galaxy's centre.
Though it is known that black holes draw in nearby material, it will be the first chance to see one consume such a cloud.
As it is torn apart, the turbulent area around the black hole will become unusually bright, giving astronomers a chance to learn more about it.
An article in Nature suggests the spectacle should be visible in 2013.
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