ESO: A Black Hole's Dinner is Fast Approaching

[bystander]

A Black Hole's Dinner is Fast Approaching
European Southern Observatory | 2011 Dec 14

VLT spots cloud being disrupted by black hole

Simulation of gas cloud after close approach to the black hole at the centre of the Milky Way (Credit: ESO/MPE/Marc Schartmann)

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Click to play embedded YouTube video.

ESOcast 39: A Black Hole's Dinner is Fast Approaching

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Astronomers using ESO’s Very Large Telescope have discovered a gas cloud with several times the mass of the Earth accelerating fast towards the black hole at the centre of the Milky Way. This is the first time ever that the approach of such a doomed cloud to a supermassive black hole has been observed. The results will be published in the 5 January 2012 issue of the journal Nature.

During a 20-year programme using ESO telescopes to monitor the movement of stars around the supermassive black hole at the centre of our galaxy (eso0846) ^[1], a team of astronomers led by Reinhard Genzel at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has discovered a unique new object fast approaching the black hole.

Over the last seven years, the speed of this object has nearly doubled, reaching more than 8 million km/h. It is on a very elongated orbit ^[2] and in mid-2013 it will pass at a distance of only about 40 billion kilometres from the event horizon of the black hole, a distance of about 36 light-hours ^[3]. This is an extremely close encounter with a supermassive black hole in astronomical terms.

This object is much cooler than the surrounding stars (only about 280 degrees Celsius), and is composed mostly of hydrogen and helium. It is a dusty, ionised gas cloud with a mass roughly three times that of the Earth. The cloud is glowing under the strong ultraviolet radiation from the hot stars around it in the crowded heart of the Milky Way.

The current density of the cloud is much higher than the hot gas surrounding the black hole. But as the cloud gets ever closer to the hungry beast, increasing external pressure will compress the cloud. At the same time the huge gravitational pull from the black hole, which has a mass four million times that of the Sun, will continue to accelerate the inward motion and stretch the cloud out along its orbit.

“The idea of an astronaut close to a black hole being stretched out to resemble spaghetti is familiar from science fiction. But we can now see this happening for real to the newly discovered cloud. It is not going to survive the experience,” explains Stefan Gillessen (MPE) the lead author of the paper.

The cloud’s edges are already starting to shred and disrupt and it is expected to break up completely over the next few years ^[4]. The astronomers can already see clear signs of increasing disruption of the cloud over the period between 2008 and 2011.

The material is also expected to get much hotter as it nears the black hole in 2013 and it will probably start to give off X-rays. There is currently little material close to the black hole so the newly-arrived meal will be the dominant fuel for the black hole over the next few years.

One explanation for the formation of the cloud is that its material may have come from nearby young massive stars that are rapidly losing mass due to strong stellar winds. Such stars literally blow their gas away. Colliding stellar winds from a known double star in orbit around the central black hole may have led to the formation of the cloud.

“The next two years will be very interesting and should provide us with extremely valuable information on the behaviour of matter around such remarkable massive objects,” concludes Reinhard Genzel.

1. Notes:
2. The black hole at the centre of the Milky Way is formally known as Sgr A* (pronounced Sagittarius A star). It is the closest supermassive black hole known by far and hence is the best place to study black holes in detail.
3. The observations were made using the NACO infrared adaptive optics camera and the SINFONI infrared spectrograph, both attached to the ESO Very Large Telescope in Chile. The centre of the Milky Way lies behind thick dust clouds that scatter and absorb visible light and must be observed at infrared wavelengths where the clouds are more transparent.
4. A light-hour is the distance that light travels in one hour. It is a little more than the distance from the Sun to the planet Jupiter in the Solar System. For comparison the distance between the Sun and the nearest star is more than four light-years. The cloud will pass at less than ten times the distance from the Sun to Neptune from the black hole
5. This effect well known from the physics of fluids and can be seen when for example pouring syrup in a glass of water. The flow of syrup downwards through the water will be disrupted and the droplet will break apart — effectively diluting the syrup in the water.
   
   A gas cloud on its way toward the super-massive black hole in the Galactic Centre - S. Gillessen et al
   • Nature (online 2011 Dec 14) DOI: 10.1038/nature10652 (eso pdf)

A big meal coming up for galactic black hole
Max Planck Gesellschaft | 2011 Dec 14

Astronomers discover a gas cloud which will soon be swallowed up by the object Saggitarius A*

Wandering cloud: This composite image shows the positions of the gas cloud in 2002, 2007, and 2011 marked in colour. (© ESO/MPE)

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At the moment, the black hole at the heart of the Milky Way is going hungry. But its diet may soon be over: a gas cloud has ventured too close to the super massive black hole and will be devoured by it over the next few years. The feeding of the black hole will be observed by astronomers at first-hand, who should also be able to note a largely increased X-ray emission at the time. Even now they can see how the huge gravitational pull of the black hole is causing some distortion to the gas cloud. The cloud was discovered by an international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics.

On a clear night, a shimmering band appears to span the skies: the Milky Way. It is part of our galaxy – that system of gas and dust and at least 200 billion stars, of which our sun is also a member. The constellation Saggitarius (from the Latin for the Archer), is particularly rich with stars. Here, hidden behind cosmic clouds, lies the centre of our galaxy. This is where astronomers observe the strong radio source Sagittarius A*. And the scientists suspect that behind it resides a black hole.

At the heart of our Milky Way resides a black hole with about 4.3 million solar masses, as has been proven with long term observations of the motions of stars orbiting this gravitational monster.

At a distance of 26,000 light years, Sagittarius A* is the only super massive black hole close enough to be observed it in detail. Long-term studies of the stellar orbits around this gravitational monster show that it has about 4.3 million solar masses. For most of the time, the black hole lays dormant, emitting modest flares only occasionally. While black holes cannot emit radiation directly by their very nature, the emission from the galactic centre originates from matter falling towards the event horizon, releasing potential energy and heating up.

Analysing very sharp images and detailed observations of the galactic centre with the Very Large Telescope of the European Southern Observatory (ESO), the astronomers have now detected for the first time a gas cloud that is falling into the accretion zone of the black hole. The scientists discovered that the orbit of the cloud is highly eccentric. In the year 2013, it will be closest to the black with a distance of 40 billion kilometres – a very close encounter in astronomical terms.

“Only two stars so far have come that close to the black hole since we started our observations in 1992,” says Stefan Gillessen, lead author of the paper describing the detection and analysis of the gas cloud. "The crucial difference to these stars - that passed unharmed through their closest approach - is that the gas cloud will be completely ripped apart by the tidal forces around the black hole." As a result the gas inflow into the black hole should increase substantially, as should the level of radiation from it.

The gas cloud can be seen in all long-wavelength infrared images from 2002 onwards, and for the past three years already shows signs of being disrupted. As the cloud falls towards the black hole – its current velocity is about 2350 kilometres per second – it will interact with the hot gas present in the accretion flow around the black hole and become disrupted by turbulent interaction.

“Because the mass of the gas cloud is larger than the mass of the hot gas within the area of closest approach to the black hole, the accretion near the event horizon will be temporarily dominated by the accretion of the cloud itself,” explains Reinhard Genzel, director at the Max Planck Institute for Extraterrestrial Physics and head of the galactic centre research group. “This will provide stringent constraints on the physics of black hole accretion, since we have an unusually good knowledge of the mass available.”

In 2013, the distance of the gas cloud to the black hole will be 36 light hours, which is about 3100 times the event horizon of the black hole. In terms of our solar system, this would be about 250 times the distance Earth-Sun, and the event horizon of the black hole is about 20 times the size of the Sun but with a mass 4.3 million times larger.

As the gas cloud falls towards the black hole, the hot gas present in the accretion disk around the black hole is expected to drive a shock wave, which will slowly compress the cloud. This will lead to a growing, dense shell surrounding the inner zone of the gas cloud. Due to the black hole’s tidal forces, the cloud becomes elongated along its direction of motion, until it is completely disrupted due to instabilities at the contact area.

Just before pericentre the gas cloud intersects with the shock front and the post-shock temperature may increase rapidly to several million Kelvin. This should lead to increased emission in particularly in high-energy X-rays.

Due to the long-term observations at many different wavelengths, the astronomers can constrain the properties of the cloud very well. The temperature of the warm dust cloud is about 280 °C and its density is 300 times greater than that of the surrounding hot gas, with a total mass of about three Earth masses (1.7 10^25 kg). With this information, the scientists were able so simulate the time evolution of the size and velocities in a model, the main effects being the gravitational pull of the super massive black hole and the interaction with the surrounding hot gas.

From this simulation and hydrodynamic calculations, the astronomers predict that the temperature of the gas cloud should increase rapidly to several million Kelvin near the black hole, leading to X-ray emission that should initially be somewhat larger than the current X-ray luminosity of the galactic centre. Over the following years it could potentially brighten by a large factor.

“Detailed observations of the radiation from the galactic centre over the next years will give us the unique opportunity to probe the properties of the accretion flow and observe the feeding process of a super massive black hole in real time,” predicts Stefan Gillessen.

Galactic Black Hole disrupts Gas Cloud
Max Planck Institute for Extraterrestrial Physics | 2011 Dec 14

Warning: Black Hole Dead Ahead!
Science NOW | Ken Croswell | 2011 Dec 14

Disaster looms for gas cloud falling into Milky Way’s central black hole
University of California, Berkeley | 2011 Dec 14

[bystander]

ScienceShot: Probing a Black Hole
Science NOW | Bruce Dorminey | 2011 Dec 20

[i]Credit: NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al[/i]

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How do you probe a supermassive black hole? Take a look at the pulsars that orbit it. These rapidly spinning neutron stars flash regular radio pulses, and in an upcoming issue of The Astrophysical Journal astronomers say that the timing of such pulses could provide a new understanding of the 4 million solar mass black hole at the center of the Milky Way^*. Scientists have speculated that physics as we know it could break down in the presence of such a strong gravitational force. If that's the case, any flashes from nearby pulsars would appear to speed up or slow down when viewed from Earth, with their clocklike arrival times running early or late and likely dependent on where their orbits were in relation to the black hole. In the process, the astronomers also hope to determine Sagittarius A*'s spin rate and true mass down to an accuracy of about 1 part in a million. First, though, they have to find pulsars close enough to this gravitational monstrosity to be useful. And that's not expected to happen until the Square Kilometer Array comes online early next decade.

• Prospects for Probing the Spacetime of Sgr A* with Pulsars - K. Liu, N. Wex, M. Kramer, J. M. Cordes, T. J. W. Lazio
  • arXiv.org > astro-ph > arXiv:1112.2151 > 09 Dec 2011

[bystander]

Milky Way's black hole getting ready for snack
Lawrence Livermore National Laboratories | 2012 Oct 22

[i]3D volume visualization, spanning the period 2010 to 2020, of the gas and dust cloud G2 as it approaches the Sgr A* black hole near the center of the Milky Way galaxy.[/i]

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Get ready for a fascinating eating experience in the center of our galaxy.

The event involves a black hole that may devour much of an approaching cloud of dust and gas known as G2.

A supercomputer simulation prepared by two Lab physicists and a former postdoc suggests that some of G2 will survive, although its surviving mass will be torn apart, leaving it with a different shape and questionable fate.

The findings are the work of computational physicist Peter Anninos and astrophysicist Stephen Murray, both of AX division within the Weapons and Complex Integration Directorate (WCI), along with their former postdoc Chris Fragile, now an associate professor at the College of Charleston in South Carolina, and his student, Julia Wilson.

They came up with six simulations, using the Cosmos++ computer code developed by Anninos and Fragile, which required more than 50,000 computing hours on 3,000 processors on the Palmetto supercomputer at Clemson University in Columbia, S.C.

Previous simulations of the upcoming event had been done in two-dimensions, but the Cosmos++ code includes 3D capability, as well as a unique "moving mesh" enhancement, allowing the simulation to more-efficiently follow the cloud's progression toward the black hole.

The black hole is known as Sgr A*. "Sgr" is the abbreviation for Sagittarius, the constellation near the center of the Milky Way. Most galaxies have a black hole at their center, some thousands of times bigger than this one.

"While this one is 3-to-4 million times as big as our sun, it has been relatively quiet," according to Murray. "It's not getting fed very much." Contrary to their name, black holes can appear very bright. That's because gas orbiting them loses energy via friction, getting hotter and brighter as it spirals inward before falling into the black hole.

The composition of the G2 cloud is still a mystery.

Astronomers originally noticed something in the region in 2002, but the first detailed determinations of its size and orbit came only this year. The dust in the cloud has been measured at about 550 degrees Kelvin, approximately twice as hot as the surface temperature on Earth. The gas, mostly hydrogen, is about 10,000 degrees Kelvin, or almost twice as hot as the surface of the sun.

Its origin is still unknown.

Murray says: "The speculation ranges from it having been an old star that had kind of a burp and lost some of its outer atmosphere, to something that was trying to be a planet and couldn't quite manage it because the environment was too hot."

As the cloud approaches the black hole and begins to fall into what Murray describes as "a gravity well" beginning next September, it will begin to shed energy, causing it to heat to incredibly high temperatures, visible to radio and X-ray telescopes on Earth as well as orbiting satellites such as NASA's Chandra X-ray Observatory.

But it won't be a collision course.

The point at which a stellar object can no longer escape being swallowed by a black hole is known as the Schwarzschild radius, a quantity whose value depends on the black hole's mass, the speed of light and the gravitational constant.

The cloud will actually pass far enough away that it will escape the point of no return by approximately 2,200 Schwarzschild radii, which in this case is about 200 times as far as Earth is from the sun.

But the supercomputer simulations show that the cloud will not survive the encounter.

According to Anninos: "There's too much dynamical friction that it experiences through hydrodynamic instabilities and tidal stretching from the black hole. So a lot of its kinetic energy and angular momentum will be dissipated away and it will just sort of break up into some sort of incoherent structure. Much of it will join the rest of the hot accretion disk around the black hole, or just fall and get captured by the black hole. It will lose a lot of its energy but not all of it. It will become so diffuse that it's unlikely that any remnant of the gas will continue on its orbital track."

The close encounter will take several months. The entire event is predicted to last less than a decade.

The simulation is posted on the Web. It shows the cloud modeled as a simple gas sphere, near the point in its orbit where it was first discovered. As it approaches Sgr A*, a process known as tidal stretching increasingly distorts the cloud. By the end of 2012, the cloud will be nearly five times longer than it is wide.

Along with tidal stretching, the cloud also experiences resistance in the form of ram pressure as it tries to plow through the hot interstellar gas that already fills the space around Sgr A*. The interactions of G2 with this background gas cause further disruptions to the cloud from Rayleigh-Taylor and Kelvin-Helmholtz instabilities. Collectively, these effects act to strip some material from the cloud and feed it into Sgr A*.

An article describing the simulation research will appear in an upcoming issue of the Astrophysical Journal.

3D Moving-Mesh Simulations of Galactic Center Cloud G2 - Peter Anninos, P. Chris Fragile, Julia Wilson, Stephen D. Murray

• arXiv.org > astro-ph > arXiv:1209.1638 > 07 Sep 2012

[bystander]

Galactic Gas Cloud Could Help Spot Hidden Black Holes
Universe Today | Jason Major | 2013 Feb 15

[i]Illustration of gas cloud G2 approaching Sgr A* (ESO/MPE/M.Schartmann/J.Major)[/i]

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The heart of our Milky Way galaxy is an exotic place. It’s swarming with gigantic stars, showered by lethal blasts of high-energy radiation and a veritable cul-de-sac for the most enigmatic stellar corpses known to science: black holes. And at the center of the whole mélange is the granddaddy of all the black holes in the galaxy — Sagittarius A*, a supermassive monster with 4 million times more mass than the Sun packed into an area smaller than the orbit of Mercury.

Sgr A* dominates the core of the Milky Way with its powerful gravity, trapping giant stars into breakneck orbits and actively feeding on anything that comes close enough. Recently astronomers have been watching the movement of a large cloud of gas that’s caught in the pull of Sgr A* — they’re eager to see what exactly will happen once the cloud (designated G2) enters the black hole’s dining room… it will, in essence, be the first time anyone watches a black hole eat.

But before the dinner bell rings — estimated to be sometime this September — the cloud still has to cover a lot of space. Some scientists are now suggesting that G2′s trip through the crowded galactic nucleus could highlight the locations of other smaller black holes in the area, revealing their hiding places as it passes.

In a new paper titled “G2 can Illuminate the Black Hole Population near the Galactic Center” researchers from Columbia University in New York City and the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts propose that G2, a cloud of cool ionized gas over three times more massive than Earth, will likely encounter both neutron stars and other black holes on its way around (and/or into) SMBH Sgr A*.

The team notes that there are estimated to be around 20,000 stellar-mass black holes and about as many neutron stars in the central parsec of the galaxy. (A parsec is equal to 3.26 light-years, or 30.9 trillion km. In astronomical scale it’s just over 3/4 the way to the nearest star from the Sun.) In addition there may also be an unknown number of intermediate-mass black holes lurking within the same area.

These ultra-dense stellar remains are drawn to the center region of the galaxy due to the effects of dynamical friction — drag, if you will — as they move through the interstellar material.

Of course, unless black holes are feeding and actively throwing out excess gobs of hot energy and matter due to their sloppy eating habits, they are very nearly impossible to find. But as G2 is observed moving along its elliptical path toward Sgr A*, it could very well encounter a small number of stellar- and intermediate-mass black holes and neutron stars. According to the research team, such interactions may be visible with X-ray spotting spacecraft like NASA’s Chandra and NuSTAR.

The chances of G2 encountering black holes and interacting with them in such a way as to produce bright enough x-ray flares that can be detected depends upon a lot of variables, like the angles of interaction, the relative velocities of the gas cloud and black holes, the resulting accretion rates of in-falling cloud matter, and the temperature of the accretion material. In addition, any observations must be made at the right time and for long enough a duration to capture an interaction (or possibly multiple interactions simultaneously) yet also be able to discern them from any background X-ray sources.

Still, according to the researchers such observations would be important as they could provide valuable information on galactic evolution, and shed further insight into the behavior of black holes.

G2 can Illuminate the Black Hole Population near the Galactic Center - Imre Bartos, Zoltan Haiman, Bence Kocsis, Szabolcs Marka

• arXiv.org > astro-ph > arXiv:1302.3220 > 13 Feb 2013

[bystander]

Ripped Apart by a Black Hole
ESO Science Release | VLT | 2013 Jul 17

VLT watches in real time as gas cloud makes closest approach to the monster at the centre of the Milky Way

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Click to play embedded YouTube video.

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A gas cloud being ripped apart by the black hole at the centre of the Milky Way
(Credit: ESO/S. Gillessen)

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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.

In 2011 ESO's Very Large Telescope (VLT) discovered a gas cloud with several times the mass of the Earth accelerating towards the black hole at the centre of the Milky Way (eso1151) ^[1]. This cloud is now making its closest approach and new VLT observations show that it is being grossly stretched by the black hole’s extreme gravitational field.

"The gas at the head of the cloud is now stretched over more than 160 billion kilometres around the closest point of the orbit to the black hole. And the closest approach is only a bit more than 25 billion kilometres from the black hole itself — barely escaping falling right in," explains Stefan Gillessen (Max Planck Institute for Extraterrestrial Physics, Garching, Germany) who led the observing team ^[2]. "The cloud is so stretched that the close approach is not a single event but rather a process that extends over a period of at least one year."

As the gas cloud is stretched its light gets harder to see. But by staring at the region close to the black hole for more than 20 hours of total exposure time with the SINFONI instrument on the VLT — the deepest exposure of this region ever with an integral field spectrometer ^[3] — the team was able to measure the velocities of different parts of the cloud as it streaks past the central black hole ^[4].

"The most exciting thing we now see in the new observations is the head of the cloud coming back towards us at more than 10 million km/h along the orbit — about 1% of the speed of light," adds Reinhard Genzel, leader of the research group that has been studied this region for nearly twenty years. "This means that the front end of the cloud has already made its closest approach to the black hole."

The origin of the gas cloud remains mysterious, although there is no shortage of ideas ^[5]. The new observations narrow down the possibilities.

"Like an unfortunate astronaut in a science fiction film, we see that the cloud is now being stretched so much that it resembles spaghetti. This means that it probably doesn’t have a star in it," concludes Gillessen. "At the moment we think that the gas probably came from the stars we see orbiting the black hole."

The climax of this unique event at the centre of the galaxy is now unfolding and being closely watched by astronomers around the world. This intense observing campaign will provide a wealth of data, not only revealing more about the gas cloud ^[6], but also probing the regions close to the black hole that have not been previously studied and the effects of super-strong gravity.

1. Notes
   
   [*] The black hole at the centre of the Milky Way is estimated to have a mass of about four million times that of the Sun and is formally known as Sgr A* (pronounced Sagittarius A star). It is the closest supermassive black hole known by far and hence is the best place to study black holes in detail. The study of the supermassive black hole at the centre of the galaxy and its environment is rated number one in the list of ESO's top ten astronomical discoveries.
   
   [*] The distance of closest approach is about five times the distance of the planet Neptune from the Sun. This is much too close for comfort to a black hole with a mass four million times that of the Sun!
   
   [*] In an integral field spectrometer the light recorded in each pixel is separately spread out into its component colours and so spectra are recorded for each pixel. The spectra can then be analysed individually and used to create maps of the velocities and the chemical properties of each part of the object, for example.
   
   [*] The team is also hoping to see evidence of how the rapidly moving cloud interacts with any ambient gas around the black hole. So far nothing has been found, but further observations are planned to look for such effects.
   
   [*] Astronomers thought that the gas cloud might have been created by stellar winds from the stars orbiting the black hole. Or possibly even be the result of a jet from the galactic centre. Another option was that a star was at the centre of the cloud. In this case the gas would come either from a wind from the star, or from a planet-forming disc of gas and dust around the star.
   
   [*] As this event at the centre of the galaxy unfolds, astronomers expect to see that the evolution of the cloud switches from purely gravitational and tidal to complex, turbulent hydrodynamics.

Pericenter passage of the gas cloud G2 in the Galactic Center - Stefan Gillessen et al

• arXiv.org > astro-ph > arXiv:1306.1374 > 06 Jun 2013

Pulled Apart By Black Hole Heart
Universe Today | Tammy Plotner | 2013 Jul 17

Galactic Black Hole Disrupts Gas Cloud
Max Planck Institute for Extraterrestrial Physics

Close encounter: gas cloud swings around galactic centre black hole
Max Planck Institute for Extraterrestrial Physics | 2013 Jul 16

[geckzilla]

Why was it renamed to eso1151? Sgr A* is a lot easier for me to remember. Anytime there is an object named with 4 numbers I have to look back and forth at the number in order to transcribe it. Just typing this I almost put 1511. I can't get it right unless I am looking straight at the number. Argh!

[bystander]

eso1151 is the original eso article reporting this gas cloud. See OP.

[geckzilla]

My bad, I misread "formally" as "formerly".

[bystander]

Preparing for Fireworks in Our Galactic Center
Smithsonian Astrophysical Observatory
Weekly Science Update | 2013 Jul 12

[i]NASA's Nuclear Spectroscopic Telescope Array captured these views of the supermassive black hole at the heart of our galaxy in high-energy X-rays. The background image, taken in infrared light, shows the location of our Milky Way's supermassive black hole. In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. The time series at right shows observed flaring at four different X-ray energies. A gas cloud has been spotted approaching the black hole, and new calculations predict that bright radio emission from shocks generated buy this encounter may be detected later this year. [b](Credit: NASA/JPL-Caltech (Spitzer/Chandra/NuSTAR))[/b][/i] [url=http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA16214][b][i]Pointing X-ray Eyes at our Resident Supermassive Black Hole[/i][/b][/url]

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Our Milky Way galaxy has a super-massive black hole at its core, about four million solar-masses in size. Supermassive black holes of this mass or even thousands of times bigger are found at the nuclei of most galaxies. Black holes, despite their reputation for being implacable sinks for matter and energy, can eject both radiation and matter from their vicinity. Sometimes this outpouring occurs as powerful jets of charged particles, prompted when nearby matter accretes onto to a hot disk around the black hole. In dramatic cases, like quasars for example, nuclear black holes are responsible for some of the most spectacular phenomena in the cosmos, with dramatic jets, flaring, and intense radiation at all wavelengths including X-rays and gamma-rays. The processes take the huge gravitational potential energy of infalling matter and convert it into radiation other forms of energy.

The super-massive black hole at the center of the Milky Way, however, is quiescent. It emits no jets, and matter around the source has been seen only faintly spotted. The larger volume around the black hole, however, is much busier. There are seven bright stars and perhaps a thousand fainter stars orbiting the black hole, along with hot gas and dust. Over the years astronomers have measured the orbital motions of the bright stars to determine the mass of the black hole to a precision of about 10%. About ten years ago scientists noticed what seemed to be a cloud of gas also in orbit near the black hole. Last year they announced that, if their calculations are correct, the gas cloud will approach extremely close to the black hole -- about thirty-six light-hours (about 260 astronomical units) - and will do so sometime later this year! No one knows for sure what will happen, but some dramatic fireworks are expected if the cloud is torn apart by the strong gravitational force of the black hole, or is even eaten.

CfA astronomers Aleksander Sadowski, Ramesh Narayan, Lorenzo Sironi, and Feryal Ozel have performed detailed computations to simulate this coming galactic interaction. In particular, they looked at the shocks likely to develop as the cloud reaches its closest approach, and conclude that intense radio emission from such shocks is likely to develop seven to nine months before the actual moment of closest approach. Although so far no such emission has been spotted, there are considerable uncertainties in the future path of the cloud, in part because its internal structure is very uncertain. Nevertheless, this new paper has encouraged astronomers to keep their eyes peeled; the public is also alerted to stay tuned for exciting developments to come.

Location of the bow shock ahead of cloud G2 at the Galactic Center - A. Sadowski et al

• Monthly Notices of the RAS 433(3) 2165 (2013 Aug 11) DOI: 10.1093/mnras/stt879
  arXiv.org > astro-ph > arXiv:1303.3893 > 15 Mar 2013 (v1), 29 Apr 2013 (v2)

http://asterisk.apod.com/viewtopic.php?p=186312#p186312

<< Previous Science Update

[MargaritaMc]

According to these presentations at the AAS 223rd Meeting, the G2 cloud is still being observed as it approaches the Milky Way's black hole.
 

AstroBites
Wednesday, 8 January, 10:15 am EST
http://astrobites.org/2014/01/07/astrob ... at-aas223/
Care & Feeding of Black Holes
excerpt

Keck Observations of G2: can we watch a BH eat?
Leo Meyer (University of California Los Angeles)

We’re currently watching a gas cloud, called G2, as it passes close to the supermassive black hole Sgr A* in its orbit around the center of our galaxy (check out all the astrobites on this subject!). Previous estimates concluded that the point at which G2 passes closest to Sgr A* would be this past fall, but Leo Meyer’s group calculated that time to be closer to March 2014. This is good news for those of us who have been disappointed by the lack of fireworks associated with G2′s interaction with Sgr A* so far: maybe we can still hope for things to get more exciting in the future! We still don’t know whether G2 is a gas cloud or a star enshrouded by gas, but we have seen it start to get tidally sheared as it passes around Sgr A*. There isn’t yet any evidence from Sgr A* that it’s increased its snacking habits as a result of G2′s appearance, but stay tuned in the coming months!

The Swift/X-Ray Telescope Monitoring Campaign of the Galactic Center
Nathalie Degenaar (University of Michigan)
[108.03]

Speaking of staying tuned, you can actually keep up with Sgr A*’s activity yourself by going to
http://www.swift-sgra.com. Swift is a spacecraft that is currently doing daily X-ray monitoring of the galactic center, so we’ll be ready if Sgr A* flares up as a result of eating part of G2. However, due to (un)fortunate serendipity (unfortunate for people who want to study Sgr A*, fortunate for people studying magnetars), a magnetar recently went off very near Sgr A* — and its high X-ray luminosity is currently obscuring Swift’s view of Sgr A*. The good news is that even though we’re currently not getting the information about Sgr A* that we’d hoped for, the magnetar observations should help us to get a better measurement of the magnetic field around the galactic center, and could provide a useful way of testing Einstein’s theories of space and time distortions near black holes.

M

[neufer]

Click to play embedded YouTube video.

[saturno2]

This cloud has the important speed of 2,222 km/sec
( 8 million of km/h)

[MargaritaMc]

There is a useful AstroBite here:

http://astrobites.org/2014/01/20/what-f ... test-meal/
What Fed Sgr A* its Latest Meal?

It explores this paper
http://arxiv.org/abs/1401.2990
Possible Origin of the G2 Cloud from the Tidal Disruption of a Known Giant Star by Sgr A*

M

[BDanielMayfield]

This G2 gas clump story is starting to remind me of the comet ISON hype. Nevertheless, even if G2 doesn’t cause any dramatic fireworks at our galaxy’s core, much good science will be produced because of it, and that is also like ISON.

Something that intrigues me about SMBH’s is how they don’t just swallow all matter that approaches them. Often much or most of the material gets shot out the jets. What percentages of accreted and ejected matter happen in events like this? Hopefully this incident will shed light on this question.

Bruce

[MargaritaMc]

Have you seen this article, Bruce?
http://csirouniverseblog.com/2013/11/14 ... clude=3460

It's only background stuff that you probably know already (and I have an idea I've already posted it elsewhere - but senility is setting in and I'm not sure if or where...) but I loved the very graphic and Australian style of describing how a black hole feeds.

Margarita