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Young stars close to SgrA*
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'Young' stars that seem to have formed impossibly close to our galaxy's supermassive black hole could in fact be ancient interlopers merely masquerading as youngsters, a new study claims.
Several clusters of what appear to be massive young stars have been found just a few dozen light years from the black hole at the centre of the galaxy.  But that is puzzling, since astronomers think the black hole's intense gravity should rip apart gas clouds before they have a chance to condense and form stars (although some recent work has disputed this). At the same time, such massive stars are too short-lived to have survived a journey from much farther out.

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RE: Our black hole
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In a cosmic game of pinball, black holes fling high-energy protons into space, where they zigzag around at near light-speeds before smashing into low-energy protons, a new study finds.
Then the collisions send bursts of gamma rays flying out from the centre of our galaxy, which explains for the first time the mechanism for the high-energy jets first spotted in 2004.
This proton-slinging could explain more than this cataclysmic light show deep in our galaxy. The scientists suggest other black holes in the universe could rely on the pinball mechanism to produce enormous jets of light.

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Our galaxy's supermassive black hole is responsible for the mysterious gamma-ray emission from the galactic centre, a new study suggests. Churning magnetic fields around the monster black hole may act like a giant particle accelerator, leading to high-speed collisions that produce the gamma rays.
Extremely energetic gamma rays, with energies in the tens of tera-electronvolts (1 TeV is 1012 eV) have been detected streaming from our galaxy's centre recently by ground-based gamma-ray observatories such as the High Energy Stereoscopic System (HESS) near Gambsberg, Namibia.
How such high-energy gamma rays are produced has been a mystery. Some scientists have proposed that it is the result of dark matter particles decaying, but others are not so sure.

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SgrA*
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The giant black hole at our galaxy's centre ate something about as massive as Mercury 60 years ago, new results from the Chandra X-ray Observatory suggest. The finding adds to previous evidence that the black hole, which is currently starved, does in fact devour things from time to time.

Our galaxy harbours a black hole called SgrA* that weighs nearly 4 million Suns, based on studies of the motion of stars and gas around it. But it does not appear to be gaining any weight at the moment, since it is not flaring up in X-rays, as happens when it sucks matter towards it.
There are signs that it has had larger meals in the past, however. Astronomers can infer the black hole's dietary history by looking for signs of its X-ray flares reaching and heating up gas clouds nearby. By measuring their distances from the black hole, they can tell how long it took for the X-rays to get there and thus when the flares occurred.
Several studies have shown evidence of past outbursts, including one in 2005 suggesting that SgrA* ate a large meal 350 yeas ago that made it a million times brighter than it is now.

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Sagittarius A*
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Title: The Position of Sagittarius A*: III. Motion of the Stellar Cusp
Authors: M. J. Reid, K. M. Menten, S. Trippe, T. Ott, R. Genzel

In the first two papers of this series, we determined the position of Sgr A* on infrared images, by aligning the positions of red giant stars with positions measured at radio wavelengths for their circumstellar SiO masers. In this paper, we report detections of 5 new stellar SiO masers within 50" (2 pc) of Sgr A* and new and/or improved positions and proper motions of 15 stellar SiO masers. The current accuracies are ~1 mas in position and ~0.3 mas/y in proper motion. We find that the proper motion of the central stellar cluster with respect to Sgr A* is less than 45 km/s. One star, IRS 9, has a three-dimensional speed of ~370 km/s at a projected distance of 0.33 pc from Sgr A*. If IRS 9 is bound to the inner parsec, this requires an enclosed mass that exceeds current estimates of the sum of the mass of Sgr A* and luminous stars in the stellar cusp by ~0.8 x 10^6 Msun. Possible explanations include i) that IRS 9 is not bound to the central parsec and has "fallen" from a radius greater than 9 pc, ii) that a cluster of dark stellar remnants accounts for some of the excess mass, and/or iii) that Ro is considerably greater than 8 kpc.

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Sgr A* Flare
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Title: Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma
Authors: F. Yusef-Zadeh, D. Roberts, M. Wardle, C. O. Heinke, G. C. Bower

Researchers have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A* at 43 GHz (lambda=7mm) and 22 GHz (lambda=13mm). A low level of flare activity has been detected with a duration of about 2 hours at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by about 20 to 40 minutes.
The overall characteristics of the flare emission are interpreted in terms of the plasmon model of Van der Laan (1966) by considering the ejection and adiabatically expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index nu^-alpha of alpha=1.6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically-expanding self-absorbed source.
They present the expected synchrotron light curves for an expanding blob as well as the peak frequency emission as a function of the energy spectral index constrained by the available flaring measurements in near-IR, sub-millimetre, millimetre and radio wavelengths. They note that the blob model is consistent with the available measurements, however, they can not rule out the jet of Sgr A*. If expanding material leaves the gravitational potential of Sgr A*, the total mass-loss rate of nonthermal and thermal particles is estimated to be less than 2 x 10^-8 solar masses per year^-1.
The researchers discuss the implication of the mass-loss rate since this value matches closely with the estimated accretion rate based on polarisation measurements.

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SgrA*
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16 October 2002

A video of what is currently thought to be the closest star to the supermassive black hole at the centre of our galaxy. The star, designated "S2", orbits SgrA* in a highly elliptical orbit with a period of 15 years or so, but at its closest approach it swings within 17 light hours of the black hole (around three times the distance between the Sun and Pluto).
In the video, the star ricochets past its closest approach to the black hole. This slingshot effect enabled astronomers to further pinpoint the mass of the black hole, which is confidently estimated at 2 million suns or so.
The mass observation, coupled with the size constraints observed, indicates the object at the centre of the galaxy is definitely composed of some exotically dense form of matter.

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Sagittarius A*
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Sagittarius A*: Stars Surprisingly Form in Extreme Environment Around Milky Way's Black Hole

Chandra's image of the Galactic Centre has provided evidence for a new and unexpected way for stars to form. A combination of infrared and X-ray observations indicates that a surplus of massive stars has formed from a large disk of gas around Sagittarius A*, the Milky Way's Black Hole .


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Position(2000): RA 17h 45m 40s Dec -29° 00' 28.00

According to the standard model for star formation, gas clouds from which stars form should have been ripped apart by tidal forces from the supermassive black hole. Evidently, the gravity of a dense disk of gas around Sagittarius A* offsets the tidal forces and allows stars to form. The tug-of-war between the black hole's tidal forces and the gravity of the disk has also favoured the formation of a much higher proportion of massive stars than normal.

This novel mode of star formation may solve several mysteries about supermassive black holes that reside at the centres of nearly all galaxies. When the massive stars explode as supernovas, they will enrich the central region's galaxies with heavy elements such as oxygen and iron. This may explain the large amounts of such elements observed in the disks of supermassive black holes.

Another possibility is that the massive stars around Sagittarius A* could have formed in a cluster far away from the black hole and migrated inward. A large number of low-mass stars would be expected to form in association with the massive stars - the migration model predicts that about a million low-mass stars should have migrated to the Galactic Centre along with the massive stars.

Chandra observations of the Galactic Centre show that the expected low-mass stars are not there. The conclusion is that the massive stars must have formed where we see them now - around the black hole.

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RE: Our black hole
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Black Holes Aren't So Black

Common wisdom holds that we can never see a black hole because nothing can escape it -- not even light. Fortunately, black holes aren't completely black. As gas is pulled into a black hole by its strong gravitational force, the gas heats up and radiates.
That radiation can be used to illuminate the black hole and paint its profile.
Within a few years, astronomers believe they will be able to peer close to the horizon of the black hole at the centre of the Milky Way.
Already, they have spotted light from "hot spots" just outside the black hole. While current technology is not quite ready for the final plunge, Harvard theorists Avery Broderick and Avi Loeb (Harvard-Smithsonian Centre for Astrophysics) already have modelled what observers will see when they look into the maw of this monster.

"It will be really remarkable when observers can see all the way to the edge of the Milky Way's central black hole -- a hole 10 million miles in diameter that's more than 25,000 light-years away" - Avery Broderick.

All it will take is a cross-continental array of Submillimetre telescopes to effectively create a single telescope as large as the Earth. This process, known as interferometry, has already been used to study longer wavelength radio emissions from outer space. By studying shorter wavelength Submillimetre emissions, astronomers could get a high-resolution view of the region just outside the black hole.


Comparison of the orbit-averaged, disk-subtracted images of a spot for two radio frequencies (at which opacity is important) and in the infrared (at which the disk is transparent). For reference, the photon-capture cross section for a Schwarzschild black hole is shown by the dashed white line.

"The Holy Grail of black hole astronomy is within our grasp. We could see the shadow that the black hole casts on surrounding material, and determine the size and spin of the black hole itself" - Avery Broderick.

Infrared observations using existing and near-future interferometric instruments also offer the possibility of imaging the core of our Galaxy in incredible detail, with resolutions better than one milli-arcsecond.

"Submillimetre and infrared observations are complementary. We need to use both to tackle the problem of getting high-resolution observations. It's the only way to get a complete picture of the Galactic centre" - Lincoln Greenhill, Smithsonian astronomer.

The black hole at the centre of the Milky Way is the best target for interferometric observations because it spans the largest area in the sky of any known black hole. Nevertheless, its angular size of tens of micro-arcseconds poses a major challenge to observers, requiring resolution 10,000 times better than the Hubble Space Telescope provides in visible light.

"When astronomers achieve it, that first image of the black hole's shadow and inner accretion disk will enter textbooks, and will test our current notions on gravity in the regime where space-time is strongly curved" - Avi Loeb.

"Ultimately, we want to test Einstein's general theory of relativity in the strong field limit -- within a strong gravitational field like that of a black hole" - Avery Broderick.

In preparation for that observational leap, Broderick and Loeb created a computer program to simulate the view. Emissions from the Galaxy's central black hole are known to fluctuate, probably due to clumps of material being swallowed. The researchers modelled those clumps of hot gas and predicted the up-close appearance.
They also summed the total light from the "hot spots" to simulate low-resolution observations possible with current technology.

New observational results are starting to come out and already are proving consistent with Broderick and Loeb's prediction.

"Observations to date only span a limited time interval. With routine monitoring, astronomers will be able to collect many examples of flares and start deriving the characteristics of the black hole itself" -Avi Loeb.


AVI movie
The difference between the two photos is that the Left photo shows the case of a non-spinning black hole and the photo on the right shows the case of a maximally spinning black hole. At present we do not know by how much the black hole in the Galactic centre is spinning. Future observations will enable astronomers to infer the spin of the black hole, and to test the validity of Einstein's theory of gravity.


Imaging Optically-Thin Hot Spots Near the Black Hole Horizon of Sgr A* at Radio and Near-Infrared Wavelengths

Images from the vicinity of the black hole horizon at the Galactic centre (Sgr A*) could be obtained in the near future with a Very Large Baseline Array of sub-millimetre telescopes. The recently observed short-term infrared and X-ray variability of the emission from Sgr A* implies that the accretion flow near the black hole is clumpy or unsteady.
The researchers calculate the appearance of a compact emission region (bright spot) in a circular orbit around a spinning black hole as a function of orbital radius and orientation.
They find that the mass and spin of the black hole can be extracted from their generic signatures on the spot image as well as on the lightcurves of its observed flux and polarisation. The strong-field distortion remains noticeable even when the spot image is smoothed over the expected ~20 microarcsecond resolution of future sub-millimetre observations.

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A group of young stars has been spotted dangerously close to the giant black hole at the centre of our Galaxy - only the second such group known to be braving the region's extreme conditions.
Two years ago, astronomers found the first cluster of young stars 0.7 light years from the black hole. How they got there is unclear, because the black hole's gravity ought to tear apart the clouds of gas and dust from which new stars form.
The cluster may have formed farther out in the galaxy and migrated inwards, held together from within by the gravity of a middleweight black hole.
The latest group of five young stars, found by Jessica Lu of the University of California in Los Angeles and her colleagues using the Keck I telescope in Hawaii, is moving in convoy even closer to the galactic centre - just 0.26 light years away. The stars appear to be only about 10 million years old and don't seem to be held together by a smaller black hole.


"What's amazing is that this little group of stars can survive in this hostile environment. You would think that the stars would be quickly torn apart." - Jessica Lu.

Lu speculates that star clusters might be able to form near the supermassive black hole despite the intense gravity, radiation, pressure and heat.
"The process would be very foreign to what we currently think of as star formation."
Alternatively, the stars might be remnants of a larger cluster that formed farther out and was stripped of most of its stars as it spiralled towards the black hole.
Super-massive black holes can create jets of charged particles when matter falling towards them gets ejected along strong magnetic fields. The particles slice through space at nearly the speed of light, emitting radio waves as they go.
It is possible that violent jet of particles shot out from the black hole triggered star birth in nearby neutral hydrogen gas.


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The black hole in the middle of the Milky Way - known as Sagittarius A* - weighs 3 million times as much as the Sun and has plenty of material nearby to gobble up. But curiously, the area around the black hole is relatively dim, emitting no more energy than a mediocre star like the Sun.
However, that has not always the case; Sagittarius A* was incredibly active 350 years ago, blasting out intense gamma rays. These have now reached the nearby B2 cloud, which is absorbing the radiation and fluorescing in X-rays as a result.
The black hole, as viewed from Earth, was a million times brighter than it is now.



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-- Edited by Blobrana at 09:08, 2005-05-13

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