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Posts: 131433
Black Hole "seeds"

Astronomers announced today that they have found the first sample of intermediate-mass black holes in active galaxies - a discovery that will help in understanding the early universe.

"These are local analogues of the `seed' black holes from which supermassive black holes formed" - Ms. Jenny E. Greene, the Harvard-Smithsonian Centre for Astrophysics (CfA).

Greene presented these results with Dr. Luis C. Ho of Carnegie Observatories at the 207th meeting of the American Astronomical Society in Washington, D.C.

"Supermassive black holes (with masses of millions to billions of times the mass of the Sun) are found in the centres of most, if not all, massive galaxies, and the black hole masses scale with the galaxy masses, so that larger black holes reside in larger galaxies. We want to understand how this connection is established, and more specifically, what role black holes play in the evolution of galaxies" - Ms. Jenny E. Greene.

Black holes probably evolve as material, such as gas, dust, stars and even other black holes, gets sucked in by the strong gravitational pull.

"However, we cannot observe the starting conditions of the black holes directly. How massive were they? How and when were they made? These are crucial questions to answer if we want to understand how black holes impact the growth of galaxies" - Dr. Luis C. Ho.

The black hole "seeds" originally may have formed from the explosions of the first stars or from the collapse of clumps of gas in the early universe. Each of these different formation scenarios leads to very different numbers of intermediate-mass black holes left over in the universe today. Until now, few good candidates had been found.
Greene sifted for intermediate-mass black holes in the first data release from the Sloan Digital Sky Survey, a multi-year comprehensive survey of one quarter of the sky. (The first public data release from the SDSS contains information on 50 million objects, including spectra and redshifts for almost 200,000 objects.)
In her thesis work, Greene identified objects with black holes by detecting the light from gas moving at extremely high velocities close to the black hole. Using the speed of the gas, and an estimate of the distance of the gas from the black hole, it was possible to estimate a black hole mass for each galaxy. She then selected all of the objects with masses less than one million solar masses, yielding a total of 19 new black holes.

"This sample provides the only currently available observational constraints on the properties of seed black holes in the early universe" - Ms. Jenny E. Greene.

Besides the formation of supermassive black holes seen today, this data set may help with another question - the re-ionisation of the universe.
Present theory holds that soon after the Big Bang, the universe was filled mostly with hydrogen and helium that was ionised - too hot to remain in a stable state. Over about 300,000 years, the universe expanded and cooled, and the gases began to recombine and stabilise to neutral states. This neutral gas acted as an opaque fog blocking the transmission of light. The universe then entered the dark ages, estimated to have lasted about half a billion years.
At the same time, matter was clumping together to form the first stars, galaxies and quasars. (Quasars are incredibly bright objects powered by supermassive black holes.) The radiation from these new objects made the opaque gas of the universe become transparent by splitting atoms of hydrogen into free electrons and protons, thus re-ionising the universe.

"These seed black holes presumably occasionally lit up as 'mini-quasars.' It is still an open question whether the emission from small black holes played an important role in re-ionising the universe, ending the cosmic dark ages. Our measurements of the light radiated by low-mass black holes will help us decide whether or not black holes in this mass range could have contributed significantly to re-ionisation" - Ms. Jenny E. Greene.

Gravity waves also could point to an early population of intermediate-sized black holes. When two black holes merge, their coalescence sends out gravity waves, or ripples in space-time.

"Gravitational wave experiments, especially the Laser Interferometric Space Antenna (LISA) expect to be very sensitive to the merging of 100,000-solar-mass black holes. The objects we identified will give clues to help the LISA team determine how many black hole collisions they may expect to find" - Ms. Jenny E. Greene.




Posts: 131433
RE: blackhole spectra

Scientists have found new evidence that black holes are performing the disappearing acts for which they are known.

A team from MIT and Harvard has found that a certain type of X-ray explosion common on neutron stars is never seen around their black hole cousins, as if the gas that fuels these explosions has vanished into a void.
This is strong evidence, the team said, for the existence of a theoretical border around a black hole called an event horizon, a point from beyond which nothing, not even light, can escape.

Ron Remillard of the MIT Kavli Institute in Cambridge, Mass., led the analysis and is discussing his team's result today at a press conference at the 207th meeting of the American Astronomical Society in Washington, D.C. His colleagues are Dacheng Lin of MIT and Randall Cooper and Ramesh Narayan of the Harvard-Smithsonian Centre for Astrophysics in Cambridge.
The scientists studied a complete sample of transient X-ray sources detected with NASA's Rossi X-ray Timing Explorer during the last nine years. They detected 135 X-ray bursts from the 13 sources believed to be neutron stars, but none from the 18 suspected black holes.
Gas released by a nearby star can accumulate on the hard surface of a neutron star, and it will eventually erupt in a thermonuclear explosion. The more massive compact objects in this study suspected of being black holes appeared to have no surface. Gas falling toward the black hole seems to disappear.

"Event horizons are invisible by definition, so it seems impossible to prove their existence. Yet by looking at dense objects that pull in gas, we can infer whether that gas crashes and accumulates onto a hard surface or just quietly vanishes. For the group of suspected black holes we studied, there is a complete absence of surface explosions called X-ray bursts" - Ron Remillard

A black hole forms when a very massive star runs out of fuel. Without energy to support its mass, the star implodes. If the star is more than 25 times more massive than our sun, the core will collapse to a point of infinite density with no surface. Within a boundary of about 50 miles from the black hole centre, gravity is so strong that not even light can escape its pull. This boundary is the theoretical event horizon.
Stars of about 10 to 25 solar masses will collapse into compact spheres about 10 miles across, called neutron stars. These objects have a hard surface and no event horizon.

Black holes and their neutron star cousins are sometimes located in binary systems, orbiting a relatively normal star companion. Gas from these stars, lured by strong gravity, can flow toward the compact object periodically. This process, called accretion, releases large amounts of energy, predominantly in the form of X-rays.
Gas can accumulate on a neutron star surface, and when conditions are ripe, the gas will ignite in a thermonuclear explosion that is visible as a one-minute event called a Type I X-ray burst. The suspected black holes -- that is, the more massive types of compact objects in this study -- behave as if they have no surface and are located behind event horizons.
The idea of using the absence of X-ray bursts to confirm the presence of event horizons in black holes was proposed in 2002 by Harvard's Narayan and Jeremy Heyl of the University of British Columbia in Vancouver.

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Posts: 131433

MIT scientists and colleagues have found a black hole that has chiselled a remarkably stable indentation in the fabric of space and time, like a dimple in one's favourite spot on the sofa.

The finding may help scientists measure a black hole's mass and how it spins, two long-sought measurements, by virtue of the extent of this indentation. Using NASA's Rossi X-ray Timing Explorer, the team saw identical patterns in the X-ray light emitted near the black hole nine years apart, as captured in archived data from 1996 and in a new, unprecedented 550-hour observation from 2005.
Black hole regions are notoriously chaotic, generating light at a range of frequencies. Similarities seen nine years apart imply something very fundamental is producing a pair of observed frequencies, namely the warping of space and time predicted by Einstein but rarely seen in such detail.
Jeroen Homan of the MIT Kavli Institute and his colleagues from the University of Michigan, Amsterdam University and MIT are presenting this result today at the annual meeting of the American Astronomical Society in Washington, D.C.

"The fact that we found the exact same frequency of X-ray oscillations nine years later is likely no coincidence. The black hole is still singing the same tune. The oscillations are created by a groove hammered into space-time by the black hole. This phenomenon has been suspected for a while, but now we have strong evidence to support it" - Jeroen Homan .

Position(2000): RA: 16 54 0.1 Dec: -39 50 44
Size 14'1 x 14'1

A black hole forms when a very massive star runs out of fuel. Without the power to support its mass, the star implodes and the core collapses to a point of infinite density. Black holes have a theoretical border called an event horizon. Gravity is so strong within the event horizon that nothing, not even light, can escape its pull. Outside the event horizon, light can still escape.

The X-ray brightness of the black hole X-ray binary GROJ165540, varying throughout its 2005 outburst. The high frequency oscillations were found between days ~75 and ~100.
Credit Jeroen Homan/MIT

Homan's team -- which includes Jon Miller of the University of Michigan, Rudy Wijnands of Amsterdam University and Walter Lewin of MIT -- observed a region less than 100 miles from the event horizon of a black hole system called GRO J1655-40. Here, matter can orbit a black hole relatively stably, but occasionally it wobbles at certain precise frequencies. This is a direct result of how the black hole deforms space and time, a four-dimensional concept that Einstein called space-time.
The team observed GRO J1655-40 twice a day on average for eight months, for a total of more than 550 hours. Gas from a companion star was falling toward the black hole, heating to high temperatures and causing the entire region to glow in X-ray light.
During the long observation, the team uncovered fluctuations in the X-ray light, called quasi-periodic oscillations, or QPOs. These are thought to be from wobbling blobs of gas whipping around the black hole. The team observed QPOs at frequencies of 300 Hz and 450 Hz -- the same as those observed nine years ago. This was by far the longest observation of a black hole during an outburst. Previous observations have determined that GRO J1655-40 is about 6.5 times more massive than the sun.

"The precise frequencies are determined by the mass of the black hole and also by how fast it spins. Those measurements -- mass and spin -- have been difficult to obtain. Fortunately, we already have an estimate of the mass of this black hole. By understanding the behaviour of matter so close to the black hole's edge, we can now begin to determine the spin and thus, for the first time, completely describe the black hole." - Jon Miller.

Graph showing the variability 'spectrum' of GRO J1655-40. The strength of the variability is plotted as a function of frequency. The fast oscillations we discovered at 300 Hz and 450 Hz are very weak and can only be seen by 'zooming' in - see lower panel.
Credit Jeroen Homan/MIT.

Making this detection possible, the team said, was the long and intensive observing program with the Rossi X-ray Timing Explorer, a unique and durable observatory launched on Dec. 30, 1995.

"Had we not observed in this way, we would probably not have detected the pair of QPOs again. We need time. X-ray light from black holes typically shows many types of fluctuations. Often we see black holes brighten and weaken a few times per second, but the rate at which this happens changes from day to day. What is so special about the fluctuations that we observed is not only that they are much faster than the ordinary fluctuations -- a few hundred times per second! -- but also that the rate of the fluctuations is exactly the same as when we last saw them, nine years ago." - Rudy Wijnands.


-- Edited by Blobrana at 03:46, 2006-01-10



Posts: 131433
RE: blackhole spectra

A research group at Cambridge think that the universe might once have been packed full of tiny black holes. Dr Martin Haehnelt, a researcher in the group led by Astronomer Royal Martin Rees, will present new evidence to support this controversial idea at the Institute of Physics conference Physics 2005 in Warwick.

Most cosmologists believe that supermassive black holes grew up in big galaxies, accumulating mass as time went on. But there is increasing evidence for a different view - that small black holes grew independently and merged to produce the giants which exist today.

Haehnelt points to evidence from recent studies of the cosmic microwave background (CMB).
This radiation, sometimes called `the echo of the big bang` has been travelling unaltered through space since the universe was just 400,000 years old. At that moment the universe cooled through a critical point, letting CMB radiation travel freely for the first time - as though a cosmic fog had lifted.

But new evidence shows that 10 to 15 percent of this radiation has been scattered since then. This indicates a re-warming of the universe which nobody had expected.
This could indicate an era in which small black holes were commonplace.

"Matter accreting around a black hole heats up. and this heating could be a sign that small black holes were widespread in the Universe at that time"- Dr Martin Haehnelt.

If small black holes merged to form the supermassive variety found at the centres of galaxies, there could be telltale evidence. Such a merger begins with two black holes going into orbit around each other, spiralling ever closer together. In the cataclysmic blast of energy when they finally merge, any asymmetry can send the resulting black hole flying off into space.

"If this happened, we might find the occasional galaxy with its central supermassive black hole missing" - Dr Martin Haehnelt
The evidence is by no means conclusive. Until it is, the CMB results will remain a source of heated debate.

Dr Martin Haehnelt is a Reader in Cosmology and Astrophysics at the Institute of Astronomy in the University of Cambridge.




Posts: 131433
RE: Rapid growth of high redshift black holes

Rapid growth of high redshift black holes
Marta Volonteri, Martin J. Rees

The two researchers discuss a model for the early assembly of supermassive black holes (SMBHs) at the centre of galaxies that trace their hierarchical build-up far up in the dark halo `merger tree'.
Motivated by the observations of luminous quasars around redshift z=6 with supermassive blackhole masses of billion solar masses, they assess the possibility of an early phase of stable super-critical quasi-spherical accretion in the BlackHoles hosted by metal free halos with virial temperature larger than 10000 K.

They assume that the first `seed' black holes formed with intermediate masses following the collapse of the first generation of stars, in mini-halos collapsing at z=20-30 from high peaks of density fluctuations.

In high redshift halos with virial temperature larger than 10000 K, conditions exist for the formation of a fat disc of gas at T_gas=5000-10000 K.
Cooling via hydrogen atomic lines is in fact effective in these comparatively massive halos.
The cooling and collapse of an initially spherical configuration of gas leads to a rotationally supported disc at the centre of the halo if baryons preserve their specific angular momentum during collapse.

The conditions for the formation of the gas disc and accretion onto central black holes out of this supply of gas are investigated, as well as the feedback of the emission onto the host and onto the intergalactic medium.

They find that even a short phase of supercritical accretion eases the requirements set by the z=6 quasars.

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Posts: 131433
RE: Black Holes in Astrophysics

Title: Black Holes in Astrophysics
Ramesh Narayan

Comments: To appear in a forthcoming Special Focus Issue on "Spacetime 100 Years Later" published by the New Journal of Physics

This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience.
Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star.
These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei.
A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons.
Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analogue of the Penrose process.

Movies here.
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Posts: 131433
blackhole spectra

Spectra obtained using the Chandra x-ray telescope show more than 300 supermassive black holes in the centres of galaxies.
A team of astronomers has been able to determine the amount of iron near the black holes. The black holes were all located in the North and South Chandra Deep Fields (located in the constellation Ursa Major), where the faintest and most-distant X-ray objects can be identified.

Chandra Deep Field-North xray image
Chandra Deep Field-South xray image

This graphic shows portions of X-ray spectra from a subset of 50 black holes about 9 billion light years away (upper panel), and another group of 22 black holes that are about 11 billion light years away (lower panel). The peaks in the spectra are produced by X-ray emission from iron atoms, and indicate that approximately the same amount of iron was present around black holes 9 billion years and 11 billion years in the past.
Similar results from other groups of black holes show that the amount of iron around black holes has not changed significantly over the past 11 billion years.
This implies that most of the iron in the galaxies that contain these supermassive black holes was created before the universe was about 2 billion years old, when galaxies were very young.

The distinctive X-ray spectral peaks in this result are produced by the fluorescence of iron atoms in a doughnut-shaped torus orbiting a supermassive black hole. In this process, high-energy X-rays from hot gas very near the black hole excite the iron atoms to a higher energy state, and they almost immediately return to their lower energy state with the emission of a lower-energy, fluorescent X-ray.

Position(J2000) RA 12h 36m 45.70s Dec +62 13' 58.00"

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