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RE: blackhole spectra
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Title: Quasinormal modes of black holes: from astrophysics to string theory
Authors: R. A. Konoplya, A. Zhidenko

Perturbations of black holes, initially considered in the context of possible observations of astrophysical effects, have been intensively studied for the past ten years in string theory, brane-world models and quantum gravity. Through the famous gauge/gravity duality, proper oscillations of perturbed black holes, called quasinormal modes (QNMs), allow for the description of the hydrodynamic regime in the dual finite temperature field theory at strong coupling, which can be used to predict the behavior of quark-gluon plasmas in the non-perturbative regime. On the other hand, the brane-world scenarios assume the existence of extra-dimensions in nature, so that multidimensional black holes might be formed in a laboratory experiment. All this stimulated active research in the field of perturbations of higher dimensional black holes and branes during recent years. In this review we discuss recent achievements on various aspects of black hole perturbations such as decoupling of variables in the perturbation equations, quasinormal modes (with special emphasis on various numerical and analytical methods of calculations), late-time tails, gravitational stability, AdS/CFT interpretation of quasinormal modes and holographic superconductors. We also briefly touch on state-of-the-art observational possibilities for detecting quasinormal modes of black holes.

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Rotating black holes
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Spinning black holes 'twist light'

Researchers have proposed a means to spot rotating black holes.
The idea may provide the first unique signature of black holes, which - true to their name - have never been seen.
The approach, reported in Nature Physics, relies on a property of light called its orbital angular momentum.
The roiling nature of the fabric of space-time around spinning black holes should impart a "twist" in this momentum which could be detected here on Earth, the researchers say.

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Supermassive black holes
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Supermassive black holes are thought to lurk at the heart of essentially all galaxies bigger than our own. Their powerful gravity should be luring in galactic matter, feeding the black holes' voracious appetites.
However, while plenty of gas is available for these black holes to feast upon, few of them have been observed to actively accrete gas from their home galaxy, presenting astronomers with a puzzle as to why these black holes aren't eating. Something must be preventing the black holes from accreting gas, though no one has known exactly what that was.

"This has been a longstanding problem" - Q. Daniel Wang of the University of Massachusetts at Amherst.

Now, Wang and his colleagues have some possible suspects behind the starving black holes: exploding stars, or supernovas.
Wang and his team investigated the starvation of the supermassive black holes at the center of two galaxies, M31 (aka the Andromeda Galaxy, our nearest galactic neighbour) and NGC 5866. They presented their findings here this week at the 213th meeting of the American Astronomical Society.

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Black holes are the rhythm at the heart of galaxies
The powerful black holes at the center of massive galaxies and galaxy clusters act as hearts to the systems, pumping energy out at regular intervals to regulate the growth of the black holes themselves, as well as star formation, according to new data from NASAs Chandra X-Ray Observatory.

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How black is a black hole?
The classical view of a black hole is that it is an object so massive that nothing, not even light, can escape its pull. In 1974, Stephen Hawking put forth his idea that black holes are not technically blackthey emit radiation and evaporate over time. Hawking radiation was assumed to be described by ideal black body radiation, and this assumption allowed for the derivation of the temperature and entropy of a black hole. But we don't actually know how black a black hole is or, in more scientific terms, what are the bounds of the effects of greybody terms in a black hole's radiation spectrum?

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VLT and Rossi XTE satellite probe violently variable black holes
Unique observations of the flickering light from the surroundings of two black holes provide new insights into the colossal energy that flows at their hearts. By mapping out how well the variations in visible light match those in X-rays on very short timescales, astronomers have shown that magnetic fields must play a crucial role in the way black holes swallow matter.
Like the flame from a candle, light coming from the surroundings of a black hole is not constant it flares, sputters and sparkles.

"The rapid flickering of light from a black hole is most commonly observed at X-ray wavelengths. This new study is one of only a handful to date that also explore the fast variations in visible light, and, most importantly how these fluctuations relate to those in X-rays" - Poshak Gandhi, who led the international team that reports these results.

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Black holes
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Galaxy riddled with black holes
Huge black holes may riddle the galaxy hidden within clusters of stars, scientists believe.
Astronomers think most galaxies, including our own, the Milky Way, have massive black holes at their centres.
Theories suggest the Milky Way was originally formed from smaller galaxies colliding together. If so, the black holes from these original galaxies could still be around.
Two US scientists believe their locations may already have been unknowingly detected.

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No matter how hard you try to push their boundaries, black holes always seem to preserve their modesty. Indiscreet astrophysicists have simulated the most violent collisions of black holes yet, and found that the resulting black hole still has an event horizon the surface through which even light cannot escape and that hide black holes interiors.
An international team of researchers created a computer simulation of what they call the most violent collision imaginable: Two black holes of equal masses smashing into each other head-on, moving at close to the speed of light.
Previous studies have suggested that when black holes collide they merge into one larger black hole, radiating huge amounts of energy in the form of gravitational waves ripples in the very shape of space that travel at the speed of light. This studys results were no exception. But the extreme velocities of the teams simulated black holes led to waves of unprecedented energy. Up to 14 percent of the black holes masses, instead of just a few percentage points, was converted into gravitational waves, the team reports in an upcoming Physical Review Letters.

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4U 1957+11
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Title: Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole
Authors: Michael A. Nowak, Adrienne Juett, Jeroen Homan, Yangsen Yao, Joern Wilms, Norbert S. Schulz, Claude R. Canizares

We present simultaneous Chandra-HETG and RXTE observations of a moderate flux `soft state' of the black hole candidate 4U1957+11. These spectra, having a minimally discernible hard X-ray excess, are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X 10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with normalised spin a*~1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere "colour-correction" factors. Standard colour-correction factors can be attained if one additionally incorporates mild Comptonisation. We find that the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and unknown distance of 4U1957+11; however, it is also driven by the limited bandpass of Chandra and XMM. We therefore present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a*~1, even when including a mild Comptonisation component; however, they also show evolution of the disk atmosphere colour-correction factors. If the rapid spin models with standard atmosphere colour-correction factors of h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11 can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.

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Ultra-massive black holes
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There appears to be an upper limit to how big the Universe's most massive black holes can get, according to new research led by a Yale University astrophysicist and published in Monthly Notices of the Royal Astronomical Society.
Once considered rare and exotic objects, black holes are now known to exist throughout the Universe, with the largest and most massive found at the centres of the largest galaxies. These "ultra-massive" black holes have been shown to have masses upwards of one billion times that of our own Sun. Now, Priyamvada Natarajan, an associate professor of astronomy and physics at Yale University and a fellow at the Radcliffe Institute for Advanced Study, has shown that even the biggest of these gravitational monsters can't keep growing forever. Instead, they appear to curb their own growth - once they accumulate about 10 billion times the mass of the Sun.

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