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For the first time, a team of international researchers has found a way to view the accretion disks surrounding black holes and verify that their true electromagnetic spectra match what astronomers have long predicted they would be. Their work will be published in the July 24 issue of the science journal Nature. A black hole and its bright accretion disk have been thought to form a quasar, the powerful light source at the centre of some distant galaxies. Using a polarising filter, the research team, which included Robert Antonucci and Omer Blaes, professors of physics at the University of California, Santa Barbara, isolated the light emitted by the accretion disk from that produced by other matter in the vicinity of the black hole.

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How do you weigh the biggest black holes in the universe? One answer now comes from a completely new and independent technique that astronomers have developed using data from NASA's Chandra X-ray Observatory.
By measuring a peak in the temperature of hot gas in the centre of the giant elliptical galaxy NGC 4649, scientists have determined the mass of the galaxys supermassive black hole. The method, applied for the first time, gives results that are consistent with a traditional technique.

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A new study using data from Chandra and ground-based telescopes, combined with detailed theoretical models, shows that the supermassive black hole in M81 feeds just like stellar mass black holes, with masses of only about ten times that of the Sun. This discovery supports the implication of Einstein's relativity theory that black holes of all sizes have similar properties, and will be useful for predicting the properties of a conjectured new class of black holes.

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Where did the universe's biggest black holes come from?
One idea suggests the behemoths began as smaller "seed" black holes that gobbled up surrounding gas. But new computer simulations suggest these seeds were born with practically nothing around them to eat, deepening the puzzle over how the biggest black holes came to be.

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Black holes can be secretive about their past, but now there may be an easy way to tell if a monster black hole was once a pair that got cosy and fused together.
Computer simulations suggest that when two black holes spiral towards each other on a collision course, much of the gas and dust in the spinning accretion disc surrounding each of them is ripped away by the gravity of the other. Some of this material fuses into a larger third disc that surrounds both.

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Title: Mergers of Stellar-Mass Black Holes in Nuclear Star Clusters
Authors: M. Coleman Miller, Vanessa M. Lauburg (University of Maryland)

Mergers between stellar-mass black holes will be key sources of gravitational radiation for ground-based detectors. However, the rates of these events are highly uncertain, given that such systems are invisible. One formation scenario involves mergers in field binaries, where our lack of complete understanding of common envelopes and the distribution of supernova kicks has led to rate estimates that range over a factor of several hundred. A different, and highly promising, channel involves multiple encounters of binaries in globular clusters or young star clusters. However, we currently lack solid evidence for black holes in almost all such clusters, and their low escape speeds raise the possibility that most are ejected because of supernova recoil. Here we propose that a robust environment for mergers could be the nuclear star clusters found in the centres of small galaxies. These clusters have millions of stars, black hole relaxation times well under a Hubble time, and escape speeds that are several times those of globulars, hence they retain most of their black holes. We present simulations of the three-body dynamics of black holes in this environment and estimate that, if most nuclear star clusters do not have supermassive black holes that interfere with the mergers, at least several tens of events per year will be detectable with Advanced LIGO.

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Title: Weighing the Black Holes in z~2 Submillimeter-Emitting Galaxies Hosting Active Galactic Nuclei
Authors: D.M. Alexander (Durham), W.N. Brandt, I. Smail, A.M. Swinbank, F.E. Bauer, A.W. Blain, S.C. Chapman, K.E.K. Coppin, R.J. Ivison, K. Menendez-Delmestre

We place direct observational constraints on the black-hole masses of the cosmologically important z~2 submillimeter-emitting galaxy (SMG; f850>4mJy) population, and use measured host-galaxy masses to explore their evolutionary status. We employ the well-established virial black-hole mass estimator to 'weigh' the black holes of a sample of z~2 SMGs with broad Halpha or Hbeta emission. The average black-hole mass and Eddington ratio (eta) of the lower-luminosity broad-line SMGs (L_X~10^44 erg/s} are log(M_BH/M_sol)~8.0 and eta~0.2, respectively. These lower-luminosity broad-line SMGs lie in the same location of the L_X-L_FIR plane as more typical SMGs hosting X-ray obscured AGN and may be intrinsically similar systems, but orientated so that the rest-frame optical nucleus is visible. Under this hypothesis, we conclude that SMGs host black holes with log(M_BH/M_odot)~7.8; we find supporting evidence from observations of local ULIRGs. Combining these black-hole mass constraints with measured host-galaxy masses, we find that the black holes in SMGs are >3 times smaller than those found in comparably massive normal galaxies in the local Universe, albeit with considerable uncertainty, and >10 times smaller than those predicted for z~2 luminous quasars and radio galaxies. These results imply that the growth of the black hole lags that of the host galaxy in SMGs, in stark contrast with that previously suggested for radio galaxies and luminous quasars at z~2. On the basis of current host-galaxy mass constraints, we show that SMGs and their descendants cannot lie significantly above the locally defined M_BH-M_GAL relationship. We argue that the black holes in the z~0 descendents of SMGs will have log(M_BH/M_odot)~8.6, indicating that they only need to grow by a factor of ~6 by the present day

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Title: The Accretion Disk Wind in the Black Hole GRO J1655-40
Authors: J. M. Miller (1), J. Raymond (2), C. S. Reynolds (3), A. C. Fabian (4), T. R. Kallman (5), J. Homan (6) ((1) University of Michigan, (2) Harvard-Smithsonian CfA, (3) University of Maryland, (4) University of Cambridge, (5) NASA/GSFC, (6) MIT)

We report on simultaneous Chandra/HETGS and RXTE observations of the transient stellar-mass black hole GRO J1655-40, made during its 2005 outburst. Chandra reveals a line-rich X-ray absorption spectrum consistent with a disk wind. Prior modelling of the spectrum suggested that the wind may be magnetically driven, potentially providing insights into the nature of disk accretion onto black holes. In this paper, we present results obtained with new models for this spectrum, generated using three independent photoionisation codes: XSTAR, Cloudy, and our own code. Fits to the spectrum in particular narrow wavelength ranges, in evenly spaced wavelength slices, and across a broad wavelength band all strongly prefer a combination of high density, high ionisation, and small inner radius. Indeed, the results obtained from all three codes require a wind that originates more than 10 times closer to the black hole and carrying a mass flux that is on the order of 1000 times higher than predicted by thermal driving models. If seminal work on thermally-driven disk winds is robust, magnetic forces may play a role in driving the disk wind in GRO J1655-40. However, even these modelling efforts must be regarded as crude given the complexity of the spectra. We discuss these results in the context of accretion flows in black holes and other compact objects.

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A group of radio astronomers has begun looking for signals from small, exploding black holes. The search is a long shot, but if it finds anything, it could be the best evidence yet for extra dimensions beyond the paltry four that we live in.

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Title: Transient Pulses from Exploding Primordial Black Holes as a Signature of an Extra Dimension
Authors: Michael Kavic, John H. Simonetti, Sean E. Cutchin, Steven W. Ellingson, Cameron D. Patterson
(Version v2)

An evaporating black hole in the presence of an extra spatial dimension would undergo an explosive phase of evaporation. We show that such an event, involving a primordial black hole, can produce a detectable electromagnetic pulse, signalling the existence of an extra dimension of size L ~ 10^{-18}-10^{-20}~m. We derive a generic relationship between the Lorentz factor of a pulse-producing "fireball" and the TeV energy scale. For a toroidally compactified extra dimension, transient radio-pulse searches probe the electroweak energy scale (~ 0.1 TeV), enabling comparison with the Large Hadron Collider. The enormous challenges of detecting quantum gravitational effects, and exploring electroweak-scale physics, make this a particularly attractive possibility.

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