* Astronomy

Title: A black hole in a globular cluster
Authors: Thomas J. Maccarone (University of Southampton) Arunav Kundu, Stephen E. Zepf (Michigan State University) Katherine L. Rhode (Wesleyan University and Yale University)

Globular star clusters contain thousands to millions of old stars packed within a region only tens of light years across. Their high stellar densities make it very probable that their member stars will interact or collide. There has been considerable debate about whether black holes should exist in these star clusters. Some theoretical work suggests that dynamical processes in the densest inner regions of globular clusters may lead to the formation of black holes of ~1,000 solar masses. Other numerical simulations instead predict that stellar interactions will eject most or all black holes that form in globular clusters. Here we report the X-ray signature of an accreting black hole in a spectroscopically-confirmed globular cluster in the Virgo Cluster giant elliptical galaxy NGC 4472. This object has an X-ray luminosity of about 4*10^39 ergs/sec, making it brighter than any non-black hole object can be in an old stellar population. The X-ray luminosity varies by a factor of 7 in a few hours, ruling out the possibility that the object is several neutron stars superposed.

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Triple Interactions of Supermassive Black Holes Found To Be Common In Early Universe
New cosmological computer simulations produced by a team of astronomers from Northwestern University, Harvard University and the University of Michigan show for the first time that supermassive black holes (SMBHs), which exist at the centres of nearly all galaxies, often come together during triple galaxy interactions.
Frederic Rasio, a theoretical astrophysicist and professor of physics and astronomy in the Weinberg College of Arts and Sciences at Northwestern University in Evanston, Ill., will present the findings Jan. 8 at the meeting of the American Astronomical Society in Seattle.
The theoretical results are of special interest because of the recent discovery by astronomers at the California Institute of Technology of a possible triple quasar, findings that also will be reported at the Seattle meeting.

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Title: Dynamics of triple black hole systems in hierarchically merging massive galaxies
Authors: Loren Hoffman, Abraham Loeb

Galaxies with stellar bulges are generically observed to host supermassive black holes (SMBHs). The hierarchical merging of galaxies should therefore lead to the formation of SMBH binaries. Merging of old massive galaxies with little gas promotes the formation of low-density nuclei where SMBH binaries are expected to survive over long times. If the binary lifetime exceeds the typical time between mergers, then triple-black-hole systems may form. Such systems can lead to the ejection of one of the black holes (BHs) at a speed exceeding 1000 km/s, far greater than attainable through gravitational radiation recoil. We study the statistics of close triple-SMBH encounters in galactic nuclei by computing a series of three-body orbits with physically-motivated initial conditions appropriate for giant elliptical galaxies. Our simulations include a smooth background potential consisting of a stellar bulge plus a dark matter halo, drag forces due to gravitational radiation and dynamical friction on the stars and dark matter, and a simple model of the time evolution of the inner density profile under heating and mass ejection by the SMBHs. We find that the binary pair coalesces as a result of repeated close encounters in ~85% of our runs. In about 40% of the runs the lightest BH is left wandering through the galactic halo or escapes the galaxy altogether, but escape of all three SMBHs is exceedingly rare. The triple systems typically scour out cores with mass deficits ~1-2 times their total mass, which can help to account for the large cores observed in some massive elliptical galaxies, such as M87. The high coalescence rate, prevalence of very high-eccentricity orbits, and gravitational radiation "spikes" during close encounters may provide interesting signals for the future Laser Interferometer Space Antenna (LISA).

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Astronomers have found a black hole where few thought they could ever exist, inside a globular star cluster. The finding has broad implications for the dynamics of stars clusters and also for the existence of a still-speculative new class of black holes called 'intermediate-mass' black holes.
The discovery is reported in the current issue of Nature. Tom Maccarone of the University of Southampton in England leads an international team on the finding, made primarily with the European Space Agency's XMM-Newton satellite.
Maccarone's team found one such stellar-mass black hole by chance feeding in a globular cluster in a galaxy named NGC 4472, about fifty million light-years away in the Virgo Cluster.

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Title: The Galactic Black Holes Cyg X-1, GX 339-4, and 4U 1957+11: In Transition and at High Resolution
Authors: Michael A. Nowak

I review here some of the open questions regarding the geometry and emission mechanisms of galactic black hole candidates. For hard states, I concentrate on the perspective of ''disk+Compton coronae'' models (for discussions of jet models, see the papers by Sera Markoff). Specifically, I discuss the implications from our 10 year long RXTE monitoring campaign of Cyg X-1. I then present simultaneous RXTE/Chandra observations of the "soft state'' black hole candidate 4U 1957+11, and discuss to what extent it does or does not allow one to test ``relativistic disk models''. The use of such models has been claimed to measure black hole spin parameters. I then briefly present a particularly freaky-weird observation of GX 339-4, where the source ''fell off'' the usual radio/X-ray correlation in the low/hard state. Questions addressed by the above observations include: are the Compton corona models unique fits to the data? (No. Jets work equally well, and simple broken power laws work better still. We argue that the latter models indicate multiple, broad-band continuum components.) Is there good evidence for a receding disk as sources transit into the hard state? (The jury is still out.) What does the relativistically broadened Fe line tell us? (Sometimes the disk, even into quiescence, stays very close to the central object, in contrast to expectations of ADAF models.) How much better/more necessary are recently discussed relativistic disk models? (I am very doubtful that such models will ever usefully measure black hole spin.)

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