* Astronomy

Title: The "non-Kerrness" of domains of outer communication of black holes and exteriors of stars
Authors: T. Bäckdahl, J. A. Valiente Kroon

In this article we construct a geometric invariant for initial data sets for the vacuum Einstein field equations (\mathcal{S},h_{ab},K_{ab}), such that \mathcal{S} is a 3-dimensional manifold with an asymptotically Euclidean end and an inner boundary \partial \mathcal{S} with the topology of the 2-sphere. The hypersurface \mathcal{S} can be though of being in the domain of outer communication of a black hole or in the exterior of a star. The geometric invariant vanishes if and only if (\mathcal{S},h_{ab},K_{ab}) is an initial data set for the Kerr spacetime. The construction makes use of the notion of Killing spinors and of an expression for a \emph{Killing spinor candidate} which can be constructed out of concomitants of the Weyl tensor.

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Title: Measuring the Spins of Accreting Black Holes
Authors: Jeffrey E. McClintock, Ramesh Narayan, Shane W. Davis, Lijun Gou, Akshay Kulkarni, Jerome A. Orosz, Robert F. Penna, Ronald A. Remillard, James F. Steiner

A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modelling the X-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit (ISCO), whose radius depends only on the mass and spin of the black hole. In the Fe K method, which applies to both classes of black holes, one models the profile of the relativistically-broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting method, which has so far only been applied to stellar-mass black holes, one models the thermal X-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the continuum-fitting method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.

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Title: Black-Hole Mass and Growth Rate at z~4.8: A Short Episode of Fast Growth Followed by Short Duty Cycle Activity
Authors: Benny Trakhtenbrot, Hagai Netzer, Paulina Lira, Ohad Shemmer
(Version v2)

We present new Gemini-North/NIRI and VLT/SINFONI H-band spectroscopy for a flux limited sample of 40 z~4.8 active galactic nuclei, selected from the Sloan Digital Sky Survey. The sample probably contains the most massive active black holes (BHs) at this redshift and spans a broad range in bolometric luminosity, 2.7x10^46< L_bol < 2.4x10^47 erg/sec. The high-quality observations and the accurate fitting of the MgII(2800A) line, enable us to study, systematically, the distribution of BH mass (M_BH) and normalized accretion rate (L/L_Edd) at z~4.8. We find that 10^8 < M_BH < 6.6x10^9 M_sun, with a median of ~8.4x10^8 M_sun. We also find that 0.2 < L/L_Edd < 3.9 with a median of ~0.6. Most of these sources had enough time to grow to their observed mass at z~4.8 from z=20, assuming a range of seed BH masses, with ~40% that are small enough to be stellar remnants. Compared to previously studied samples at z~2.4 and 3.3, the masses of the z~4.8 BHs are typically lower by ~0.5 dex. and their L/L_Edd is higher by a similar factor. The new z~4.8 sample can be considered as the progenitor population of the most massive BHs at z~2.4 and 3.3. Such an evolutionary interpretation requires that the growth of the BHs from z~4.8 to z~3.3 and z~2.4 proceeds with short duty cycles, of about 10-20%, depending on the particular growth scenario.

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Yale professors Charles Bailyn and Meg Urry talk about the mysterious world of black holes. Predicted by Einstein's theories, their existence was only confirmed observationally in the early 1980s. Professors Bailyn and Urry begin with a description of the two types of black hole -- stellar and supermassive -- and explain how astronomers look for and investigate objects that by their nature emit no observable radiation of any kind. This investigation uses both ground-based telescopes, such as the SMARTS telescope system in Chile, as well as space-based telescopes such as NASA's Fermi Gamma Ray telescope. The discussion then turns to current research at Yale that furthers our understanding of these powerful objects, how they affect the stars and galaxies around them, and what they tell us about the nature of the Universe.