149 research outputs found

### Black Holes: from Speculations to Observations

This paper provides a brief review of the history of our understanding and
knowledge of black holes. Starting with early speculations on ``dark stars'' I
discuss the Schwarzschild "black hole" solution to Einstein's field equations
and the development of its interpretation from "physically meaningless" to
describing the perhaps most exotic and yet "most perfect" macroscopic object in
the universe. I describe different astrophysical black hole populations and
discuss some of their observational evidence. Finally I close by speculating
about future observations of black holes with the new generation of
gravitational wave detectors.Comment: 15 pages, 6 Figures; to appear in the Proceedings of the Albert
Einstein Century International Conference, Paris, France, 200

### An alternative approach to solving the Hamiltonian constraint

Solving Einstein's constraint equations for the construction of black hole
initial data requires handling the black hole singularity. Typically, this is
done either with the excision method, in which the black hole interior is
excised from the numerical grid, or with the puncture method, in which the
singular part of the conformal factor is expressed in terms of an analytical
background solution, and the Hamiltonian constraint is then solved for a
correction to the background solution that, usually, is assumed to be regular
everywhere. We discuss an alternative approach in which the Hamiltonian
constraint is solved for an inverse power of the conformal factor. This new
function remains finite everywhere, so that this approach requires neither
excision nor a split into background and correction. In particular, this method
can be used without modification even when the correction to the conformal
factor is singular itself. We demonstrate this feature for rotating black holes
in the trumpet topology.Comment: 5 pages, 4 figures, matches version published in PR

### The Innermost Stable Circular Orbit in Compact Binaries

Newtonian point mass binaries can be brought into arbitrarily close circular
orbits. Neutron stars and black holes, however, are extended, relativistic
objects. Both finite size and relativistic effects make very close orbits
unstable, so that there exists an innermost stable circular orbit (ISCO). We
illustrate the physics of the ISCO in a simple model problem, and review
different techniques which have been employed to locate the ISCO in black hole
and neutron star binaries. We discuss different assumptions and approximations,
and speculate on how differences in the results may be explained and resolved.Comment: 13 pages, 2 figures, to appear in "Astrophysical Sources of
Gravitational Radiation", edited by J. M. Centrella (AIP Press

### Critical collapse of rotating radiation fluids

We present results from the first fully relativistic simulations of the
critical collapse of rotating radiation fluids. We observe critical scaling
both in subcritical evolutions, in which case the fluid disperses to infinity
and leaves behind flat space, and in supercritical evolutions that lead to the
formation of black holes. We measure the mass and angular momentum of these
black holes, and find that both show critical scaling with critical exponents
that are consistent with perturbative results. The critical exponents are
universal; they are not affected by angular momentum, and are independent of
the direction in which the critical curve, which separates subcritical from
supercritical evolutions in our two-dimensional parameter space, is crossed. In
particular, these findings suggest that the angular momentum decreases more
rapidly than the square of the mass, so that, as criticality is approached, the
collapse leads to the formation of a non-spinning black hole. We also
demonstrate excellent agreement of our numerical data with new closed-form
extensions of power-law scalings that describe the mass and angular momentum of
rotating black holes formed close to criticality.Comment: 5 pages, 4 figures; version accepted for publication in PR

### Luminosity versus Rotation in a Supermassive Star

We determine the effect of rotation on the luminosity of supermassive stars.
We apply the Roche model to calculate analytically the emitted radiation from a
uniformly rotating, radiation-dominated supermassive configuration. We find
that the luminosity at maximum rotation, when mass at the equator orbits at the
Kepler period, is reduced by ~36% below the usual Eddington luminosity from the
corresponding nonrotating star. A supermassive star is believed to evolve in a
quasistationary manner along such a maximally rotating ``mass-shedding''
sequence before reaching the point of dynamical instability; hence this reduced
luminosity determines the evolutionary timescale. Our result therefore implies
that the lifetime of a supermassive star prior to dynamical collapse is ~56%
longer than the value typically estimated by employing the usual Eddington
luminosity.Comment: 5 pages, 2 figures, uses emulateapj.sty; to appear in Ap

### A formalism for the construction of binary neutron stars with arbitrary circulation

Most numerical models of binary stars - in particular neutron stars in
compact binaries - assume the companions to be either corotational or
irrotational. Either one of these assumptions leads to a significant
simplification in the hydrodynamic equations of stationary equilibrium. In this
paper we develop a new formalism for the construction of binary stars with
circulation intermediate between corotational and irrotational. Generalizing
the equations for irrotational flow we cast the Euler equation, which is an
algebraic equation in the case of corotational or irrotational fluid flow, as
an elliptic equation for a new auxiliary quantity. We also suggest a
parameterized decomposition of the fluid flow that allows for a variation of
the stellar circulation.Comment: 8 pages, no figures; published version with erratu

- â€¦