14199709
GRAVITATIONAL WAVES FROM STELLAR COLLAPSE: CORRELATIONS TO EXPLOSION ASYMMETRIES
The collapse of massive stars produces not only observable outbursts across the entire electromagnetic spectrum but, for Galactic (or near-Galactic) supernovae, detectable signals for ground-based neutrino and gravitational-wave detectors. Gravitational waves and neutrinos provide the only means to study the actual engine behind the optical outbursts: the collapsed stellar core. While the neutrinos are most sensitive to details of the equation of state, gravitational waves provide a means to study the mass asymmetries in this central core. We present gravitational-wave signals from a series of three-dimensional core-collapse simulations with asymmetries derived from initial perturbations caused by precollapse convection, core rotation, and low-mode convection in the explosion engine itself. A Galactic supernovae will allow us to differentiate these different sources of asymmetry. Combining this signal with other observations of the supernova, from neutrinos to gamma-rays to the compact remnant, dramatically increases the predictive power of the gravitational-wave signal. We conclude with a discussion of the gravitational-wave signal arising from collapsars, the leading engine for long-duration gamma-ray bursts.
119020610
Towards adiabatic waveforms for inspiral into Kerr black holes : A new model of the source for the time domain perturbation equation
We revisit the problem of the emission of gravitational waves from a test mass orbiting and thus perturbing a Kerr black hole. The source term of the Teukolsky perturbation equation contains a Dirac delta function which represents a point particle. We present a technique to effectively model the delta function and its derivatives using as few as four points on a numerical grid. The source term is then incorporated into a code that evolves the Teukolsky equation in the time domain as a (2+1) dimensional partial differential equation. The waveforms and energy fluxes are extracted far from the black hole. Our comparisons with earlier work show an order of magnitude gain in performance (speed) and numerical errors less than 1% for a large fraction of parameter space. As a first application of this code, we analyze the effect of finite extraction radius on the energy fluxes. This paper is the first in a series whose goal is to develop adiabatic waveforms describing the inspiral of a small compact body into a massive Kerr black hole.
16126239
Seismic gravity-gradient noise in interferometric gravitational-wave detectors
When ambient seismic waves pass near and under an interferometric gravitational-wave detector, they induce density perturbations in the earth, which in turn produce fluctuating gravitational forces on the interferometer’s test masses. These forces mimic a stochastic background of gravitational waves and thus constitute a noise source. This seismic gravitygradient noise has been estimated and discussed previously by Saulson using a simple model of the earth’s ambient seismic motions. In this paper, we develop a more sophisticated model of these motions, based on the theory of multimode Rayleigh and Love waves propagating in a multilayer medium that approximates the geological strata at the LIGO sites (Tables II–IV), and we use this model to revisit seismic gravity gradients. We characterize the seismic gravity-gradient noise by a transfer function, T (f) ≡ x̃(f)/W̃ (f), from the spectrum of rms seismic displacements averaged over vertical and horizontal directions, W̃ (f), to the spectrum of interferometric test-mass motions, x̃(f) ≡ Lh̃(f); here L is the interferometer arm length, h̃(f) is the gravitational-wave noise spectrum, and f is frequency. Our model predicts a transfer function with essentially the same functional form as that derived by Saulson, T ' 4πGρ(2πf)β(f), where ρ is the density of the earth near the test masses, G is Newton’s constant, and β(f) ≡ γ(f)Γ(f)β(f) is a dimensionless reduced transfer function whose components γ ' 1 and Γ ' 1 account for a weak correlation between the interferometer’s two test masses (Fig. 1) and a slight reduction of the noise due to the height of the test masses above the earth’s surface. This paper’s primary foci are (i) a study of how β(f) ' β(f) depends on the various Rayleigh and Love modes that are present in the seismic spectrum (Figs. 4–11 and Table I), (ii) an attempt to estimate which modes are actually present at the two LIGO sites at quiet times and at noisy times, and (iii) a corresponding estimate of the magnitude of β(f) at quiet and noisy times. We conclude that at quiet times β ' 0.35–0.6 at the LIGO sites, and at noisy times β ' 0.15–1.4. (For comparison, Saulson’s simple model gave β = β = 1/ √ 3 = 0.58.) By folding our resulting transfer function into the “standard LIGO seismic spectrum” [Eq. (1.29)], which approximates W̃ (f) at typical times, we obtain the gravity-gradient noise spectra shown in Fig. 2. At quiet times this noise is below the benchmark noise level of “advanced LIGO interferometers” at all frequencies (though not by much at ∼ 10 Hz); at noisy times it may significantly exceed the advanced noise level near 10 Hz. The lower edge of our quiet-time noise constitutes a limit, beyond which there would be little gain from further improvements in vibration isolation and thermal noise, unless one can also reduce the seismic gravity gradient noise. Two methods of such reduction are briefly discussed: monitoring the earth’s density perturbations near each test mass, computing the gravitational forces they produce, and correcting the data for those forces; and constructing narrow moats around the interferometers’ corner and end stations to shield out the fundamental-mode Rayleigh waves, which we suspect dominate at quiet times.
8108905
Untangling the merger history of massive black holes with LISA
Binary black hole coalescences emit gravitational waves that will be measurable by the space-based detector LISA to large redshifts. This suggests that LISA may be able to observe black holes grow and evolve as the Universe evolves, mapping the distribution of black hole masses as a function of redshift. An immediate difficulty with this idea is that LISA measures certain redshifted combinations of masses with good accuracy: if a system has some mass parameter m, then LISA measures (1 + z)m. This mass-redshift degeneracy makes it difficult to follow the mass evolution. In many cases, LISA will also measure the luminosity distance D of a coalescence accurately. Since cosmological parameters (particularly the mean density, the cosmological constant and the Hubble constant) are now known with moderate precision, we can obtain z from D and break the degeneracy. This makes it possible to untangle the mass and redshift and to study the mass and merger history of black holes. Mapping the black hole mass distribution could open a window on to an early epoch of structure formation.
36773341
Circular and non-circular nearly horizon-skimming orbits in Kerr spacetimes
We have performed a detailed analysis of orbital motion in the vicinity of a nearly extremal Kerr black hole. For very rapidly rotating black holes—spin parameter aJ=M > 0:9524M —we have found a class of very strong-field eccentric orbits whose orbital angular momentum Lz increases with the orbit's inclination with respect to the equatorial plane, while keeping latus rectum and eccentricity fixed. This behavior is in contrast with Newtonian intuition, and is in fact opposite to the normal behavior of black hole orbits. Such behavior was noted previously for circular orbits; since it only applies to orbits very close to the black hole, they were named ''nearly horizon-skimming orbits.'' Our current analysis generalizes this result, mapping out the full generic (inclined and eccentric) family of nearly horizon- skimming orbits. The earlier work on circular orbits reported that, under gravitational radiation emission, nearly horizon-skimming orbits exhibit unusual inspiral, tending to evolve to smaller orbit inclination, toward prograde equatorial configuration. Normal orbits, by contrast, always demonstrate slowly growing orbit inclination —orbits evolve toward the retrograde equatorial configuration. Using up-to-date Teukolsky-based fluxes, we have concluded that the earlier result was incorrect —all circular orbits, including nearly horizon-skimming ones, exhibit growing orbit inclination under radiative backreaction. Using kludge fluxes based on a Post-Newtonian expansion corrected with fits to circular and to equatorial Teukolsky-based fluxes, we argue that the inclination grows also for eccentric nearly horizon-skimming orbits. We also find that the inclination change is, in any case, very small. As such, we conclude that these orbits are not likely to have a clear and peculiar imprint on the gravitational waveforms expected to be measured by the space-based detector LISA.
1891241
How black holes get their kicks: Radiation recoil in binary black hole mergers
Gravitational waves from the coalescence of binary black holes carry linear momentum, causing center of mass recoil. This ``radiation rocket'' has important implications for systems with escape speeds of order the recoil velocity. We describe new recoil calculations using high precision black hole perturbation theory to estimate the magnitude of the recoil for the slow ``inspiral'' coalescence phase; coupled with a cruder calculation for the final ``plunge'', we estimate the total recoil imparted to a merged black hole. We find that velocities of many tens to a few hundred km/sec can be achieved fairly easily. The recoil probably never exceeds about 500 km/sec.
18693900
TeV Gamma-Ray Observations of the Perseus and Abell 2029 Galaxy Clusters
Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large-scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and A2029 using the 10 m Whipple Cerenkov telescope during the 2003-2004 and 2004-2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeV emission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeV and theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of -2.1, our results imply that the cosmic-ray proton energy density is less than 7.9% of the thermal energy density for the Perseus Cluster.
11717821
Multiwavelength observations of 1ES 1959+650, 1 year after the strong outburst of 2002
In 2003 April-May, the blazar 1ES 1959+650 showed an increased level of X-ray activity. This prompted a multiwavelength observation campaign with the Whipple 10 m γ-ray telescope, the Rossi X-Ray Timing Explorer, the Bordeaux Optical Observatory, and the University of Michigan Radio Astronomy Observatory. We present the multiwavelength data taken from 2003 May 2 to June 7 and compare the source characteristics with those measured during observations taken during the years 2000 and 2002. The X-ray observations gave a data set with high signal-to-noise ratio light curves and energy spectra; however, the γ-ray observations did not reveal a major TeV γ-ray flare. Furthermore, we find that the radio and optical fluxes do not show statistically significant deviations from those measured during the 2002 flaring periods. While the X-ray flux and X-ray photon index appear correlated during subsequent observations, the apparent correlation evolved significantly among the years 2000, 2002, and 2003. We discuss the implications of this finding for the mechanism that causes the flaring activity.
120455459
Long-period variables in the Large Magellanic Cloud. II. Infrared photometry, spectral classification, AGB evolution, and spatial distribution
Infrared JHK photometry and visual spectra have been obtained for a large sample of long-period variables (LPVs) in the Large Magellanic Cloud (LMC). Various aspects of the asymptotic giant branch (AGB) evolution of LPVs are discussed using these data. The birth/death rate of LPVs of different ages in the LMC is compared with the birth rates of appropriate samples of planetary nebulas, clump stars, Cepheids, and OH/IR stars. It appears that there are much fewer large-amplitude LPVs per unit galactic stellar mass in the LMC than in the Galaxy. It is suggested that this may be due to the fact that the evolved intermediate-age AGB stars in the LMC often turn into carbon stars, which tend to have smaller pulsation amplitudes than M stars. There is also a major discrepancy between the number of LPVs in the LMC (and in the Galaxy) and the number predicted by the theories of AGB evolution, pulsation, and mass loss. A distance modulus to the LMC of 18.66 + or - 0.05 is derived by comparing the LMC LPVs with P about 200 days with the 47 Tucanae Mira variables in the (K, log P) plane. 64 refs.
14821200
RINGS AND BALLS
We examine the various linkings in space-time of “ball-like” and “ring-like” topological solitons in certain nonlinear sigma models in 2+1 and 3+1 dimensions. By going to theories where soliton overlaps are forbidden, these linkings become homotopically nontrivial and can be studied by investigating the topology of the corresponding configuration spaces. Our analysis reveals the existence of topological terms which give the linking number of the world tubes of distinct species of ball solitons in 2+1 dimensions, or which in 3+1 dimensions count the number of times a ball or ring soliton threads through the center of a ring of a different species. We explicitly construct these terms for our models, and generalize them to cases where soliton overlaps are no longer strictly forbidden so the terms are no longer purely topological. One of the (3+1)-dimensional theories we consider also has topological solitons which consist of two rings (of distinct species) linked in space.
15777392
NICMOS Observations of Extragalactic Cepheids. I. Photometry Database and a Test of the Standard Extinction Law
We present the results of near-infrared observations of extragalactic Cepheids made with the Near-Infrared Camera and Multi-Object Spectrometer on board the Hubble Space Telescope (HST). The variables are located in the galaxies IC 1613, IC 4182, M31, M81, M101, NGC 925, NGC 1365, NGC 2090, NGC 3198, NGC 3621, NGC 4496A, and NGC 4536. All fields were observed in the F160W bandpass; additional images were obtained in the F110W and F205W filters. Photometry was performed using the DAOPHOT II/ALLSTAR package. Self-consistent distance moduli and color excesses were obtained by fitting period-luminosity relations in the H, I, and V bands. Our results support the assumption of a standard reddening law adopted by the HST Key Project on the Extragalactic Distance Scale. A companion paper will determine true distance moduli and explore the effects of metallicity on the Cepheid distance scale.