78089676
Spacetime and orbits of bumpy black holes
Our Universe contains a great number of extremely compact and massive objects which are generally accepted to be black holes. Precise observations of orbital motion near candidate black holes have the potential to determine if they have the spacetime structure that general relativity demands. As a means of formulating measurements to test the black hole nature of these objects, Collins and Hughes introduced ''bumpy black holes'': objects that are almost, but not quite, general relativity's black holes. The spacetimes of these objects have multipoles that deviate slightly from the black hole solution, reducing to black holes when the deviation is zero. In this paper, we extend this work in two ways. First, we show how to introduce bumps which are smoother and lead to better behaved orbits than those in the original presentation. Second, we show how to make bumpy Kerr black holes--objects which reduce to the Kerr solution when the deviation goes to zero. This greatly extends the astrophysical applicability of bumpy black holes. Using Hamilton-Jacobi techniques, we show how a spacetime's bumps are imprinted on orbital frequencies, and thus can be determined by measurements which coherently track the orbital phase of a small orbiting body. We find thatmore » in the weak field, orbits of bumpy black holes are modified exactly as expected from a Newtonian analysis of a body with a prescribed multipolar structure, reproducing well-known results from the celestial mechanics literature. The impact of bumps on strong-field orbits is many times greater than would be predicted from a Newtonian analysis, suggesting that this framework will allow observations to set robust limits on the extent to which a spacetime's multipoles deviate from the black hole expectation.« less
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.
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.
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.
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.
119525918
The Extragalactic Distance Scale Key Project. VIII. The Discovery of Cepheids and a New Distance to NGC 3621 Using the Hubble Space Telescope
We report on the discovery of Cepheids in the field spiral galaxy NGC 3621, based on observations made with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 3621 is one of 18 galaxies observed as a part of the HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. Sixty-nine Cepheids with periods in the range 9-60 days were observed over 12 epochs using the F555W filter, and over four epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent packages, DAOPHOT II/ALLFRAME and DoPHOT. Period-luminosity relations in the V and I bands were constructed using 36 fairly isolated Cepheids present in our set of 69 variables. Extinction-corrected distance moduli relative to the LMC of 10.63 ? 0.09 and 10.56 ? 0.10 mag were obtained using the ALLFRAME and DoPHOT data, respectively. True distance moduli of 29.13 ? 0.18 and 29.06 ? 0.18 mag, corresponding to distances of 6.3 ? 0.7 and 6.1 ? 0.7 Mpc, were obtained by assuming values of ?0 = 18.50 ? 0.10 mag and E(V - I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.
4322218
A Cepheid distance to the Fornax cluster and the local expansion rate of the Universe
Both galaxy distances and velocities are required for the determination of the expansion rate of the Universe, as described by the Hubble constant H0. The radial velocities of galaxies arise not just from this expansion but also from random components and large-scale flows. To reach out to distances dominated by the overall cosmic expansion, it is necessary to probe large physical scales where galaxy–galaxy and galaxy–cluster interactions become lessimportant. But accurate distances of nearby galaxies and clusters (commonly measured using Cepheid variable stars) are nevertheless required to calibrate the indirect distance indicators generally used to measure these large scales. Here we report aCepheid distance of 18.6 ± 1.9 (statistical error) ± 1.9 Mpc (systematic error) for the galaxy NGC1365 in Fornax, a cluster of galaxies in the Southern Hemisphere. We find a value of H0 = 70 km s−1 Mpc−1 from Fornax alone, and 73 km s−1 Mpc−1 from the intervening galaxy flow, each corrected for infall into the Virgo cluster. These values are consistent with the Hubble constant measured in the far field using secondary methods. Our data support previous suggestions that the local small-scale velocity field has very small scatter (∼±70 km s−1).
15202727
AGAPEROS: Searching for variable stars in the LMC Bar with the Pixel Method - I. Detection, astrometry and cross-identification
We extend the work developed in previous pa- pers on microlensing with a selection of variable stars. We use the Pixel Method to select variable stars on a set of 2.5 10 6 pixel light curves in the LMC Bar presented elsewhere. The previous treatment was done in order to optimise the detection of long timescale variations (larger than a few days) and we further optimise our analysis for the selection of Long Timescale and Long Period Variables (LT&LPV). We choose to perform a selection of variable objects as comprehensive as possible, independent of pe- riodicity and of their position on the colour magnitude diagram. We detail the dierent thresholds successively applied to the light curves, which allow to produce a cat- alogue of 632 variable objects. We present a table with the coordinate of each variable, its EROS magnitudes at one epoch and an indicator of blending in both colours, together with a nding chart. A cross-correlation with various catalogues shows that 90% of those variable objects were undetected before, thus enlarging the sample of LT&LPV previously known in this area by a factor of 10. Due to the limitations of both the Pixel Method and the data set, additional data { namely a longer baseline and near infrared photometry { are re- quired to further characterise these variable stars, as will be addressed in subsequent papers.
119377823
The Hubble Space Telescope Extragalactic Distance Scale Key Project. XXIII. The Discovery of Cepheids in NGC 3319
The distance to NGC 3319 has been determined from Cepheid variable stars as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen and four epochs of observations, using filters F555W (V) and F814W (I), respectively, were made with the Wide Field Planetary Camera 2. Thirty-three Cepheid variables between periods of 8 and 47 days were discovered. Adopting a Large Magellanic Cloud distance modulus of 18.50 ± 0.10 mag and extinction of E(V-I) = 0.13 mag, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.78 ± 0.14 (random) ± 0.10 (systematic) mag and the extinction of E(V-I) = 0.06 mag were determined for NGC 3319. This galaxy is the last galaxy observed for the HST H0 Key Project.