Relativistic Effects on the Appearance of a Clothed Black Hole

For an accretion disk around a black hole, the strong relativistic effects affect every aspect of the radiation from the disk, including its spectrum, light-curve, and image. This work investigates in detail how the images of a thin disk around a black hole will be distorted, and what the observer will see from different viewing angles and in different energy bands.Comment: 4 pages, 5 figures. Based on the poster presented at the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17, 2002). Color versions of figures are given separatel

Strong deflection limit of black hole gravitational lensing with arbitrary source distances

The gravitational field of supermassive black holes is able to strongly bend light rays emitted by nearby sources. When the deflection angle exceeds $\pi$, gravitational lensing can be analytically approximated by the so-called strong deflection limit. In this paper we remove the conventional assumption of sources very far from the black hole, considering the distance of the source as an additional parameter in the lensing problem to be treated exactly. We find expressions for critical curves, caustics and all lensing observables valid for any position of the source up to the horizon. After analyzing the spherically symmetric case we focus on the Kerr black hole, for which we present an analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review

The gravitational wave spectrum of non-axisymmetric, freely precessing neutron stars

Evidence for free precession has been observed in the radio signature of several pulsars. Freely precessing pulsars radiate gravitationally at frequencies near the rotation rate and twice the rotation rate, which for rotation frequencies greater than $\sim 10$ Hz is in the LIGO band. In older work, the gravitational wave spectrum of a precessing neutron star has been evaluated to first order in a small precession angle. Here we calculate the contributions to second order in the wobble angle, and we find that a new spectral line emerges. We show that for reasonable wobble angles, the second-order line may well be observable with the proposed advanced LIGO detector for precessing neutron stars as far away as the galactic center. Observation of the full second-order spectrum permits a direct measurement of the star's wobble angle, oblateness, and deviation from axisymmetry, with the potential to significantly increase our understanding of neutron star structure.Comment: 22 pages, 1 figure. Minor changes in the text, typos correcte

Aqueye optical observations of the Crab Nebula pulsar

We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye) which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a 2 day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative are in agreement with those from the Jodrell Bank radio ephemerides archive. We also found evidence of the optical peak leading the radio one by ~230 microseconds. The distribution of phase-residuals of the whole dataset is slightly wider than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape, such as the average count rate and background level are those of the Crab pulsar observed with Aqueye. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in 2 days only. The time of arrival of the optical peak of the Crab pulsar leads the radio one in agreement with what recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).Comment: 7 pages, 7 figures. Accepted for publication in Astronomy and Astrophysic

Optical Spectrum of Main-, Inter- and Off-pulse Emission from Crab Pulsar

A dedicated stroboscopic device was used to obtain optical spectra of the Crab main-pulse and inter-pulse as well as the spectrum of the underlying nebula when the pulsar is turned off. As the nebular emission is very inhomogeneous, our ability to effectively subtract the nebular background signal is crucial. No spectral lines intrinsic to the pulsar are detected. The main-pulse and the inter-pulse behave as power laws, both with the same de-reddened index Alpha = +0.2 +- 0.1. This value was obtained by subtracting the nebular spectrum at the exact position of the pulsar. The underlying nebula is redder, Alpha = -0.4 +- 0.1. Its emission lines are split into approaching (sim. -1200 km/s) and receding (sim. +600 km/s) components. The strength of emission line components and the flux in nebular continuum vary on arcsec scale. The nebular line and continuum intensities along the N-S slit are given.Comment: Accepted for publication in the Astrophysical Journal. 10 pages, 3 Tables, 4 Figure

Generic Tracking of Multiple Apparent Horizons with Level Flow

We report the development of the first apparent horizon locator capable of finding multiple apparent horizons in a ``generic'' numerical black hole spacetime. We use a level-flow method which, starting from a single arbitrary initial trial surface, can undergo topology changes as it flows towards disjoint apparent horizons if they are present. The level flow method has two advantages: 1) The solution is independent of changes in the initial guess and 2) The solution can have multiple components. We illustrate our method of locating apparent horizons by tracking horizon components in a short Kerr-Schild binary black hole grazing collision.Comment: 13 pages including figures, submitted to Phys Rev

Relativistic positioning: four-dimensional numerical approach in Minkowski space-time

We simulate the satellite constellations of two Global Navigation Satellite Systems: Galileo (EU) and GPS (USA). Satellite motions are described in the Schwarzschild space-time produced by an idealized spherically symmetric non rotating Earth. The trajectories are then circumferences centered at the same point as Earth. Photon motions are described in Minkowski space-time, where there is a well known relation, Coll, Ferrando & Morales-Lladosa (2010), between the emission and inertial coordinates of any event. Here, this relation is implemented in a numerical code, which is tested and applied. The first application is a detailed numerical four-dimensional analysis of the so-called emission coordinate region and co-region. In a second application, a GPS (Galileo) satellite is considered as the receiver and its emission coordinates are given by four Galileo (GPS) satellites. The bifurcation problem (double localization) in the positioning of the receiver satellite is then pointed out and discussed in detail.Comment: 16 pages, 9 figures, published (online) in Astrophys. Space Sc

Precession Interpretation of the Isolated Pulsar PSR B1828-11

Pulse timing of the isolated pulsar PSR B1882-11 shows strong Fourier power at periods ~1000, 500 and 250 d, correlated with changes in the pulse profile (Stairs, Lyne & Shemar 2000). We study the extent to which these data can be explained by precession of the star's rigid crust coupled to the magnetic dipole torque. We find that the correlated changes in the pulse duration and spin period derivative can be explained as precession at a period of ~500 d with a wobble angle of ~3 deg if the star's dipole moment is nearly orthogonal to its symmetry axis. The dipole torque produces a harmonic at ~250 d. Comparison of the predicted spin dynamics with the observed pulse durations requires the radio beam to have a non-standard ``hour-glass'' shape. We make predictions of variations in beam polarization and pulse profile with which to test this interpretation. The precession interpretation of PSR B1828-11 seriously challenges the current understanding of the liquid interior of the neutron star. In particular, if the internal liquid is in a superfluid state, its rotational vortices cannot be significantly pinned to the crust.Comment: 15 pages, 5 figure