Gravitational strings. Do we see one?

I present a class of objects called gravitational strings (GS) for their similarity to the conventional cosmic strings: even though the former are just singularities in flat spacetime, both varieties are equally "realistic", they may play equally important cosmological r\^ole and their lensing properties are akin. I argue that the enigmatic object CSL-1 is an evidence in favor of the existence of GS.Comment: The published version. Minor correction

Scaling in a SU(2)/Z_3 model of cosmic superstring networks

Motivated by recent developments in superstring theory in the cosmological context, we examine a field theory which contains string networks with 3-way junctions. We perform numerical simulations of this model, identify the length scales of the network that forms, and provide evidence that the length scales tend towards a scaling regime, growing in proportion to time. We infer that the presence of junctions does not in itself cause a superstring network to dominate the energy density of the early Universe.Comment: 12pp, 3 fig

The parallax distorsion via a weak microlensing effect

Parallax measurements allow distances to celestial objects to be determined. Coupled with measurement of their position on the celestial sphere, it gives a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a remote source affected by a weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. It is shown that the distortions to appear may change strongly the parallax values in case they amount to several microseconds of arc. In particular, at this accuracy many measured values of the parallaxes must be negative.Comment: 34 LaTeX pages, 12 PostScript figure (epsfig.sty

Microarcsecond instability of the celestial reference frame

The fluctuation of the angular positions of reference extragalactic radio and optical sources under the influence of the irregular gravitational field of visible Galactic stars is considered. It is shown that these angular fluctuations range from a few up to hundreds of microarcseconds. This leads to a small rotation of the celestial reference frame. The nondiagonal coefficients of the rotation matrix are of the order of a microarcsecond. The temporal variation of these coefficients due to the proper motion of the foreground stars is of the order of one microsecond per 20 years. Therefore, the celestial reference frame can be considered inertial and homogeneous only to microarcsecond accuracy. Astrometric catalogues with microarcsecond accuracy will be unstable, and must be reestablished every 20 years.Comment: 5 pages, 2 figures, accepted to MNRA

Probing fundamental physics with pulsars

Pulsars provide a wealth of information about General Relativity, the equation of state of superdense matter, relativistic particle acceleration in high magnetic fields, the Galaxy's interstellar medium and magnetic field, stellar and binary evolution, celestial mechanics, planetary physics and even cosmology. The wide variety of physical applications currently being investigated through studies of radio pulsars rely on: (i) finding interesting objects to study via large-scale and targeted surveys; (ii) high-precision timing measurements which exploit their remarkable clock-like stability. We review current surveys and the principles of pulsar timing and highlight progress made in the rotating radio transients, intermittent pulsars, tests of relativity, understanding pulsar evolution, measuring neutron star masses and the pulsar timing array.Comment: 6 pages, 1 figure, to appear in the proceedings of IAU XXVII GA - JD3 - Neutron Stars: Timing in Extreme Environments XXVII IAU General Assembly, Rio de Janeiro, Brazil, 3-14 August 200

Gravitational-Wave Stochastic Background from Kinks and Cusps on Cosmic Strings

We compute the contribution of kinks on cosmic string loops to stochastic background of gravitational waves (SBGW).We find that kinks contribute at the same order as cusps to the SBGW.We discuss the accessibility of the total background due to kinks as well as cusps to current and planned gravitational wave detectors, as well as to the big bang nucleosynthesis (BBN), the cosmic microwave background (CMB), and pulsar timing constraints. As in the case of cusps, we find that current data from interferometric gravitational wave detectors, such as LIGO, are sensitive to areas of parameter space of cosmic string models complementary to those accessible to pulsar, BBN, and CMB bounds.Comment: 24 pages, 3 figure

Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion

Pulsar timing arrays (PTAs) will be sensitive to a finite number of gravitational wave (GW) "point" sources (e.g. supermassive black hole binaries). N quiet pulsars with accurately known distances d_{pulsar} can characterize up to 2N/7 distant chirping sources per frequency bin \Delta f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta \gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are poorly known, a PTA with F frequency bins can still characterize up to (2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing residuals in the vicinity of a GW source still allows the source to be localized quasi-topologically within roughly the smallest quadrilateral of quiet pulsars that encircles it on the sky, down to a limiting resolution \delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be unconfused, even at the lowest frequencies, with matched filtering always appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio

Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries

We calculate the expected nHz--$\mu$Hz gravitational wave (GW) spectrum from coalescing Massive Black Hole (MBH) binaries resulting from mergers of their host galaxies. We consider detection of this spectrum by precision pulsar timing and a future Pulsar Timing Array. The spectrum depends on the merger rate of massive galaxies, the demographics of MBHs at low and high redshift, and the dynamics of MBH binaries. We apply recent theoretical and observational work on all of these fronts. The spectrum has a characteristic strain $h_c(f)~10^{-15} (f/yr^{-1})^{-2/3}$, just below the detection limit from recent analysis of precision pulsar timing measurements. However, the amplitude of the spectrum is still very uncertain owing to approximations in the theoretical formulation of the model, to our lack of knowledge of the merger rate and MBH population at high redshift, and to the dynamical problem of removing enough angular momentum from the MBH binary to reach a GW-dominated regime.Comment: 31 Pages, 8 Figures, small changes to match the published versio