Scattering of gravitational radiation. Intensity fluctuations

Aims. The effect of gravitational microlensing on the intensity of gravitational radiation as it propagates through an inhomogeneous medium is considered. Lensing by both stars and a power law spectrum of density perturbations is examined. Methods. The long wavelengths characteristic of gravitational radiation mandate a statistical, physical-optics approach to treat the effect of the lensing. Results. A model for the mass power spectrum of a starfield, including the effects of clustering and allowing for a distribution of stellar masses, is constructed and used to determine both the amplitude of fluctuations in the gravitational wave strain and its associated temporal fluctuation spectrum. For a uniformly distributed starfield the intensity variance scales linearly with stellar density, σ, but is enhanced by a factor ≳σr^2_F when clustering is important, where r_F is the Fresnel scale. The effect of lensing by a power law mass spectrum, applicable to lensing by small scale fluctuations in gas and dark matter, is also considered. For power law mass density spectra with indices steeper than −2 the wave amplitude exhibits rms fluctuations 1.3^(1/4)(D_(eff)/1 Gpc)^(1/2)%, where is the variance in the mass surface density measured in M^2_⊙ pc^(−4) and D_(eff) is the effective distance to the lensing medium. For shallower spectra the amplitude of the fluctuations depends additionally on the inner length scale and power law index of the density fluctuations. The intensity fluctuations are dominated by temporal fluctuations on long timescales. For lensing material moving at a speed v across the line of sight the fluctuation timescale exceeds v^(−1)(D_(eff)λ)^(1/2). Lensing by small scale structure induces at most ≈15% rms variations if the line of sight to a gravitational wave source intersects a region with densities ~100 M_⊙ pc^(−2), which are typically encountered in the vicinity of galaxy clusters

Intra-Day Variability and the Interstellar Medium Towards 0917+624

The intra-day variable source 0917+624 displays annual changes in its timescale of variability. This is explained in terms of a scintillation model in which changes in the variability timescale are due to changes in the relative velocity of the scintillation pattern as the Earth orbits the sun. (see also astro-ph/0102050)Comment: 4 pages, 1 figure. Accepted for A&A Letter

Rapidly Evolving Circularly Polarized Emission during the 1994 Outburst of GRO J1665-40

We report the detection of circular polarization during the 1994 outburst of the Galactic microquasar GRO J1655-40. The circular polarization is clearly detected at 1.4 and 2.4GHz, but not at 4.8 and 8.4GHz, where its magnitude never exceeds 5 mJy. Both the sign and magnitude of the circular polarization evolve during the outburst. The time dependence and magnitude of the polarized emission can be qualitatively explained by a model based on synchrotron emission from the outbursts, but is most consistent with circular polarization arising from propagation effects through the relativistic plasma surrounding the object.Comment: 8 pages, 3 figs., A&A accepte

All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities

We demonstrate a new technique for detecting radio transients based on interferometric closure quantities. The technique uses the bispectrum, the product of visibilities around a closed-loop of baselines of an interferometer. The bispectrum is calibration independent, resistant to interference, and computationally efficient, so it can be built into correlators for real-time transient detection. Our technique could find celestial transients anywhere in the field of view and localize them to arcsecond precision. At the Karl G. Jansky Very Large Array (VLA), such a system would have a high survey speed and a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth. The ability to localize dispersed millisecond pulses to arcsecond precision in large volumes of interferometer data has several unique science applications. Localizing individual pulses from Galactic pulsars will help find X-ray counterparts that define their physical properties, while finding host galaxies of extragalactic transients will measure the electron density of the intergalactic medium with a single dispersed pulse. Exoplanets and active stars have distinct millisecond variability that can be used to identify them and probe their magnetospheres. We use millisecond time scale visibilities from the Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect dispersed pulses and reject local interference. The computational and data efficiency of the bispectrum will help find transients on a range of time scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include discussion of non-Gaussian statistics of techniqu

Science with the Australian Square Kilometre Array Pathfinder

The future of centimetre and metre-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline an ambitious science program for ASKAP, examining key science such as understanding the evolution, formation and population of galaxies including our own, understanding the magnetic Universe, revealing the transient radio sky and searching for gravitational waves

Scatter broadening of compact radio sources by the ionized intergalacticmedium: prospects for detection with Space VLBI and the SquareKilometre Array

We investigate the feasibility of detecting and probing various components of the ionized intergalactic medium (IGM) and their turbulent properties at radio frequencies through observations of scatter broadening of compact sources. There is a strong case for conducting targeted observations to resolve scatter broadening (where the angular size scales as ~ν−2) of compact background sources intersected by foreground galaxy haloes and rich clusters of galaxies to probe the turbulence of the ionized gas in these objects, particularly using Space very long baseline interferometry (VLBI) with baselines of 350 000 km at frequencies below 800 MHz. The sensitivity of the Square Kilometre Array (SKA) allows multifrequency surveys of interstellar scintillation (ISS) of ~100 μJy sources to detect or place very strong constraints on IGM scatter broadening down to ~1 μas scales at 5 GHz. Scatter broadening in the warm–hot component of the IGM with typical overdensities of ~30 cannot be detected, even with Space VLBI or ISS, and even if the outer scales of turbulence have an unlikely low value of ~1 kpc. None the less, intergalactic scatter broadening can be of the order of ~100 μas at 1 GHz and ~3 μas at 5 GHz for outer scales ~1 kpc, assuming a sufficiently high-source redshift that most sight-lines intersect within a virial radius of at least one galaxy halo (z >~ 0.5 and 1.4 for 10 10 and 10 11 M⊙ systems, following McQuinn 2014). Both Space VLBI and multiwavelength ISS observations with the SKA can easily test such a scenario, or place strong constraints on the outer scale of the turbulence in such regions

High brightness temperatures and circular polarisation in extra-galactic radio sources

Some rapidly variable extra-galactic radio sources show very high brightness temperatures T_B>10^{12}K and high degrees of circular polarisation (1%). Standard synchrotron models that assume a power-law electron distribution cannot produce such high temperatures and have much lower degrees of intrinsic circular polarisation. We examine the synchrotron and inverse Compton radiation from a monoenergetic electron distribution using standard synchrotron theory. Constraints on the source parameters are found by formulating the results as functions of the source size, Doppler boosting factor, optical depth to synchrotron self-absorption, maximum frequency of synchrotron emission, and the strength of the inverse Compton radiation. The model gives brightness temperatures T_B=10^{13}K to 10^{14}K for moderate (<10) Doppler boosting factors and intrinsic degrees of circular polarisation at the percent level. It predicts a spectrum I_\nu\propto\nu^{1/3} between the radio and the infra-red as well as emission in the MeV to GeV range. We find the conditions under which electrons do not cool within the source, enabling the GHz emission to emerge without absorption and the potentially catastrophic energy losses by inverse Compton scattering to be avoided. We suggest that sources such as PKS 1519 -273, PKS 0405 -385 and J 1819 +3845 can be understood within this scenario without invoking high Doppler boosting factors, coherent emission mechanisms, or the dominance of proton synchrotron radiation.Comment: 4 pages, to appear in A&A Letter