Analysing Astronomy Algorithms for GPUs and Beyond

Astronomy depends on ever increasing computing power. Processor clock-rates have plateaued, and increased performance is now appearing in the form of additional processor cores on a single chip. This poses significant challenges to the astronomy software community. Graphics Processing Units (GPUs), now capable of general-purpose computation, exemplify both the difficult learning-curve and the significant speedups exhibited by massively-parallel hardware architectures. We present a generalised approach to tackling this paradigm shift, based on the analysis of algorithms. We describe a small collection of foundation algorithms relevant to astronomy and explain how they may be used to ease the transition to massively-parallel computing architectures. We demonstrate the effectiveness of our approach by applying it to four well-known astronomy problems: Hogbom CLEAN, inverse ray-shooting for gravitational lensing, pulsar dedispersion and volume rendering. Algorithms with well-defined memory access patterns and high arithmetic intensity stand to receive the greatest performance boost from massively-parallel architectures, while those that involve a significant amount of decision-making may struggle to take advantage of the available processing power.Comment: 10 pages, 3 figures, accepted for publication in MNRA

The Galactic Exoplanet Survey Telescope (GEST)

The Galactic Exoplanet Survey Telescope (GEST) will observe a 2 square degree field in the Galactic bulge to search for extra-solar planets using a gravitational lensing technique. This gravitational lensing technique is the only method employing currently available technology that can detect Earth-mass planets at high signal-to-noise, and can measure the frequency of terrestrial planets as a function of Galactic position. GEST's sensitivity extends down to the mass of Mars, and it can detect hundreds of terrestrial planets with semi-major axes ranging from 0.7 AU to infinity. GEST will be the first truly comprehensive survey of the Galaxy for planets like those in our own Solar System.Comment: 17 pages with 13 figures, to be published in Proc. SPIE vol 4854, "Future EUV-UV and Visible Space Astrophysics Missions and Instrumentation

Mirage: A New Package for the Simulation of Gravitationally Microlensed Quasars

We present Mirage, a new package for simulating gravitationally lensed quasars that allows simulation of arbitrarily sized emitting regions of the quasar’s accretion disk. We develop a robust, large-scale simulator, wirtten in Python, to model gravitationally lensed quasars. Numerical simulation of gravitationally microlensed quasars provides a tool to determine the physical size and temperature profile of quasars accretion disks which is impossible through direct observation. The method consists of ray-tracing approximately 1010 paths through a simulated starfield, taking advantage of the latest technologies in cluster computing,to calculate flux received by the observer from each lensed image from different regions of the accretion disk as the quasar moves relative to the lensing galaxy. We compare our simulations to observations of QSO2237+0305 in optical and X-ray wavebands to place constraints on the relative size of the x-ray and optical emitting regions of the quasar’s accretion disk

Gravitational microlensing

The formulation of the Theory of General Relativity and the observational evidence for the expansion of the universe provided the basis for much of the work carried out in the field of cosmology over the past hundred years. Huge volumes of research have been conducted to find reliable values for cosmological parameters and to describe the amount and nature of the matter in the universe. Chapter 1 of this thesis attempts to summarise current theoretical and observational thinking on these matters and, in particular, examines the wide-ranging application of gravitational lensing to the search for so-called dark matter. The use of gravitational microlensing to investigate a cosmological population of compact objects, their effects on the long term variability of the apparent luminosity of quasars and on the results of the on-going observations of high redshift supernovae is discussed. Such investigation forms the basis for this thesis.The main tool for this investigation is a computer model which simulates the gravitational lensing effect of a population of compact object over a period of time. Chapter 2 sets out the theoretical background for this simulation. In particular, the methods used to set the physical parameters of the simulation, such as its volume, the redshifts of the lenses and their masses, are outlined.Chapter 3 presents the implementation of the computer model. Modelling techniques used by other researchers are discussed, as are alternative approaches considered for the implementation of this model. In order to simulate the evolving distribution of the lensing objects over time, the simulation was designed to run on high performance parallel supercomputers. The method by which the simulation was designed to take advantage of this type of computing platform is also discussed.In order to examine the effects of a cosmological distribution of compact objects on high redshift sources properly, it is necessary to have observational data. For this thesis, the observational data consists of a set of lightcurves from high redshift quasars observed over a 25 year period. This data set is outlined in Chapter 4. The results from the computer simulation are then presented, including both example light curves and power spectra for a variety of cosmological models, source sizes, source redshifts and lens masses. This observational data is compared with the simulation data and is found to have comparable levels of power for a number of simulation models.Chapter 5 examines the effect of a cosmological population of compact objects on the ongoing high redshift supernovae searches. The effects of such objects are modelled for a number of cosmological models for the range of redshifts proposed for the SNAP and VISTA searches. It is found that the proposed number counts for supernovae detection in each redshift bin are sufficient to differentiate between the different cosmological models

Observational signatures of microlensing in gravitational waves at LIGO/Virgo frequencies

Microlenses with typical stellar masses (a few ${\rm M}_{\odot}$) have traditionally been disregarded as potential sources of gravitational lensing effects at LIGO/Virgo frequencies, since the time delays are often much smaller than the inverse of the frequencies probed by LIGO/Virgo, resulting in negligible interference effects at LIGO/Virgo frequencies. While this is true for isolated microlenses in this mass regime, we show how, under certain circumstances and for realistic scenarios, a population of microlenses (for instance stars and remnants from a galaxy halo or from the intracluster medium) embedded in a macromodel potential (galaxy or cluster) can conspire together to produce time delays of order one millisecond which would produce significant interference distortions in the observed strains. At sufficiently large magnification factors (of several hundred), microlensing effects should be common in gravitationally lensed gravitational waves. We explore the regime where the predicted signal falls in the frequency range probed by LIGO/Virgo. We find that stellar mass microlenses, permeating the lens plane, and near critical curves, can introduce interference distortions in strongly lensed gravitational waves. For those lensed events with negative parity, (or saddle points, never studied before in the context of gravitational waves), and that take place near caustics of macromodels, they are more likely to produce measurable interference effects at LIGO/Virgo frequencies. This is the first study that explores the effect of a realistic population of microlenses, plus a macromodel, on strongly lensed gravitational waves.Comment: 16 page