119191664
Accelerated Bayesian model-selection and parameter-estimation in continuous gravitational-wave searches with pulsar-timing arrays
We describe several new techniques which accelerate Bayesian searches for continuous gravitational-wave emission from supermassive black-hole binaries using pulsar timing arrays. These techniques mitigate the problematic increase of search-dimensionality with the size of the pulsar array which arises from having to include an extra parameter per pulsar as the array is expanded. This extra parameter corresponds to searching over the phase of the gravitational-wave as it propagates past each pulsar so that we can coherently include the pulsar-term in our search strategies. Our techniques make the analysis tractable with powerful evidence-evaluation packages like MultiNest. We find good agreement of our techniques with the parameter-estimation and Bayes factor evaluation performed with full signal templates, and conclude that these techniques make excellent first-cut tools for detection and characterisation of continuous gravitational-wave signals with pulsar timing arrays. Crucially, at low to moderate signal-to-noise ratios the factor by which the analysis is sped up can be > 100, permitting rigorous programs of systematic injection and recovery of signals to establish robust detection criteria within a Bayesian formalism.
118618521
Results of the First IPTA Closed Mock Data Challenge
The 2012 International Pulsar Timing Array (IPTA) Mock Data Challenge (MDC) is designed to test current Gravitational Wave (GW) detection algorithms. Here we will briefly outline two detection algorithms for a stochastic background of gravitational waves, namely, a first-order likelihood method and an optimal statistic method and present our results from the closed MDC data sets.
118358917
A Bayesian analysis pipeline for continuous GW sources in the PTA band
The direct detection of gravitational waves (GWs) by pulsar timing arrays (PTAs) is very likely within the next decade. While the stochastic GW background is a promising candidate for detection it is also possible that single resolvable sources may be detectable as well. In this work we will focus on the detection and characterization of single GW sources from supermassive black hole binaries. We introduce a fully Bayesian data analysis pipeline that is meant to carry out a search, characterization, and evaluation phase. This will allow us to rapidly locate the global maxima in parameter space, map out the posterior, and finally weigh the evidence of a GW detection through a Bayes factor. Here we will make use of an adaptive metropolis algorithm and parallel tempering. We test this algorithm on realistic simulated data that are representative of modern PTAs.
119259587
The stochastic background: scaling laws and time to detection for pulsar timing arrays
We derive scaling laws for the signal-to-noise ratio of the optimal cross-correlation statistic, and show that the large power-law increase of the signal-to-noise ratio as a function of the observation time T that is usually assumed holds only at early times. After enough time has elapsed, pulsar timing arrays enter a new regime where the signal to noise only scales as . In addition, in this regime the quality of the pulsar timing data and the cadence become relatively unimportant. This occurs because the lowest frequencies of the pulsar timing residuals become gravitational-wave dominated. Pulsar timing arrays enter this regime more quickly than one might naively suspect. For T = 10 yr observations and typical stochastic background amplitudes, pulsars with residual root-mean-squares of less than about 1 μs are already in that regime. The best strategy to increase the detectability of the background in this regime is to increase the number of pulsars in the array. We also perform realistic simulations of the NANOGrav pulsar timing array, which through an aggressive pulsar survey campaign adds new millisecond pulsars regularly to its array, and show that a detection is possible within a decade, and could occur as early as 2016.
279355
Time-domain implementation of the optimal cross-correlation statistic for stochastic gravitational-wave background searches in pulsar timing data
Supermassive black hole binaries, cosmic strings, relic gravitational waves from inflation, and first-order phase transitions in the early Universe are expected to contribute to a stochastic background of gravitational waves in the 10^(−9) –10^(−7)  Hz frequency band. Pulsar timing arrays (PTAs) exploit the high-precision timing of radio pulsars to detect signals at such frequencies. Here we present a time-domain implementation of the optimal cross-correlation statistic for stochastic background searches in PTA data. Due to the irregular sampling typical of PTA data as well as the use of a timing model to predict the times of arrival of radio pulses, time-domain methods are better-suited for gravitational-wave data analysis of such data. We present a derivation of the optimal cross-correlation statistic starting from the likelihood function, a method to produce simulated stochastic background signals, and a rigorous derivation of the scaling laws for the signal-to-noise ratio of the cross-correlation statistic in the two relevant PTA regimes: the weak-signal limit where instrumental noise dominates over the gravitational-wave signal at all frequencies, and a second regime where the gravitational-wave signal dominates at the lowest frequencies.
119306478
AN EFFICIENT APPROXIMATION TO THE LIKELIHOOD FOR GRAVITATIONAL WAVE STOCHASTIC BACKGROUND DETECTION USING PULSAR TIMING DATA
Direct detection of gravitational waves by pulsar timing arrays will become feasible over the next few years. In the low frequency regime (10–7 Hz-10–9 Hz), we expect that a superposition of gravitational waves from many sources will manifest itself as an isotropic stochastic gravitational wave background. Currently, a number of techniques exist to detect such a signal; however, many detection methods are computationally challenging. Here we introduce an approximation to the full likelihood function for a pulsar timing array that results in computational savings proportional to the square of the number of pulsars in the array. Through a series of simulations we show that the approximate likelihood function reproduces results obtained from the full likelihood function. We further show, both analytically and through simulations, that, on average, this approximate likelihood function gives unbiased parameter estimates for astrophysically realistic stochastic background amplitudes.
26207540
Support for temporally varying behavior of the Pioneer anomaly from the extended Pioneer 10 and 11 Doppler data sets.
The Pioneer anomaly is a small sunward anomalous acceleration found in the trajectory analysis of the Pioneer 10 and 11 spacecraft. As part of the investigation of the effect, the analysis of recently recovered Doppler data for both spacecraft has been completed. The presence of a small anomalous acceleration is confirmed by using data spans more than twice as long as those that were previously analyzed. We examine the constancy and direction of the Pioneer anomaly and conclude that (i) the data favor a temporally decaying anomalous acceleration (~2×10(-11) m/s(2)/yr) with an over 10% improvement in the residuals compared to a constant acceleration model, (ii) although the direction of the acceleration remains imprecisely determined, we find no support in favor of a Sun-pointing direction over the Earth-pointing or along the spin-axis directions, and (iii) support for an early "onset" of the acceleration remains weak in the pre-Saturn Pioneer 11 tracking data. We present these new findings and discuss their implications for the nature of the Pioneer anomaly.
115466327
Liquids-in-capillaries: New fiber detectors for high energy physics applications
We are developing scintillating-fiber detectors incorporating organic liquid scintillation cocktails in glass capillaries. The organic solvents have high refractive index, making them suitable as core materials for scintillation waveguides. Only one fluorescent dye at high concentration (1%) is utilized in the solution. This ensures that the dominant energy transfer between solvent and solute will be nonradiative, and that fluorescent emission from the dye will be local to the ionization deposition in the material. Hence such structures might be suitable for high resolution tracking devices. Liquid scintillation solutions may also provide advantages in radiation resistance and replaceability. 8 refs., 6 tabs.
12088216
Anomalous orbital-energy changes observed during spacecraft flybys of earth.
We report and characterize anomalous orbital-energy changes observed during six Earth flybys by the Galileo, NEAR, Cassini, Rosetta, and MESSENGER spacecraft. These anomalous energy changes are consistent with an empirical prediction formula which is proportional to the total orbital energy per unit mass and which involves the incoming and outgoing geocentric latitudes of the asymptotic spacecraft velocity vectors. We use this formula to predict a potentially detectable flyby velocity increase of less than 1 mm/s for a second Rosetta flyby on November 13, 2007.
122401337
Development of new scintillating fiber detectors for high energy physics applications
The development of novel scintillating fiber detectors for colliding-beam and fixed target applications is considered. The authors present initial results from a developmental study of high-refractive-index liquid scintillators and the incorporation of these liquids in glass capillaries. Advantages of liquid-in-capillary fiber detectors include high efficiency and fast decay; the potential for high-resolution tracking, as light emission is expected to be local to the deposited ionization in liquids containing single solutes; the potential for good radiation resistance; and replaceability. >
122075191
Liquid capillary scintillation detectors
Liquid-in-capillary detectors for tracking applications in high-energy physics experiments are discussed. The detectors consist of glass capillaries of low refractive index filled with liquids of sufficiently high refractive index to produce an efficient waveguides. The preparation of scintillating core liquids from the solvent 1-phenylnaphthalene and single solutes of selected fluorescent dyes is described. >