by Adams, Matthew R. and Cornish, Neil J.
10 Pages, 10 Figures

The detection of a stochastic background of gravitational waves could significantly impact our understanding of the physical processes that shaped the early Universe. The challenge lies in separating the cosmological signal from other stochastic processes such as instrument noise and astrophysical foregrounds. One approach is to build two or more detectors and cross correlate their output, thereby enhancing the common gravitational wave signal relative to the uncorrelated instrument noise. When only one detector is available, as will likely be the case with the Laser Interferometer Space Antenna (LISA), alternative analysis techniques must be developed. Here we show that models of the noise and signal transfer functions can be used to tease apart the gravitational and instrument noise contributions. We discuss the role of gravitational wave insensitive “null channels” formed from particular combinations of the time delay interferometry, and derive a new combination that maintains this insensitivity for unequal arm length detectors. We show that, in the absence of astrophysical foregrounds, LISA could detect signals with energy densities as low as $latex \Omega_{\rm gw} = 6 \times 10^{-13}$ with just one month of data. We describe an end-to-end Bayesian analysis pipeline that is able to search for, characterize and assign confidence levels for the detection of a stochastic gravitational wave background, and demonstrate the effectiveness of this approach using simulated data from the third round of Mock LISA Data Challenges.

by McKenzie, Kirk and Spero, Robert E. and Shaddock, Daniel A.
28 pages, 36 figures

For the laser interferometer space antenna (LISA) to reach it’s design sensitivity, the coupling of the free running laser frequency noise to the signal readout must be reduced by more than 14 orders of magnitude. One technique employed to reduce the laser frequency noise will be arm locking, where the laser frequency is locked to the LISA arm length. This paper details an implementation of arm locking, studies orbital effects, the impact of errors in the Doppler knowledge, and noise limits. The noise performance of arm locking is calculated with the inclusion of the dominant expected noise sources: ultra stable oscillator (clock) noise, spacecraft motion, and shot noise. Studying these issues reveals that although dual arm locking [A. Sutton & D. A Shaddock, Phys. Rev. D 78, 082001 (2008).] has advantages over single (or common) arm locking in terms of allowing high gain, it has disadvantages in both laser frequency pulling and noise performance. We address this by proposing a hybrid sensor, retaining the benefits of common and dual arm locking sensors. We present a detailed design of an arm locking controller and perform an analysis of the expected performance when used with and without laser pre-stabilization. We observe that the sensor phase changes beneficially near unity-gain frequencies of the arm-locking controller, allowing a factor of 10 more gain than previously believed, without degrading stability. We show that the LISA frequency noise goal can be realized with arm locking and Time-Delay Interferometry only, without any form of pre-stabilization.

by Marassi, Stefania and Schneider, Raffaella and Ferrari, Valeria
11 pages, 9 figures, 1 table accepted by MNRAS

Using the results of a numerical simulation which follows the evolution, metal enrichment and energy deposition of both Population III and Population II stars, we predict the redshift dependence of the formation rate of black hole remnants of Population III stars with masses 100- 500Msun and of neutron stars(black holes) remnants of Population II stars with masses 8-20Msun (20-40Msun). We describe the gravitational wave spectrum produced by Population III and Population II sources adopting the most appropriate signals available in the literature and we compute the stochastic backgrounds resulting from the cumulative emission of these sources throughout the history of the Universe. With the aim of assessing whether these backgrounds might act as foregrounds for signals generated in the Inflationary epoch, we compare their amplitudes with the sensitivity of currently planned and future ground/space-based interferometers.