Michele's publications

Searches for gravitational waves from cosmic-string cusps using Markov Chain Monte Carlo and Nested Sampling

Cosmic strings

With Michael Cohen and Curt Cutler

A network of observable, macroscopic cosmic (super-)strings may have formed in the early Universe. If so, the cusps that generically develop on cosmic-string loops emit bursts of gravitational radiation that could be detectable by both ground- and space-based gravitational-wave interferometers. Here we report on two versions of a LISA-oriented string-burst search pipeline that we have developed and tested within the context of the Mock LISA Data Challenges.

The two versions rely on the publicly available MultiNest and PyMC software packages, respectively. To reduce the effective dimensionality of the search space, our implementations use the F-statistic to analytically maximize over the signal's amplitude and polarization, A and psi, and use the FFT to search quickly over burst arrival times t_C. The standard F-statistic is essentially a frequentist statistic that maximizes the likelihood; we also demonstrate an approximate, Bayesian version of the F-statistic that incorporates realistic priors on A and psi.

We calculate how accurately LISA can expect to measure the physical parameters of string-burst sources. To understand LISA's angular resolution for string-burst sources, we draw maps of the waveform fitting factor [maximized over (A psi, t_C)] as a function of sky position; these maps dramatically illustrate why (for LISA) inferring the correct sky location of the emitting string loop will often be practically impossible. We also identify and elucidate several symmetries that are imbedded in this search problem, and we derive the distribution of cut-off frequencies f_max for observable bursts.

Fitting-factor maps

(Displayed with zoom.it; use mouse to zoom and pan, rightmost button to go fullscreen.)

  • Figure 2 in the paper, reproduced below in a zoomable version, displays the broad degeneracy of burst signals across the sky, which will limit the resolution of sky position even for high-SNR signals. Each of the small square maps visualizes the fitting factor (the true signal vs. template overlap, maximized over the amplitude, polarization, and time of arrival of the template, and normalized by the amplitude of the true signal) between a true signal at the sky location marked by the blue dot, and templates across the sky.
  • Different square maps correspond to different true-source LISA-plane latitudes (β), longitudes (λ), and polarizations (ψ). Inside each map the vertical and horizontal axes span the target LISA-plane latitude (β') and longitude (λ'), respectively, with the poles at the top and bottom and the LISA plane at the center. The upper and lower formations of maps correspond to bursts with high- and low-frequency content, respectively.

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© M. Vallisneri 2014 — last modified on 2012/05/17