All-sky search for persistent gravitational waves with folded data

This research project involves development of the all-sky search for persistent gravitational waves. It uses radiometery and data folding.

Boris Goncharov, Eric Thrane

Method paper: Arxiv.org, Physical Review D | Search paper: in preparation

Motivation

By now LIGO has detected several gravitational wave signals from coalescing and merging compact binaries, with black holes and neutron stars.Accretion neutron star However, there are other interesting astrophysical phenomena that can produce gravitational waves detectable in the experiment. One example is neutron star matter oscillations. They can be a result of the following: accretion from a companion star, influence of magnetic fields, dynamical instabilities (i.e. R-modes). Gravitational waves emmited in these scenarios can be approximated as persistent, lasting for a long time, and narrowband, emmited with almost the same gravitational wave frequency.

Other research groups have developed several searches for continuous gravitational waves, targeting neutron stars. These searches account for frequency modulation of gravitational-wave signal emmited by neutron star, and are designed to be statistically optimal. However, these searches depend on theoretical models. Deviations from these models, i.e. so-called neutron star glitches, can lead to a gravitational wave signal being missed by continuous wave searches. We develop a robust search that complements continuous wave seraches, and that is sensitive to other unknown sources of gravitational radiation as well.

Radiometry

Radiometry is a technique of cross-correlating Fourier-transformed strain, measured by two or more gravitational-wave detectors. Information about sky location of a source of gravitational radiation is encoded in the time delay between signal's arrival in different detectors. This time delay is encoded in the phase of the complex-valued cross-correlated data, while the signal strength is encoded in the magnitude of the cross-correlated data.

Previous radiometer searches for LIGO were either all-sky, but averaged over all frequencies, or all-frequency, but targeted to specific directions. We employ "folding", a method of data compression, that for the first time makes possible an all-sky all-frequency search for persistent gravitational waves with radiometer.

Data folding

If the signal is persistent and narrowband, it stays within one frequency bin for the time of the search. As LIGO is a ground based experiment, and as Earth rotates around the sun, astrophysical signal's pattern in the measured gravitational wave strain would repeat itself every sidereal day. Folding is essentially weightenly averaging all sidereal days of data together. As a result, we have to work with only one day of data instead of several months of data from LIGO's observing runs.

Current status

Paper on the topic is getting ready for publication.

Related publications

Goncharov, B., & Thrane, E. (2018). An all-sky radiometer for narrowband gravitational waves using folded data. arXiv preprint arXiv:1805.03761.

Recent Work