All-sky search for persistent gravitational waves with folded data

In this project we performed the end-to-end study of the all-sky search for persistent gravitational waves with radiometry 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 with ground-based interferometers. It is predicted that some neutron stars could be spherically asymmetric, emitting gravitational waves as they spin. Possible mechanisms for asymmetry are: accretion from a companion star, influence of magnetic fields, dynamical instabilities (i.e. R-modes). Gravitational waves emitted in these scenarios can be approximated as persistent, lasting for a long time, and narrowband, emitted with almost the same gravitational wave frequency.

Other research groups have developed several searches for continuous gravitational waves, targeting neutron stars. Coherent and semi-coherent searches account for frequency modulation of the gravitational-wave signal emitted by a neutron star. Matched-filter searches are designed to be statistically optimal. Deviations from theoretical predictions of neutron star rotation period, i.e. neutron star glitches, can lead to a gravitational wave signal being missed by continuous-wave searches. We develop a robust search that both complements continuous-wave searches and sensitive to other unknown sources of gravitational radiation as well.

Radiometry

Radiometry is a technique of cross-correlating strain data measured by two or more gravitational-wave detectors. Information about the sky location of a source of gravitational radiation is encoded in the time delay between the signal's arrival in different detectors.

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 the radiometer.

Data folding

We choose our frequency bins to be wide enough to allow a persistent narrowband signal to be present only in one frequency bin for the time of the search. As ground-based interferometers rotate around the Earth, the time-delay pattern of a signal would repeat itself every sidereal day. Folding is essentially weighted averaging all time segments of data that correspond to the same orientation of detectors with respect to the source. 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

The method paper has been published in Physical Review D. The search is currently in preparation.

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