LIGO Fails to Find Continuous Gravitational Waves From Pulsars

In February 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history by announcing the first-ever detection of gravitational waves (GW). These waves, predicted by Einstein’s Theory of General Relativity, are created when massive objects collide (neutron stars or black holes), causing ripples in spacetime that can be detected millions or billions of light years away. Since their discovery, astrophysicists have been finding applications for GW astronomy, which include probing the interiors of neutron stars.

For instance, scientists believe that probing the continuous gravitational wave (CW) emissions from neutron stars will reveal data on their internal structure and equation of state and can provide tests of General Relativity. In a recent study, members of the LIGO-Virgo-KAGRA (LVK) Collaboration conducted a search for CWs from 45 known pulsars. While their results showed no signs of CWs emanating from their sample of pulsars, their work does establish upper and lower limits on the signal amplitude, potentially aiding future searches.

The LVK Collaboration is an international consortium of scientists from hundreds of universities and institutes worldwide. This collaboration combines data from the Laser Interferometer Gravitational-Wave Observatory’s (LIGO) twin observatories, the Virgo Observatory, and the Kamioka Gravitational Wave Detector (KAGRA). The preprint of the paper, “Search for continuous gravitational waves from known pulsars in the first part of the fourth LIGO-Virgo-KAGRA observing run,” recently appeared online.

First discovered in 1967, pulsars are a class of neutron stars that have strong magnetic fields, causing them to emit beams of electromagnetic radiation from their poles. They also rotate rapidly, creating a strobing effect reminiscent of a lighthouse. Given their stability and predictability, pulsars present an opportunity to search for continuous gravitational waves (CWs). Unlike transient GW, which are produced by binary black hole and neutron star mergers, CWs are long-lasting signals expected to come from massive, spinning objects (like pulsars).

To date, all GW events observed by astronomers have been transient in nature. To find evidence of these events, the team searched for signals from 45 known pulsars (and a narrowband search for 16 pulsars) from the first part of the fourth LIGO-Virgo-KAGRA observing run (O4a). They also employed three independent data analysis methods and two different emission models. As they indicated in their paper, no CW signals were detected, but the results were still informative:

“No evidence of a CW signal was found for any of the targets. The upper limit results show that 29 targets surpass the theoretical spin-down limit. For 11 of the 45 pulsars not analyzed in the last LVK targeted search, we have a notable improvement in detection sensitivity compared to previous searches. For these targets, we surpass or equal the theoretical spin-down limit for the single-harmonic emission model. We also have, on average, an improvement in the upper limits for the low-frequency component of the dual-harmonic search for all analyzed pulsars.”

The team also conducted a search for polarization that is consistent with a theory of gravitation alternative to General Relativity (Brans–Dicke theory). While CWs remain unconfirmed, the team predicts that a full analysis of the full O4 dataset will improve the sensitivity of targeted/narrowband searches for pulsars and CWs.

Further Reading: arXiv