The Quest for Continuous Gravitational Waves: What We're Learning from Neutron Stars

* **Introduction**:

* This episode discusses the search for continuous gravitational waves (CWs) emitted by neutron stars, specifically known pulsars, using data from the LIGO-Virgo-KAGRA (LVK) collaboration's fourth observing run (O4a).

* CWs are distinct from transient gravitational waves (GWs) like those from black hole mergers; they are nearly monochromatic signals with small variations over long periods.

* **Detecting CWs can provide insights into the internal structure of neutron stars, their equations of state, and help test general relativity.**

* **What are Pulsars?**

* Pulsars are extremely dense, rapidly rotating objects with strong magnetic fields that emit beams of electromagnetic radiation.

* Their stability and predictable behavior make them ideal for CW searches.

* Electromagnetic (EM) observations of pulsars help predict and correct for modulations of any CW signal, like the Doppler effect from Earth's motion.

* **The Search for Continuous Gravitational Waves:**

* CWs are expected from neutron stars due to non-axisymmetric mass distributions. This can be caused by strain in the star's crust, accretion, strong magnetic fields, or fluid oscillations.

* The search focuses on known pulsars using "targeted searches," where the GW signal is assumed to be locked to the pulsar’s rotation as determined by EM observations.

* This paper presents results from three independent search methods (Bayesian, 5-vector, and F/G/D-statistic). These methods are used to look for both single-harmonic (at twice the pulsar's spin frequency) and dual-harmonic (at both the spin frequency and twice the spin frequency) emissions.

* A subset of pulsars were analyzed with a "narrowband search," which relaxes the assumption that the GW phase evolution exactly matches the EM solution, and searches in a narrow band around the expected frequencies.

* **Results**

* **No CW signal was detected in the O4a data.**

* The search set upper limits on the signal amplitude and the ellipticity (a measure of asymmetry in the star’s mass distribution) for each target.

* For 29 targets, the upper limit on the amplitude is below the theoretical "spin-down limit," which is calculated by assuming all the pulsar's rotational energy loss is due to GW emission.

* The lowest upper limit on the amplitude is 6.4 x 10^-27 for pulsar J0537-6910.

* The lowest constraint on the ellipticity is 8.8 x 10^-9 for pulsar J0437-4715.

* **No evidence was found for non-standard polarizations predicted by the Brans-Dicke theory**.

* The narrowband search also found no signals.

* **Comparison with Past Searches**:

* The results from this search are comparable to previous searches using data from the second and third observing runs (O2 and O3).

* The improved sensitivity of the detectors during O4a is balanced by a shorter observation period, resulting in similar overall sensitivity.

* Some pulsars have improved upper limits compared to previous searches and some have worse, which is expected, with low frequency searches showing more improvement.

* **Significance of the Findings:**

* These results continue to push into the regime of astrophysical interest for neutron star parameters, such as ellipticity, which is related to the "mountain" size on the neutron star.

* **The fact that these searches are not detecting signals yet is important, as it places constraints on the properties of these objects**.

* **Future Directions:**

* The full O4 dataset, when analyzed, will provide improved sensitivity.

* Ongoing improvements in detectors and data analysis techniques will enhance future search capabilities.

* **Reference:** A. G. Abac et al. (2025). Search for continuous gravitational waves from known pulsars in the first part of the fourth LIGO-Virgo-KAGRA observing run. https://arxiv.org/abs/2501.01495

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: C. Reed, PennState