Search for Continuous Gravitational Waves from Known Pulsars in the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run

Abstract

Continuous gravitational waves (CWs) emission from neutron stars carries information about theirinternal structure and equation of state, and it can provide tests of General Relativity. We present asearch for CWs from a set of 45 known pulsars in the first part of the fourth LIGO–Virgo–KAGRAobserving run, known as O4a. We conducted a targeted search for each pulsar using three independentanalysis methods considering the single-harmonic and the dual-harmonic emission models. We find noevidence of a CW signal in O4a data for both models and set upper limits on the signal amplitudeand on the ellipticity, which quantifies the asymmetry in the neutron star mass distribution. For thesingle-harmonic emission model, 29 targets have the upper limit on the amplitude below the theoreticalspin-down limit. The lowest upper limit on the amplitude is 6.4×10−27 for the young energetic pulsarJ0537−6910, while the lowest constraint on the ellipticity is 8.8×10−9 for the bright nearby millisecondpulsar J0437−4715. Additionally, for a subset of 16 targets we performed a narrowband search that ismore robust regarding the emission model, with no evidence of a signal. We also found no evidence ofnon-standard polarizations as predicted by the Brans-Dicke theory.