Stochastic gravitational-wave background searches and constraints on neutron-star ellipticity

Abstract

Rotating neutron stars (NSs) are promising sources of gravitational waves (GWs) in the frequency band of ground-based detectors. They are expected to emit quasi-monochromatic, long-duration GW signals, called continuous waves (CWs), due to their deviations from spherical symmetry. The degree of such deformations, and hence the information about the internal structure of a NS, is encoded in a dimensionless parameter 𝜀 called ellipticity. Searches for CW signals from isolated Galactic NSs have shown to be sensitive to ellipticities as low as 𝜀 ∼ O (10 −9 ). These searches are optimal for detecting and characterising GWs from individual NSs, but they are not designed to measure the properties of NSs as population, such as the average ellipticity 𝜀 av . These ensemble properties can be determined by the measurement of the stochastic gravitational-wave background (SGWB) arising from the superposition of GW signals from individually-undetectable NSs. In this work, we perform a cross-correlation search for such a SGWB using the data from the first three observation runs of Advanced LIGO and Virgo. Finding no evidence for a SGWB signal, we set upper limits on the dimensionless energy density parameter Ω gw ( 𝑓 ). Using these results, we also constrain the average ellipticity of Galactic NSs and five NS "hotspots", as a function of the number of NSs emitting GWs within the frequency band of the search 𝑁 band . We find 𝜀 av < ∼ 1.8 × 10 −8 , with 𝑁 band = 1.6 × 10 7 , for Galactic NSs, and 𝜀 av < ∼ [3.5 − 11.8] × 10 −7 , with 𝑁 band = 1.6 × 10 10 , for NS hotspots.