Observational constraints on the spin/anisotropy of the CCOs of Cassiopeia A, Vela Jr. and G347.3-0.5 and a single surviving continuous gravitational wave candidate

TL;DR

The paper reports a deep, directed search for continuous gravitational waves from three young neutron stars in supernova remnants Cas A, Vela Jr., and G347.3-0.5, using O3/O4a data and Einstein@Home. It employs a hierarchical 5-stage follow-up pipeline to grow phase coherence and suppress noise, achieving the most stringent upper limits on CW amplitude, ellipticity, r-mode amplitude, and crustal anisotropy to date. One low-frequency G347.3 candidate survives all follow-ups but remains marginal (false-alarm probability $\sim$27% after accounting for trials), and no robust CW detection is claimed. The results impose tight astrophysical constraints, translating CW limits into bounds on neutron-star ellipticity, crustal anisotropy, and spin evolution across plausible birth spin scenarios, with significant implications for the internal physics of young NSs.

Abstract

We carry out the deepest and broadest search for continuous gravitational-wave signals with frequencies between 20-1500 Hz, from three neutron stars at the center of the supernova remnants Cassiopeia A, Vela Jr., and G347.3-0.5. This search was made possible by the computing power shared by thousands of Einstein@Home volunteers. After the initial Einstein@Home search, we perform a multi-stage follow-up of the most promising $\approx$ 45 million signal candidates. In the last stages, we use independent data to further investigate the remaining candidates from the previous stages. We set the most stringent constraints to date on the gravitational-wave amplitude, equatorial ellipticity, r-mode saturation amplitude, and -- for the first time -- the neutron-star crustal anisotropy. For spin periods lower than 2 ms we constrain the ellipticity to be smaller than $4\times 10^{-7}$ for all targets. We exclude the crustal anisotropy to be smaller than $5\times 10^{-3}$ for spin periods between 1.3-100 ms. Only one candidate -- from the low frequency G347.3 search -- survives all follow-ups. We illustrate properties of this candidate. Investigations on new data will aid in clarifying its nature. Such ``new" data exists and would be optimal for this purpose, but they are not publicly accessible at the time of writing.

Figures (11)

• Figure 1: Number of templates searched in 1-Hz bands as a function of signal frequency. In the legend we also show the total number of templates searched for each target in the low and high frequency bands.
• Figure 2: Distributions of $R^{a}$ of $R^{a}$ from all the Cas A searches. In orange color are the distributions of the growth ratio $R^{a}$ for the search candidates; in blue are the results for the test-signals drawn from the target population. The vertical dashed line indicates the per-stage threshold $R^{a}_{\mathrm{thr}}$. As the hierarchical follow-ups progress, noise-dominated outliers increasingly cluster at lower $R^{a}$, while candidates associated with signals shift toward larger values, yielding a progressively clearer separation between the two populations.
• Figure 3: Distributions of $R^{a}$ from all the Vela Jr. searches.
• Figure 4: Distributions of $R^{a}$ from all the G347.3 searches.
• Figure 5: Upper limits on the gravitational wave amplitude of continuous gravitational wave signals from our search for Cas A (red triangles) as a function of frequency, compared to other recent results. The horizontal lines show the indirect age-based upper limits corresponding to braking indexes of 5 and 7.