Search for continuous gravitational waves from neutron stars in five globular clusters in the first part of the fourth LIGO-Virgo-KAGRA observing run

Abstract

We present the results of directed searches for continuous gravitational waves from unknown neutron stars in five Milky Way globular clusters. We carry out the searches in the LIGO data from the first eight months of the fourth LIGO-Virgo-KAGRA observing run using the WEAVE semi-coherent program, which sums matched-filter detection-statistic values over many time segments spanning the observation period. No gravitational wave signal is detected in the search band of 20-475 Hz for assumed source ages greater than 300 years. Injections of simulated continuous wave signals in the data indicate that we achieve the most sensitive results to date across most of the explored parameter space volume, obtaining median 95% confidence level upper limits as low as $\sim 4.2 \times 10^{-26}$ near 282 Hz for NGC 6397.

Figures (8)

• Figure 1: Data collected from the H1 and L1 detectors during the first eight months of the fourth observing run (O4a), spanning from May 24, 2023 (15:00:00 UTC) to January 16, 2024 (16:00:00 UTC). The shaded segments represent the 7.5-day coherence segments used in the initial stage of the search (detailed in Section \ref{['sec:method']}).
• Figure 2: Central density as a function of core radius for Milky Way globular clusters from Harris1996. The color of each point indicates the number of pulsars observed in that cluster. Blue lines indicate lines of constant formation rate $\Gamma$ (Eq. \ref{['eq:encounter merit']}), and red lines indicate constant binary encounter rate $\gamma$ (Eq. \ref{['eq:binary encounter merit']}). The targets selected for this search are marked with black crosses. The figures of merit (Eqs. \ref{['eq:encounter merit']}-\ref{['eq:binary encounter merit']}) used to prioritize target selection also consider the distance $d$ to each cluster, not shown in this diagram.
• Figure 3: Comparison of search range and the required spin-down rate $\dot{f}_\mathrm{age}$ (Eq. \ref{['eq:fdot sd limit']}, blue) to exceed the age-based strain limit, assuming a source with an age of 300 years. The black solid line indicates the maximum spin-down covered in this search. The search range fully encompasses the physically motivated region.
• Figure 4: H1 spectrogram near 283.9 Hz (bright) with injected signal using the NGC 6397 outlier's parameters (dark). The spectrogram is generated by taking the daily sum of the cumulative averaged power across all 1800-second SFTs within each day. In the last 60-day segment, the simulated signal overlaps with an instrumental line artifact, causing it to appear as an outlier at the $T_{\textrm{\tiny{coh}}}=120$ days follow-up stage.
• Figure 5: Gravitational wave strain amplitude upper limits (95% efficiency) in each 1-Hz band. (a) Results for Terzan 10, NGC 6544, and NGC 104 assuming signals originate from a central region with radius equal to the core radius $r_c$. (b) Results for NGC 6397, NGC 6544, and NGC 6540 assuming signals originate from within the tidal radius $r_t$. The degradation of the upper limit at higher frequencies is more pronounced for searches covering the larger radius.