Directional Search for Persistent Gravitational Waves: Results from the First Part of LIGO-Virgo-KAGRA's Fourth Observing Run

TL;DR

This work reports a comprehensive directional search for persistent gravitational waves using LIGO, Virgo, and KAGRA data from O1 through the first part of O4a. It implements four complementary analyses—ASAF, targeted-narrowband radiometer, broadband radiometer, and SPH—to probe localized, narrowband, broadband, and extended anisotropies in the GW background. No evidence for persistent directional GW signals is found, and the study delivers the most stringent upper limits to date on anisotropic GW emission across sky directions, spectral indices, and angular scales, with notable sensitivity gains over prior runs. The results constrain both astrophysical populations (e.g., compact binaries, rotating neutron stars) and cosmological scenarios (e.g., cosmic strings), informing future searches as the detector network expands and observing time increases.

Abstract

The angular distribution of gravitational-wave power from persistent sources may exhibit anisotropies arising from the large-scale structure of the Universe. This motivates directional searches for astrophysical and cosmological gravitational-wave backgrounds, as well as continuous-wave emitters. We present results of such a search using data from the first observing run through the first portion of the fourth observing run of the LIGO-Virgo-KAGRA Collaborations. We apply gravitational-wave radiometer techniques to generate skymaps and search for both narrowband and broadband persistent gravitational-wave sources. Additionally, we use spherical harmonic decomposition to probe spatially extended sources. No evidence of persistent gravitational-wave signals is found, and we set the most stringent constraints to date on such emissions. For narrowband point sources, our sensitivity estimate to effective strain amplitude lies in the range $(0.03 - 8.4) \times 10^{-24}$ across all sky and frequency range $(20 - 160)$ Hz. For targeted sources -- Scorpius X-1, SN 1987A, the Galactic Center, Terzan 5, and NGC 6397 -- we constrain the strain amplitude with best limits ranging from $\sim 1.1 \times 10^{-25}$ to $6.5 \times 10^{-24}$. For persistent broadband sources, we constrain the gravitational-wave flux $F_{α, \hat{n}}^{95\%, \mathrm{UL}}(25\, \mathrm{Hz}) < (0.008 - 5.5) \times 10^{-8}\, \mathrm{erg\, cm^{-2}\, s^{-1}\, Hz^{-1}}$, depending on the sky direction $\hat{n}$ and spectral index $α=0,\,2/3,\,3$. Finally, for extended sources, we place upper limits on the strain angular power spectrum $C_\ell^{1/2} < (0.63 - 17) \times 10^{-10} \,\mathrm{sr}^{-1}$.

Figures (12)

• Figure 1: Bayesian sensitivity estimate to Effective Strain Amplitude $h_{\rm eff}$: Left panel: Sky-averaged sensitivity from the O1-O3 and O1-O4a datasets are shown as blue and yellow solid lines, respectively. The shaded regions represent 1-sigma variations in the estimates across the sky. The overall shape of the curves reflects the detectors' sensitivity profiles. Gaps indicate vetoed frequency bins, while narrow vertical peaks correspond to instrumental spectral lines that were not vetoed, as their impact on the analysis was minimal. Top Right Panel: Skymap of frequency-averaged sensitivity using O1–O4a data. The observed pattern reflects the typical sky sensitivity of an HL-dominated network. As expected, the sensitivity is worse near the poles and equator, with improved sensitivity in the mid-declination regions. Bottom Right Panel: Skymap showing the ratio of frequency-averaged sensitivity from O1-O4a to that from O1-O3.
• Figure 2: Plots showing the UL for the five targets of the O1-O4a NBR search. In each plot, the black solid line shows the Bayesian UL set at 95% confidence level, while the gray line is the $1\sigma$ sensitivity estimation in the hypothesis of no signal. The shaded blue area shows the $1\sigma$ sensitivity estimation with data up to the O3 run, highlighting the sensitivity improvement granted by adding the O4a run to the analyses.
• Figure 3: 95% confidence Bayesian UL skymaps from a BBR search for point-like sources. The maps are presented in the equatorial coordinate system and show UL skymaps of the GW energy flux from the combination of all the data from O1 to O4a LIGO observing runs and the Virgo O3 data. $\alpha =$ 0, 2/3, and 3 are represented from left to right.
• Figure 4: $95\%$UL on the two $C_\ell$ estimators for different $\alpha$ using combined O1+O2+O3+O4a data.
• Figure 5: ASAF search results --- Significance: Left Panel: Distribution of SNR for the zero-lag (blue) and time-shifted (magenta) datasets with O4a observing run. The time-shifted (unphysical) distribution is consistent with a Gaussian (gray line) with mean $-5\times10^{-4}$ and standard deviation 0.98. Poisson 1- and 2-sigma uncertainties for the time-shifted histogram are shown as a yellow-shaded region. The zero-lag histogram is largely consistent with the time-shifted data, except for non-Gaussian excess at negative SNR. Vertical brown lines (solid and dashed) indicate the 1- and 2-sided $5\%$ global $p$-value SNR thresholds. Although an astrophysical signal is expected to yield a positive SNR, we investigate the origin of the observed negative SNR feature. Right Panel: Same as the figure in the right panel but with O1-O4a data.