All-sky Searches for Continuous Gravitational Waves from Isolated Neutron Stars in the Data from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run

Abstract

We present results from an all-sky search for continuous gravitational waves, using three different methods applied to the first eight months of LIGO data from the fourth LIGO-Virgo-KAGRA Collaboration s observing run. We aim at signals potentially emitted by rotating, non-axisymmetric isolated neutron star in the Milky Way. The analysis spans a frequency range from 20 Hz to 2000 Hz and accommodates frequency derivative magnitudes up to $10^{-8}$ Hz/s. No statistically significant periodic gravitational wave signals were detected. We establish 95% confidence-level (CL) frequentist upper limits on the dimensionless strain amplitudes. The most stringent population-averaged strain upper limits reach 9.7 $\times$ $10^{-26}$ near 290 Hz, matching the best previous constraints from 250 to $\sim$1700 Hz while extending coverage to a much broader spin-down range. At higher frequencies, the new limits improve upon previous results by factors of approximately $\sim$1.6. These constraints are applied to three astrophysical scenarios: 1) the distribution of galactic neutron stars as a function of spin frequency and ellipticity; 2) the contribution of millisecond pulsars to the GeV excess near the galactic center; and 3) the possible dark matter fraction composed of nearby inspiraling primordial binary black holes with asteroid-scale masses.

Figures (17)

• Figure 1: Comparison of frequency and spin-down ranges for the templated searches presented here, along with those of previous Einstein@Home searches bib:cwallskyO3EatHbib:cwallskyO3EatHbucketbib:cwallskyO3EatHHighFreq of the O3 data discussed in section \ref{['sec:constraints']}. The dark-green-shaded rectangle with fine gridding bars shows the 20--2000 Hz and $-10^{-8}$--$10^{-9}$ Hz/s range for the O4a PowerFlux search. The slightly taller but narrower yellow rectangle shows the region searched in the O4a FrequencyHough analysis. The smaller rectangles with diagonal hatching show the three Einstein@Home search ranges for the O3 data bib:cwallskyO3EatHbib:cwallskyO3EatHbucketbib:cwallskyO3EatHHighFreq, which have small variations in $\dot f_{\rm GW}$ ranges searched. The white curves show corresponding nominal ellipticities implied by a given frequency and spin-down for the gravitar model in which all rotational energy loss can be attributed to gravitational wave emission due to a mass quadrupole. Searching for high ellipticity values at a given frequency requires a high $\dot f_{\rm GW}$ range. (color online)
• Figure 2: Top panel: Population-averaged 95% confidence level (CL) estimated upper limits on gravitational strain amplitude for the three searches presented here: PowerFlux (dark green), FrequencyHough (yellow) and SOAP (magenta dashed). The PowerFlux limits are estimates based on strict circular-polarization upper limits (see section \ref{['sec:powerflux_upperlimits']}). The FrequencyHough and SOAP estimated limits are somewhat conservative (see sections \ref{['sec:fhulresults']} and \ref{['sec:soap_sensitivity']}). Also shown are results from previous searches in the full LIGO O3 data set: Time-domain $\mathcal{F}$-statisticbib:cwallskyO3FourPipelines (red); FrequencyHoughbib:cwallskyO3FourPipelines (blue); SkyHoughbib:cwallskyO3FourPipelines (magenta); PowerFlux with loose coherence bib:TripatheeRilesO3 (cyan); and Einstein@Homebib:cwallskyO3EatHbib:cwallskyO3EatHbucketbib:cwallskyO3EatHHighFreq (dark olive, violet, light olive, 90% CL, covering $\sim$26% of the spindown range of the other templated searches). Bottom panel: Magnification in the frequency range 20-500 Hz to show finer detail. (color online)
• Figure 3: Ellipticity sensitivities of the PowerFlux and FrequencyHough searches for neutron stars spinning down solely due to gravitational radiation ("gravitars") under different assumed distances. Each family of three curves (orchid, dark green dotted, yellow diamonds) show the sensitivities for the O4a PowerFlux circular-polarization, population-averaged and FrequencyHough population-averaged sensititivies that apply for four different assumed distances of $d$ = 10 pc, 100 pc, 1 kpc and 10 kpc. The smooth red dotted curve shows, however, the maximum detectable ellipticity for the maximum allowed spindown magnitude of 10$^{-8}$ Hz/s used here, while the lower, dashed red curve shows the corresponding maximum detectable ellipticity for the maximum allowed spindown magnitude of 2.7$\times$10$^{-9}$ Hz/s used by the O3 Einstein@Home search bib:cwallskyO3EatHbib:cwallskyO3EatHbucketbib:cwallskyO3EatHHighFreq. Where the search sensitivity curves for optimum stellar orientation (circular-polarization, shown for PowerFlux) lie above the $\dot f_{\rm GW}$ boundary, one cannot actually detect the corresponding ellipticity for that range. See Fig. \ref{['fig:range']} for graphs of range sensitivity for different assumed ellipticities, taking into account the $\dot f_{\rm GW}$ constraint. (color online)
• Figure 4: Ranges (kpc) of the PowerFlux and FrequencyHough searches for the gravitar model under different assumptions. The four sets of three curves (orchid, dark green dotted, yellow diamonds) that generally rise with frequency are the ranges for which the O4a PowerFlux circular-polarization, population-averaged and FrequencyHough population-averaged sensititivies apply for four different assumed equatorial ellipticities of $\epsilon$ = $10^{-4}$, $10^{-5}$, $10^{-6}$ and $10^{-7}$. In each case, the curve stops at the frequency at which the implied spindown magnitude at the corresponding range exceeds the maximum range allowed by the maximum spindown magnitude ($10^{-8}$ Hz/s) used here. The three gray curves (light dotted, medium dashed, dark solid) that peak below 200 Hz represent the maximum possible ranges for which the O4a PowerFlux circular-polarization limits apply under the assumption that the maximum spindown magnitude is ($10^{-8}$, $10^{-10}$, $10^{-12}$) Hz/s. Searching for high spindown magnitudes simultaneously probes higher ellipticies and larger regions of the galaxy, reaching beyond the galactic center ($\sim$ 8.5 kpc) for $\epsilon = 10^{-6}$ over the full search band. More realistically, a neutron star's spin-down may be dominated by electromagnetic radiation, in which case searching for high spindown magnitude achieves a shorter search range, but enables finding stars that would not be detectable under the gravitar assumption. (color online)
• Figure 5: Contours of constant $\log_{10}(N^{90\%})$ as a function of gravitational-wave frequency $f_{\rm GW}$ and ellipticity $\epsilon$, for the PowerFlux (solid lines), FrequencyHough (dashed lines), and SOAP (dashed-dotted lines) searches. Here, $N$ denotes the total number of neutron stars with frequency $f_{\rm GW}$ and ellipticity $\geq \epsilon$. In the absence of detection, we place upper limits on N, and $N^{90\%}$ represents the corresponding 90% confidence upper limit. The color of each contour indicates the value of $\log_{10}(N^{90\%})$, as shown in the color bar. At each point on the 2D grid, we adopt the most conservative constraint among four spatial distributions viz., the Progenitor Paczynski and the three "Reed" Reed distributions. The filled region in the top right corner is the excluded parameter space for a maximum spin-down magnitude of $10^{-8}$ Hz/s. The vertical dotted line in black demarcates a representative maximum ellipticity the NS crust can support morales2022