Search for anisotropic gravitational-wave backgrounds using data from Advanced LIGO and Advanced Virgo’s first three observing runs

We report results from searches for anisotropic stochastic gravitational-wave backgrounds using data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. For the first time, we include Virgo data in our analysis and run our search with a new efficient pipeline called pystoch on data folded over one sidereal day. We use gravitational-wave radiometry (broadband and narrow band) to produce sky maps of stochastic gravitational-wave backgrounds and to search for gravitational waves from point sources. A spherical harmonic decomposition method is employed to look for gravitational-wave emission from spatially-extended sources. Neither technique found evidence of gravitational-wave signals. Hence we derive 95% confidence-level upper limit sky maps on the gravitational-wave energy flux from broadband point sources, ranging from $F_{\alpha ,\mathrm{\Theta }}<(0.013–7.6)\times {10}^{-8}\mathrm{erg}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{Hz}}^{-1}$, and on the (normalized) gravitational-wave energy density spectrum from extended sources, ranging from ${\mathrm{\Omega }}_{\alpha ,\mathrm{\Theta }}<(0.57–9.3)\times {10}^{-9}{\mathrm{sr}}^{-1}$, depending on direction ($\mathrm{\Theta }$) and spectral index ($\alpha$). These limits improve upon previous limits by factors of 2.9–3.5. We also set 95% confidence level upper limits on the frequency-dependent strain amplitudes of quasimonochromatic gravitational waves coming from three interesting targets, Scorpius X-1, SN 1987A and the Galactic Center, with best upper limits range from $h_0<(1.7–2.1)\times {10}^{-25}$, a factor of $\ge 2.0$ improvement compared to previous stochastic radiometer searches.

Figure 1

Power spectral density spectrogram of the Hanford detector data around the 410.3 Hz calibration line using a short stretch of gated data. Each vertical column in the above plot corresponds to a power spectral density estimate using a 192 second long segment. The purple dotted lines show the region of the standard frequency notch around the calibration line and the orange dashed lines show the frequency region notched based on the nonstationarity level. We see that the latter removes a good portion of the nonstationarity region around the calibration line.

Figure 2

Top row: SNR maps from a BBR search for pointlike sources. Bottom row: upper limit (UL) sky maps of the gravitational-wave energy flux. Both sets of maps, presented in equatorial coordinate system, are derived by combining all three LIGO observing runs and the Virgo O3 data. $\alpha =0,2/3$, and 3 are represented from left to right.

Figure 3

Top row: SNR maps from the SHD search for extended sources. Bottom row: sky maps representing 95% upper limit on the normalized gravitational-wave energy density ${\mathrm{\Omega }}_{\alpha }(\mathrm{\Theta })[{\mathrm{sr}}^{-1}]$. Both sets of maps, presented in equatorial coordinate system, are derived by combining all three observing runs of LIGO-Virgo data (Virgo was incorporated only for O3). $\alpha =0,2/3$, and 3 are represented from left to right.

Figure 4

95% upper limits on $C_{\ell }$ for different $\alpha$ using combined $\mathrm{O}1+\mathrm{O}2+\mathrm{O}3$ data.

Figure 5

Comparison between the Fisher matrix condition numbers for the HL and HLV networks for different values of $\alpha$. The vertical dashed lines mark the divisions between the two thirds of eigenvalues that are included in the analysis and one third that are excluded when inverting the Fisher matrix.

Figure 6

Upper limits on the dimensionless strain amplitude $h_0$, using the data from three observing runs of LIGO-Virgo detectors, at the 95% confidence level for the narrow-band radiometer search are indicated by the gray bands for Scorpius X-1 (left), SN 1987A (middle) and the Galactic Center (right). The dark line shows the $1\sigma$ sensitivity of the search for each direction.

Figure 7

Broadband radiometer maps illustrating search sensitivity for pointlike sources from O3 data only. Each row shows maps of the $1\sigma$ sensitivity for HL, HV and LV baselines, from top to bottom, for three different power-law indices, $\alpha =0,2/3$ and 3, from left to right.

Figure 8

The spherical harmonics $1\sigma$ sensitivity maps produced from O3 illustrating a search for extended sources using each of HL, HV, LV baselines (top to bottom rows respectively). Three different power law indices, $\alpha =0,2/3$ and 3, are represented by columns from left to right.

Figure 9

The uncertainty associated with the NBR search estimator is shown. The uncertainty on the estimated gravitational-wave strain as a function of frequency is plotted for three directions, Scorpius X-1, SN 1987 and Galactic Center from left to right using O3 data for all the three baselines. HL baselines are indicated by the black lines, HV with sky blue lines and LV using orange.