Search of the early O3 LIGO data for continuous gravitational waves from the Cassiopeia A and Vela Jr. supernova remnants

We present directed searches for continuous gravitational waves from the neutron stars in the Cassiopeia A (Cas A) and Vela Jr. supernova remnants. We carry out the searches in the LIGO detector data from the first six months of the third Advanced LIGO and Virgo observing run using the weave semicoherent method, 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 20–976 Hz for assumed source ages greater than 300 years for Cas A and greater than 700 years for Vela Jr. Estimates from simulated continuous wave signals indicate we achieve the most sensitive results to date across the explored parameter space volume, probing to strain magnitudes as low as $\sim 6.3\times {10}^{-26}$ for Cas A and $\sim 5.6\times {10}^{-26}$ for Vela Jr. at frequencies near 166 Hz at 95% efficiency.

Figure 1

Upper panel: example of “strain histogram” graph for Cas A used in vetoing outliers for which instrumental contamination is apparent. The curves show the O3a-run-averaged H1 (red dashed) and L1 (blue solid) amplitude spectral densities in a narrow band containing an artifact at 48.000 Hz. The dotted curves show histograms of expected strain excess from H1 (black) and L1 (magenta) signal templates added to smooth backgrounds interpolated from neighboring frequency bands. In this depiction, the strain amplitude of the signal template has been magnified by an arbitrary factor large enough to make the signal’s structure clear. The large excess power in the H1 data, not seen in the L1 data, despite comparable strain sensitivities and comparable sidereal-averaged antenna pattern sensitivities, excludes an astrophysical source for the H1 artifact. The fact that the artifact aligns in frequency with the putative signal’s template peak in power confirms contamination of the outlier from an instrumental source. In addition, the line at precisely an integer frequency is part of a known instrumental spectral comb in the O3a H1 data. Lower panel: graph of the corresponding template signal frequencies vs. time during O3a in the H1 and L1 interferometer reference frames, in which frequency points are plotted for only those 30-minute segments used in the analysis. One sees a relatively stationary period early in the run for Cas A. The inset box shows a magnification of the frequency vs. time graph for a 15-day period starting at the midpoint of the O3a interval, one that includes a multi-day period during which no data was collected from the L1 interferometer because Hurricane Barry disrupted observatory operations. The magnification makes more clear the diurnal modulation of the reference-frame frequency by the Earth’s rotation about its axis, with slightly larger modulations seen for the lower-latitude L1 interferometer than for H1.

Figure 2

Example of strain histogram and template frequency vs. time graphs for a Vela Jr. outlier with the same definitions (and colors) used for Cas A in Fig. 1. One key difference with respect to Cas A is that the interval of relatively stationary interferometer-frame frequency corresponding to the 48-Hz instrumental line occurs after the midpoint for Vela Jr. because of its different sky location from Cas A.

Figure 3

Counts vs. frequency in 1-Hz bins for the initial Cas A search outliers (blue squares), 1st-round follow-up survivors (red diamonds), and 2nd-round follow-up survivors (green triangles). The vertical gray bands denote consolidated 0.1-Hz subbands displaying saturation in the initial search. One sees high outlier counts and saturations primarily at low frequencies, near test-mass violin modes (resonant vibration modes of silica fibers around 500 Hz) and at harmonics of beam-splitter violin modes (above 300 Hz and near-integer multiples). Counts equal to zero for different stages are depicted on the vertical logarithmic scale by distinct fractions less than one.

Figure 4

Counts vs. frequency in 1-Hz bins for the initial Vela Jr. search outliers (blue squares), 1st-round follow-up survivors (red diamonds), and 2nd-round follow-up survivors (green triangles). The vertical gray bands denote consolidated 0.1-Hz subbands, as in Fig. 3 Counts equal to zero for different stages are depicted on the vertical logarithmic scale by distinct fractions less than one.

Figure 5

Aggregated distributions of sensitivity depths (Eq. (11) for Cas A (upper) and Vela Jr. (lower) based on 84 and 71 samples, respectively, of 0.1-Hz search sub-bands spanning the full 20–976 search band. The widths of the distributions are dominated by the depth variation with respect to frequency, which we empirically fit to a linear function of negative slope.

Figure 6

Top panel: estimated gravitational wave strain amplitude sensitivities (95% efficiency) in each 0.1-Hz sub-band for the Cas A (red band) and Vela Jr. (cyan band) searches. Conservative uncertainty bands of $\pm 7\%$ are indicated, to account for statistical and systematic uncertainties in estimating sensitivity depths, including calibration uncertainties. Black triangles (upright—Cas A, inverted—Vela Jr.) denote 0.1-Hz bands for which rigorous upper limits are used to determine estimated sensitivity vs. frequency. Sensitivities are estimated for only subbands with no saturation of the candidate top-list (see Figs. 3 and 4). Sensitivities are based on the absence of any outlier exceeding the frequency-dependent threshold and surviving all stages of follow-up, using the sensitivity depths (see Fig. 5) estimated in sample bands and rescaled according to the run-average amplitude spectral noise density [H1 and L1 data combined, see Eq. (12)]. Additional results from prior searches for Cas A and Vela Jr. are also shown: O1 Einstein@Home 90% C.L. upper limits for Cas A (magenta curve) and for Vela Jr. (green curve) [8]; O3a Cas A and Vela Jr. 95% C.L. upper limits using a model-robust Viterbi method (orange curve) [11]; O3a Vela Jr. 95% C.L. upper limits using the Band-Sampled-Data directed Frequency Hough method (black curve) [11]. The solid red horizontal line indicates the age-based upper limit on Cas A strain amplitude. The dashed (dotted) horizonal blue lines indicate the optimistic (pessimistic) age-based upper limit on Vela Jr. strain amplitude, assuming an age and distance of 700 yr and 0.2 kpc (5100 yr and 1.0 kpc). Bottom panel: magnification of the sensitivity bands from this analysis over most of the search band ($\sim 40–976\mathrm{Hz}$), with $1\text{−}\sigma$ statistical uncertainties shown for the individual sparsely sampled upper limits used to estimate the depth.

Figure 7

Estimated equatorial ellipticity sensitivities (95% efficiency) in each 0.1-Hz subband for the Cas A (red) and Vela Jr. (blue, magenta) searches, derived from the strain amplitude sensitivities shown in Fig. 6 assuming a source distance of 3.3 kpc for Cas A, and assuming source distances of 1.0 kpc and 0.2 kpc for Vela Jr.

Figure 8

Estimated $r$-mode s amplitude $\alpha$ sensitivities (95% efficiency) in each 0.1-Hz subband for the Cas A (red) and Vela Jr. (blue, magenta) searches, derived from the strain amplitude sensitivities shown in Fig. 6 assuming a source distance of 3.3 kpc for Cas A, and assuming source distances of 1.0 kpc and 0.2 kpc for Vela Jr.