[Joan-Rene Merou, Sofia Bussieres, Alicia M. Sintes]

The fasttracks all sky search for continuous waves from isolated neutron stars (600 Hz to 800 Hz) produced, after applying the known lines and prominence vetoes, 29 frequency regions that still showed very high candidate counts. Since following up these candidates would be expensive, we cross-checked each region against the unvetted line lists for H1 and L1 on the full O4 run-averaged spectra that was used in the generation of the official lines lists and searched for relations among the peaks found there. The attached document contains the full investigation with the sets of frequencies where we had these statistical peaks.

The main result is the identification of a previously uncatalogued H1 comb: Unknown H1 lines at 719.56806, 749.53750 and 779.50764 Hz (Sets 6, 5 and 1 in the document) sit at a constant offset of +0.300 Hz above consecutive harmonics (n = 24, 25, 26) of the known  29.96952 Hz PSL enclosure video camera comb . Fitting the full H1 WIP list, 13 currently unidentified entries match f = n x 29.969509 + 0.300 Hz, at n = 1, 15, 19, 20, 21, 24, 25, 26, 28, 30, 32, 35, 37: 30.26972, 449.84292, 569.72097, 599.69056, 629.66000, 719.56806, 749.53750, 779.50764, 839.44667, 899.38569, 959.32403, 1049.23319 and 1109.17222 Hz. The same +0.3 Hz shift applied to the 100 Hz camera comb (99.99848 Hz spacing, 70.029 Hz offset) explains the unknown line at 770.31806 Hz (Set 3) as the n = 7 tooth plus 0.2997 Hz. Since the line list already contains a camera comb variant with a 0.60065 Hz offset (= 2 x 0.3003, annotated as possible mixing), this family appears to be intermodulation sidebands of the camera combs at multiples of ~0.3003 Hz. We recommend adding this comb to the O4 H1 line list.

We also found a family of H1 doublets split by 0.15Hz to 0.18 Hz among the unidentified lines in this band: 633.35583/633.52806, 646.78014/646.95000, 652.50000/652.66722, 669.30528/669.46833, 722.63333/722.78514, 759.75069/759.92778 and 779.34931/779.50764 Hz.

On the L1 side, four of the sets coincide with strong harmonics of the known 0.032777 Hz comb (665.83222 = 20314 x , 642.62528 = 19606 x , 637.18458 = 19440 x , 635.88194 = 19400 x  the fundamental. The comb is very dense, so this may not be enough evidence to classify these peaks as belonging to this comb.

The remaining regions correspond to broad, drifting or clustered noise bumps appearing in a single detector, which are not consistent with a CW signal. Only two regions (around 658.9 and 615.3 Hz) have no instrumental attribution and will be followed up.

Lines/bands that we intend on vetoing for the search:

H1: new cameracomb sideband comb (n x 29.969509 + 0.30021 Hz): 30.26972, 449.84292, 569.72097, 599.69056, 629.66000, 719.56806, 749.53750, 779.50764, 839.44667, 899.38569, 959.32403, 1049.23319, 1109.17222 Hz; plus the 100 Hz camera comb sidebands at 770.31806 and 970.315 Hz.

H1: ~0.17 Hz doublet family: 633.35583, 633.52806, 646.78014, 646.95000, 652.50000, 652.66722, 669.30528, 669.46833, 722.63333, 722.78514, 759.75069, 759.92778, 779.34931 Hz.

H1: broad unidentified peaks: 776.548 to 776.768, 758.22 to 759.94 (broad structure with multiple peaks), 700.798 to 700.834, 697.027 to 697.071, 671.401 to 671.419, 650.935 to 650.952, 640.718 to 640.729 Hz, and a widened contamination window up to ~670.28 Hz for the known camera to comb line at 670.0186 Hz.

L1: 0.032777 Hz comb harmonics: 665.83222, 642.62528, 637.18458, 635.88194 Hz.

L1: broad unidentified peaks/clusters: 660.75014, 657.24222, 655.48736 (+655.47847), 650.31833 (+650.3075, 650.31139), 649.6806 (within cluster 649.65 to 649.95), 648.51375, 648.77236, 646.34139, 645.85167, and the cluster at 759.216 to 759.221 Hz.