Search of the Early O3 LIGO Data for Continuous Gravitational Waves from the Cassiopeia A and Vela Jr. Supernova Remnants LIGO Scientific Collaboration and Virgo Collaboration (compiled November 29, 2021) 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 data from the first six months of the third Advanced LIGO and Virgo observing run using the Weave semi-coherent 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 ∼6.3 × 10 −26 for Cas A and ∼5.6 × 10 −26 for Vela Jr. at frequencies near 166 Hz at 95% efficiency. I. INTRODUCTION We report the results of the deepest search to date for continuous gravitational waves from the neutron stars at the centers of the Cassiopeia A (Cas A, G111.7−2.1) [1] and Vela Jr. (G266.2−1.2) [2] supernova remnants. Cas A is just over 300 years old [3, 4], and Vela Jr. may be as young as 700 years old [2]. These extremely young ob- jects have been the target of multiple searches for contin- uous gravitational waves since 2010 [5–11] because they may retain high rotation frequencies and may possess ap- preciable non-axisymmetries from their recent births [12– 19]. Continuous emission due to unstable r-modes is also possible in such young stars [20–24]. In this search, we analyze the first six months of data from the third observing run (O3a period) of the Ad- vanced Laser Interferometer Gravitational wave Obser- vatory (Advanced LIGO [25, 26]). We achieve signifi- cantly improved sensitivity for Vela Jr. with respect to a recent O3a search using a different method [11] and dramatically improved sensitivity for Cas A with respect to previous searches of O1, O2 and O3a LIGO and Virgo data [5–11]. The improvement with respect to similar, previous analyses of O1 data [8, 9] comes largely from the improved detector noise due to a variety of instru- ment upgrades [27], including a (∼3 db) improvement achieved with quantum squeezing [28]. Given the immense pressure on its nuclear matter, one expects a neutron star to assume a highly spherical shape in the limit of no rotation and, with rotation, to form an axisymmetric oblate spheroid. A number of physical pro- cesses can disrupt the symmetry, however, to produce quadrupolar gravitational waves from the stellar rota- tion. Those processes include crustal distortions from cooling or accretion, buried magnetic field energy and excitation of r-modes. Comprehensive reviews of contin- uous gravitational wave emission mechanisms from neu- tron stars can be found in [29, 30] Central compact objects (CCOs) at the cores of su- pernova remnants present interesting potential sources, especially those in remnants inferred from their sizes and expansion rates to be young. Both the Cas A and Vela Jr. remnants contain such objects, thought to be young neutron stars. One can derive an estimated age-based upper limit 1 on a CCO’s continuous-wave strain ampli- tude by assuming the star’s current rotation frequency is much lower than its rotation frequency at birth and that the star’s spin-down since birth has been dominated by gravitational wave energy loss (“gravitar” emission) [31]: h age = (2.3 × 10 −24 )  1 kpc r    1000 yr τ  I zz I 0  , (1) where r is the distance to the source, τ is its age and I zz is the star’s moment of inertia about its spin axis, with a fiducial value of I 0 = 10 38 kg· m 2 . Cas A is perhaps the most promising example of a potential gravitational wave CCO source in a supernova remnant. Its birth aftermath may have been observed by Flamsteed [3] ∼340 years ago in 1680, and the expansion of the visible shell is consistent with that date [4]. Hence Cas A, which is visible in X-rays [32, 33] but shows no pulsations [34], is almost certainly a very young neutron star at a distance of about 3.3 kpc [35, 36]. From equa- tion 1, one finds an age-based strain limit of ∼1.2×10 −24 , which is readily accessible to LIGO and Virgo detectors in their most sensitive band. The Vela Jr. CCO is observed in X-rays [37] and is potentially quite close (∼0.2 kpc) and young (690 yr) [2], for which one finds a quite high age-based strain limit of ∼1.4 × 10 −23 . Some prior continuous gravitational wave searches have also conservatively assumed a more pessimistic distance (∼1 kpc) and age (5100 yr), based on other measurements [38], for which the age-based strain limit is ∼1.0 × 10 −24 , still comparable to that of Cas A. 1 This strain estimate gives a rough benchmark upper limit on what is possible in an optimistic scenario; its assumption that current rotation frequency is small relative to the star’s birth frequency becomes less plausible for the highest frequencies searched in this analysis.