Gravitational waves from coalescing neutron stars encode information about nuclear matter at extreme densities, inaccessible by laboratory experiments. The late inspiral is influenced by the presence of tides, which depend on the neutron star equation of state. Neutron star mergers are expected to often produce rapidly rotating remnant neutron stars that emit gravitational waves. These will provide clues to the extremely hot post-merger environment. This signature of nuclear matter in gravitational waves contains most information in the 2–4 kHz frequency band, which is outside of the most sensitive band of current detectors. We present the design concept and science case for a Neutron Star Extreme Matter Observatory (NEMO): a gravitational-wave interferometer optimised to study nuclear physics with merging neutron stars. The concept uses high-circulating laser power, quantum squeezing, and a detector topology specifically designed to achieve the high-frequency sensitivity necessary to probe nuclear matter using gravitational waves. Above 1 kHz, the proposed strain sensitivity is comparable to full third-generation detectors at a fraction of the cost. Such sensitivity changes expected event rates for detection of post-merger remnants from approximately one per few decades with two A+ detectors to a few per year and potentially allow for the first gravitational-wave observations of supernovae, isolated neutron stars, and other exotica.

On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the gravitational wave (GW) signal S190814bv with a false alarm rate of 1 in 10 25 years. The GW data indicated (with >99% probability) that the event had M 1 ≥ 5 and M 2 ≤ 3M , suggesting that it resulted from a neutron star-black hole (NSBH) merger (or potentially a low-mass binary black hole merger). Due to the low false alarm rate and the precise localization (23 deg 2 at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to an NSBH merger. To search for potential counterparts, our collaboration (GROWTH) performed real-time image subtractions on 6 nights of public Dark Energy Camera (DECam) i-and z-band images that were acquired in the three weeks following the merger. The images covered >98% of the integrated probability area. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of potential counterpart candidates discovered in the DECam data. Combining these data with a photometric redshift catalog that is >97% complete in the volume of interest, we ruled out each candidate as the counterpart to S190814bv. Here we present deep and uniform photometric limits on the optical emission associated with the event. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be M ej < 0.04 M at polar viewing angles, or M ej < 0.03 M if the opacity is κ < 2 cm 2 g −1 . Assuming a tidal deformability for the neutron star compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be χ < 0.7 for mass ratios Q < 6.