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tokens: could be inaccurate if the system inclination is low or the pulsar’s companion is rapidly rotating (Freire et al. 2008 ). In sum, it is not clear if GW190814's secondary is a BH or an NS. GW190814 poses a challenge for our understanding of the population of merging compact binaries. In what follows, we estimate the merger rate density of the compact binary subpopulation represented by this source, investigate the nature of its secondary component and possible implications for the NS equation of state, discuss how the system may have formed, and study its implications for cosmology. 6.1. Merger Rate Density Given the unprecedented combination of component masses found in GW190814, we take the system to represent a new class of compact binary mergers, and use our analysis of its source properties to estimate a merger rate density for GW190814-like events. Following a method described in Kim et al. ( 2003 ), we calculate a simple, single-event rate density estimate according to our sensitivity to a population of systems drawn from the parameter-estimation posteriors. As in Abbott et al. ( 2020a ), we calculate our surveyed spacetime volume semianalytically, imposing single-detector and network S/N thresholds of 5 and 10, respectively (Tiwari 2018 ). The semianalytic for GW190814 is then multiplied by a calibration factor to match results from the search pipelines assuming a once-per-century FAR threshold. The sensitivity of a search pipeline is estimated using a set of simulated signals. For computational efficiency, this was done using preexisting search pipeline simulations and the mass properties were not highly optimized. However, given that we are estimating a rate based on a single source, the calibration errors are much smaller than the statistical errors associated with the estimate. The simulated sources were uniformly distributed in comoving volume, component masses, and component spins aligned with the orbital angular momentum. For O1 and O2, the simulated BH mass range was 5–100 M ⊙ , but for the first part of O3 we are analyzing here, the injected range was 2.5–40 M ⊙ (following our updated knowledge of the BH mass distribution); the NS mass range was 1–3 M ⊙ , and component spins are 0.95. As GW190814 occurred when LIGO Hanford was not in nominal observing mode, it is not included in the production P y CBC results, and we use G st LAL results to calculate the merger rate. We assume a Poisson likelihood over the astrophysical rate with a single count and we apply a Jeffreys prior to obtain rate posteriors. The analysis was done using samples from the Phenom PHM posterior and separately from the EOBNR PHM posterior, producing the same result in both cases. We find the merger rate density of GW190814-like systems to be . As a consistency check, we used the P y CBC search results to calculate an upper limit. Repeating the rate calculation with a P y CBC-based calibration and zero event count, we obtain an upper limit consistent (to