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The upgraded Advanced LIGO and Advanced Virgo began their third observing run (O3) on April 1, 2019. This observation, however, was suspended on March 27, 2020 owing to the impact of the COVID-19 pandemic, leaving the follow-up arrangements undetermined. The LIGO Scientific Collaboration and Virgo Collaboration (LVC) has so far announced 56 gravitational wave candidates on the GraceDB website, coupled with two gravitational wave events analyzed in detail [1,2]. GW190425, the first gravitational wave event announced in O3, is likely the second observed gravitational wave event of a binary neutron star (BNS) inspiral, with its distance much farther than that of GW170817 [3]. GW190412, the second gravitational wave event announced in O3, actually happened earlier than GW190425 but took a longer time for data analysis due to its complexity. Compared with other GW candidates, GW190412 has several unique characteristics, including the large mass ratio, the higher-order mode clearly evidenced [4], and the strongest constraint on precession and primary black hole’s spin observed so far. In the first two observing runs (O1/O2), 10 binary black hole (BBH) gravitational wave events that are classified into the first Gravitational-Wave Transient Catalog (GWTC-1) by LVC share a common feature, that is, the black holes in the binary system have comparable masses [5]. But this trend was broken by the GW event of the binary black hole observed on April 12, 2019 at 05:30:44 UTC. This gravitational wave signal GW190412 was jointly detected by two Advanced LIGO detectors and one Advanced Virgo detector. The signal-to-noise ratio of the detector network is 19, and the false alarm rate (FAR) is less than 1 per 10×10 years by the GstLAL pipeline. Bayesian parameter estimation indicates that GW190412 was generated by the merger of two black holes respectively with 30 times solar mass and 8 times solar mass. According to the prediction of the general relativity, GW signals produced by a BBH with a high massratio are expected to have higher-order modes. This prediction has been confirmed by the observation of GW190412 (Figure 1). In the context of GW data analysis, the luminosity distance and inclination angle of the source binary are usually degenerated, so it is usually difficult to accurately measure both parameters. This degeneracy may be changed by the existence of higher-order modes [2], for instance in GW190412. When using the binary black hole gravitational wave template containing higher-order modes for parameter estimation, the posterior probability distribution obtained (the green line) is significantly better than the result without higher-order modes (the blue line), breaking the V-shaped distribution. More interestingly, the posterior distribution of these two parameters is further narrowed by precession (the orange line). Both the effective-one-body numerical relativity (EOBNR) templates and phenomenological model (Phenom)

GW190412: Gravitational wave from an unequal mass binary black hole with precession