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tokens: constrained by the GW signal. We can then obtain that is . Consequently, the absolute charge Q NS is e.s.u. The dimensionless charge of an NS can be estimated as (Zhang 2019a )In order to satisfy the observational constraint, one requires . This implies that the neutron star has to be a millisecond magnetar before the merger. The condition to form such a magnetar in BNS mergers is contrived, so this scenario is disfavored. Similarly, for a charged BH–BH merger system we adopt the following two most straightforward assumptions: (1) the lighter BH has a mass of 2.8 M ⊙ , which is less than 3 M ⊙ and falls into the BH mass regime; (2) only the lighter BH carries a constant dimensionless charge (for the same absolute charge Q , a lighter BH carries a higher which is more relevant). The mass ratio q of this system should be different from the range constrained above assuming an NS–BH merger, but this ratio does not enter the problem in view of assumption (2) above. We constrain the black hole charge as and the corresponding absolute charge e.s.u. The demanded dimensionless charge is comparable to the one required to explain the putative γ -ray event (Connaughton et al. 2016 ) associated with the first BH–BH merger event (Zhang 2016 ). Contrived conditions are again needed for a BH to carry such a large charge. 4.2.2. cCBC with an Increasing Charge This scenario involves a plunging BH–NS system in which the BH is immersed in the magnetic field of the NS and gains charge via the Wald mechanism (Wald 1974 ) in an initial electro-vacuum approximation. Levin et al. ( 2018 ) suggested that the BH can be charged stably to carry the Wald’s charge quantity Q W until it could transit from the electro-vacuum state to the force-free state thanks to abundant pair production induced by the strong electric field. At this point, the BH may reach the maximal Wald charge. In this scenario, there are four possible pre-merger mechanisms (first and second magnetic dipole radiation, electric dipole radiation, and magnetic reconnection close to BH’s equatorial plane; Dai 2019 ) and two possible post-merger mechanisms (magnetic reconnection at polar regions and the BZ mechanism; Zhong et al. 2019 ) to generate γ -ray emission. Following Dai ( 2019 ) and Zhong et al. ( 2019 ), we calculate (Figure 12 ) that the subthreshold GRB could be produced by the pre-merger magnetic reconnection or the post-merger BZ mechanism if the NS surface magnetic field satisfies or , respectively, given the following conditions: the radiative efficiency η γ = 1, the mass ratio is q = 5, the minimal separation between the BH and the NS is a min = 2 GM BH / c 2 + r NS , the NS mass is M NS = 1.4 M ⊙ and its radius is r NS = 12 km. The following two points are worth mentioning in our calculation: (1) We consider that the pre-merger magnetic reconnection in Equation (19) of Dai ( 2019 ) should be the BH’s magnetic field produced by the Wald charge Q W rather than that of the NS. This is because the