Electromagnetic Emission Post Spinning Black Hole Magnetized Neutron Star Mergers

Abstract

For a binary composed of a spinning black hole (BH) (with mass 7M ⊙ ) and a strongly magnetized neutron star (NS) (with surface magnetic field strength B S,NS 10 12 G and mass ∼ 1.4M ⊙ ), the NS as a whole will possibly eventually plunge into the BH. During the inspiral phase, the spinning BH could be charged to the Wald charge quantity Q W until merger in an electro-vacuum approximation. During the merger, if the spinning charged BH creates its own magnetosphere due to an electric field strong enough for pair cascades to spark, the charged BH would transit from electro-vacuum to force-free cases and could discharge in a time 1 ms. As the force-free magnetosphere is full of a highly conducting plasma, the magnetic flux over the NS's caps would be retained outside the BH's event horizon under the frozen-in condition. Based on this scenario, we here investigate three possible energy-dissipation mechanisms that could produce electromagnetic (EM) counterparts in a time interval of the BH's discharge post a BH-NS merger-induced gravitational wave event: (1) magnetic reconnection at the BH's poles would occur, leading to a millisecond bright EM signal, (2) a magnetic shock in the zone of closed magnetic field lines due to the detachment and reconnection of the entire BH magnetic field would probably produce a bright radio emission, e.g., a fast radio burst, and (3) the Blandford-Znajek mechanism would extract the BH's rotational energy, giving rise to a millisecond-duration luminous high-energy burst. We also calculate the luminosities due to these mechanisms as a function of BH's spin for different values of B S,NS .

Multi-Messenger Constraints on Magnetic Fields in Merging Black Hole-Neutron Star Binaries

“…They consider EM signatures at radio frequencies, but other manifestations of this energy transport are possible and require further study. A number of recent works have also begun to develop possible signatures due to charging and discharging of the BH via the Wald (1974) mechanism (Chen & Dai 2021;Levin et al 2018;Dai 2019;Zhang 2019;Pan & Yang 2019;Zhong et al 2019;Yang et al 2020b). Interestingly many of these mechanisms still have total power that scales with the square of the magnetic field strength, but expected energies of emission will likely vary.…”

Coalescence of black hole–neutron star binaries

“…Specifically, the tidal disruption is required to occur outside the innermost stable circular orbit of the black hole for inducing astrophysically interesting outcomes. If the neutron star is not disrupted, as is likely the case of GW200105 and GW200115, it behaves like a point particle throughout the coalescence, and the merger process will be indistinguishable from that of (highly asymmetric) binary black holes (Foucart et al 2013a) except for possible electromagnetic emission associated with crust shattering (Tsang et al 2012), magnetospheric activities (Hansen and Lyutikov 2001;McWilliams and Levin 2011;Lai 2012;Paschalidis et al 2013;D'Orazio et al 2016;Carrasco and Shibata 2020;Wada et al 2020;East et al 2021;Carrasco et al 2021, see also Ioka and Taniguchi 2000 for earlier work on binary neutron stars), or charged black holes (Levin et al 2018;Zhang 2019;Dai 2019;Pan and Yang 2019;Zhong et al 2019). These two possibilities for the fate of merger are summarized schematically in Fig.…”

Physical Implications of the Sub-threshold GRB GBM-190816 and its Associated Sub-threshold Gravitational Wave Event

“…For an NS-BH merger with a relatively large q (e.g., ∼ 5), the NS would plunge into the BH as a whole so alternative mechanisms (e.g., McWilliams & Levin 2011;Tsang et al 2012;D'Orazio et al 2016;Levin et al 2018;Zhang 2019a;Dai 2019;Pan & Yang 2019;Zhong et al 2019) have to be introduced to explain the observed GRB. One group of the mechanisms which recently receives increasing interest are the electric and magnetic dipole radiation, magnetic reconnection, and BZ mechanism of the charged objects in the binary system.…”

“…The charged compact binary coalescence (cCBC) models involve at least one member of the binary carries either a constant (Zhang 2016(Zhang , 2019a or increasing (Levin et al 2018;Dai 2019) charge. The high-energy EM emission can be produced either before (Zhang 2019a;Dai 2019) or after (Pan & Yang 2019;Zhong et al 2019) the merger.…”

“…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 BZ mechanism; Zhong et al 2019) to generate γ-ray emission. Following Dai (2019) and Zhong et al (2019), we calculate (Figure 11) that the sub-threshold GRB could be produced by the pre-merger magnetic reconnection or the post-merger BZ mechanism if the NS' surface magnetic field log(B S,NS /G) > 13.4 and log(B S,NS /G) ∼ 13.5 − 14.5, respectively, given the following conditions: the radiative efficiency η γ = 1, the mass ratio q = 5.5, the minimal separation between the BH and the NS a min = 2GM BH /c 2 + r NS , and the NS mass M NS = 1.4 M and its radius r NS = 12 km. The following two points 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.…”