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

Abstract

The LIGO-Virgo-KAGRA Collaboration recently detected gravitational waves (GWs) from the merger of black-hole-neutron-star (BHNS) binary systems GW200105 and GW200115. No coincident electromagnetic (EM) counterparts were detected. While the mass ratio and BH spin in both systems were not sufficient to tidally disrupt the NS outside of the BH event horizon, other, magnetospheric mechanisms for EM emission exist in this regime and depend sensitively on the NS magnetic field strength. Combining GW measurements with EM flux upper limits, we place upper limits on the NS surface magnetic field strength above which magnetospheric emission models would have generated an observable EM counterpart. We consider fireball models powered by the black-hole battery mechanism, where energy is output in gamma-rays over $\lesssim 1$~second. Consistency with no detection by Fermi-GBM or INTEGRAL SPI-ACS constrains the NS surface magnetic field to $\lesssim 10^{15}$~G. Hence, joint GW detection and EM upper limits rule out the theoretical possibility that the NSs in GW200105 and GW200115, and the putative NS in GW190814, retain $\gtrsim 10^{15}$~G dipolar magnetic fields until merger. They also rule out formation scenarios where strongly magnetized magnetars quickly merge with BHs. We alternatively rule out operation of the BH-battery powered fireball mechanism in these systems. This is the first multi-messenger constraint on NS magnetic fields in BHNS systems and a novel approach to probe fields at this point in NS evolution. This demonstrates the constraining power that multi-messenger analyses of BHNS mergers have on BHNS formation scenarios, the magnetic-field evolution in NSs, and the physics of BHNS magnetospheric interactions.

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Cited by in corpus (6)

• Long-duration Gamma-ray Burst and Associated Kilonova Emission from Fast-spinning Black Hole--Neutron Star Mergers
• Compact Binary Coalescences: Astrophysical Processes and Lessons Learned
• Population Properties of Gravitational-Wave Neutron Star--Black Hole Mergers
• A Channel to Form Fast-spinning Black Hole--Neutron Star Binary Mergers as Multi-messenger Sources
• Electromagnetic precursors to black hole - neutron star gravitational wave events: Flares and reconnection-powered fast-radio transients from the late inspiral
• Compact Objects in close orbits as Gravitational Wave Sources: Formation Scenarios and Properties