Repeating fast radio bursts from neutron star binaries: Multiband and multimessenger opportunities

Recent observations indicate that magnetars may reside in merging compact binaries and at least part of fast radio bursts (FRBs) are sourced by magnetar activities. It is natural to speculate that a class of merging neutron star binaries may have FRB emitters. In this work, we study the observational aspects of these binaries-particularly those with FRB repeaters, which are promising multiband and multimessenger observation targets of radio telescopes and ground-based gravitational wave detectors as the former telescopes can probe the systems at a much earlier stage in the inspiral than the latter. We show that observations of FRB repeaters in compact binaries have a significant advantage in pinning down the binary spin dynamics, constraining neutron star equation of state, probing FRB production mechanisms, and testing beyond standard physics. As a proof of principle, we investigate several half-year long mock observations of FRB pulses originating from premerger neutron star binaries, and we find that using the information of FRB arriving times alone, the intrinsic parameters of this system (including the stellar masses, spins, and quadrupole moments) can be measured with high precision, and the angular dependence of the FRB emission pattern can also be well reconstructed. The measurement of stellar masses [with an error of O(10-6-10-5)] and quadrupole moments [with an error of O(1%-10%)] may be an unprecedented discriminator of nuclear equations of state in neutron stars. In addition, we find the multiband and multimessenger observations of this binary will be sensitive to alternative theories of gravity and beyond standard models, e.g., dynamical Chern-Simons gravity and axion field that is coupled to matter. For example, we find that the effect of gravitational parity violation can be probed much more accurately than existing observations/experiments by combining FRB observations with either gravitational observations or x-ray observations and applying universal relations for neutron stars that do not depend sensitively on the equations of state.