Probing fundamental physics with multiband and multimessenger observations of compact binaries with fast-radio-burst emitters

Recent observations indicate that magnetars, strong candidates for at least a part of fast radio bursts (FRBs), commonly reside in merging compact binaries. If an FRB-emitting neutron star is in a binary with another neutron star, one can probe the early part of the inspiral with radio observations while the late part of the same binary can be studied through gravitational-wave observations (thus multiband and multimessenger observations of binary neutron stars). Based on several mock observations of FRB pulsars, we find that physical quantities of neutron stars, like quadrupole moment, can be measured with an error of ~10% through radio observations. Combined with gravitational-wave observations, one can probe fundamental physics, including nuclear and gravitational physics. For example, the effect of gravitational parity violation can be probed much more accurately than existing observations/experiments through universal relations between the quadrupole moment and tidal deformability that do not depend sensitively on the equations of state.