Extreme mass-ratio inspirals as probes of fundamental dipoles

Even if globally neutral, in various scenarios compact objects can have a nonvanishing dipole moment. Examples include neutron stars with magnetic dipoles, black-hole microstates in the string-theory fuzzball scenario, and classical black holes in modified theories of gravity with spin-induced scalarization or Lorentz-violating terms. A fundamental dipole moment would give rise to rich phenomenology, for example, to intrinsic precession and extra emission channels in binary systems. We show that extreme mass-ratio inspirals (EMRIs) detectable by future gravitational-wave interferometers allow us to study a fundamental dipole on the secondary object in a model-agnostic fashion. By developing a general model for a fundamental scalar dipole, we compute the extra flux associated with it. This effect is suppressed by the square of the mass ratio relative to the case of fundamental charges, making its detection with EMRIs very challenging for the typical dipole moments predicted in various models. On the other hand, for the same reason, the impact of an extra dipole for constraints on extra fundamental charges is likely negligible, making the latter constraints more robust.