Gravitational waves from accretion-induced descalarization in massive scalar-tensor theory

Abstract

Many classes of extended scalar-tensor theories predict that dynamical instabilities can take place at high energies, leading to the formation of scalarized neutron stars. Depending on the theory parameters, stars in a scalarized state can form a solution-space branch that shares a lot of similarities with the so-called mass twins in general relativity appearing for equations of state containing first-order phase transitions. Members of this scalarized branch have a lower maximum mass and central energy density compared to Einstein ones. In such cases, a scalarized star could potentially over-accrete beyond the critical mass limit, thus triggering a gravitational phase transition where the star sheds its scalar hair and migrates over to its non-scalarized counterpart. Such an event resembles, though is distinct from, a nuclear or thermodynamic phase transition. We dynamically track a gravitational transition by first constructing hydrostatic, scalarized equilibria for realistic equations of state, and then allowing additional material to fall onto the stellar surface. The resulting bursts of monopolar radiation are dispersively-stretched to form a quasi-continuous signal that persists for decades, carrying strains of order $\gtrsim10^{-22} (\text{kpc}/L)^{3/2} \text{ Hz}^{-1/2}$ at frequencies of $\lesssim300 \text{ Hz}$, detectable with the existing interferometer network out to distances of $L\lesssim10 \text{ kpc}$, and out to a few hundred kpc with the inclusion of the Einstein Telescope. Electromagnetic signatures of such events, involving gamma-ray and neutrino bursts, are also considered.