Gravitational Waves from Accretion-Induced Descalarization in Massive Scalar-Tensor Theory

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 overaccrete beyond the critical mass limit, thus triggering a gravitational phase transition where the star sheds its scalar hair and migrates over to its nonscalarized counterpart. Such an event resembles, but 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 quasicontinuous signal that persists for decades, carrying strains of order $\gtrsim {10}^{-22}(\mathrm{kpc}/L{)}^{3/2}{\mathrm{Hz}}^{-1/2}$ at frequencies of $\lesssim 300\mathrm{Hz}$, detectable with the existing interferometer network out to distances of $L\lesssim 10\mathrm{kpc}$, 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.

Figure 1

Evolutionary track of an APR4, near-critical scalarized star under accretion: gravitational mass, $M$, as a function of central energy density, ${\epsilon }_c$ (top panel), together with the time evolution of ${\epsilon }_c$ itself (bottom panel). The blue branch represents weakly scalarized stars, while the red branch is strongly scalarized. The green and blue stars mark the onset and the termination of descalarization, respectively, while the black star marks the initial state of the accretion simulation described in the main text.

Figure 2

Effective, root PSD $\sqrt{S_f}$ [Eq. (3)] at $L=10\mathrm{kpc}$ and $T=2$ months for the strain of the scalar-induced GW mode as a function of retarded time from 1 to ${10}^3\mathrm{years}$ (black curve; the $k$th dot from the right along the curve represents ${10}^{(k-1)}\mathrm{years}$). Overlaid are the sensitivity curves of existing and upcoming GW interferometers.