Convective Excitation of Inertial Modes in Binary Neutron Star Mergers

Abstract

We present the first very long-term simulations (extending up to ∼140 ms after merger) of binary neutron star mergers with piecewise polytropic equations of state and in full general relativity. Our simulations reveal that, at a time of 30-50 ms after merger, parts of the star become convectively unstable, which triggers the excitation of inertial modes. The excited inertial modes are sustained up to several tens of milliseconds and are potentially observable by the planned third-generation gravitational-wave detectors at frequencies of a few kilohertz. Since inertial modes depend on the rotation rate of the star and they are triggered by a convective instability in the postmerger remnant, their detection in gravitational waves will provide a unique opportunity to probe the rotational and thermal state of the merger remnant. In addition, our findings have implications for the long-term evolution and stability of binary neutron star remnants.

Numerical-relativity simulations of long-lived remnants of binary neutron star mergers

“…The main focus of this work has been the analysis of the spectrum of the postmerger GW signal. Our investigation has been driven, in particular, by our aim to provide further evidence in support of our claim in [32] about the potential existence of new families of oscillation modes excited in longlived post-merger remnants (in addition to the main fluid quadrupole f -mode) at sufficiently late times of the evolution.…”

“…Notice that the saturation amplitude of the lowfrequency modes in these three periods is similar in order of magnitude. In the following sections (as in [32]) we interpret the low-frequency modes as inertial modes excited by convective instabilities in the remnant (in the model with the SLy EOS we thus observe three such episodes of inertial mode excitation, with somewhat different frequencies, but comparable saturation amplitude -whether it is the same or different inertial modes that are excited in each episode remains to be determined by a detailed mode analysis). At ∼110 ms it is not possible to recognize any mode clearly and at ∼123.6 ms a black hole is formed.…”

“…Recently we performed numerical simulations in full general relativity of BNS systems, extending the simu-lation time significantly longer than those previously reported in the literature [32]. These simulations allowed us to find new features in the GW spectrum of the remnant on longer timescales.…”

“…The panels in Figure 4 also show the design sensitivities of Advanced Virgo [95] and Advanced LIGO [96] as well as those of third-generation detectors Einstein Telescope [97] and Cosmic Explorer [98]. In [32] we discussed the prospects of detectability of the low-frequency modes excited at sufficiently long times after merger for models SLy and APR4. In the current work our simulations account for additional models with the H4 and MS1 EOS.…”

Magnetar formation through a convective dynamo in protoneutron stars

“…Last, our results are also relevant to the context of binary neutron star mergers. The cooling of a neutron star remnant (if any) will likely lead to similar convective motions (32). Because of the rapid rotation, we predict that neutron stars formed in mergers have largescale magnetic fields of magnetar strength generated by a strong field convective dynamo.…”