The Gravitational Wave Memory from Binary Neutron Star Mergers

TL;DR

We quantify the gravitational-wave memory from binary neutron star mergers by performing general relativistic magnetohydrodynamic simulations including neutrinos, across multiple equations of state and magnetic-field topologies. The study computes all memory components—GW null memory, EM null memory, neutrino null memory, and ejecta memory—and analyzes how magnetic fields, EOS, and mass affect the memory budget, showing EM and ejecta contributions can reach significant fractions of the total memory. Memory growth in BNS mergers is extended due to long postmerger emission, unlike binary black hole mergers, and the EM field can modify the GW null memory in ways that sometimes reduce the total memory compared to nonmagnetized cases. Detectability with next-generation detectors like ET and CE is discussed, indicating possible SNR up to ~5 at tens of Mpc, with degeneracies between EOS and magnetic-field effects relevant for parameter estimation. These results demonstrate that magnetic-field physics and the NS EOS must be incorporated in memory analyses to avoid biased inferences from future GW memory observations.

Abstract

The gravitational wave signal produced by the merger of two compact objects includes both an oscillatory transient and a non-oscillatory part, the so-called memory effect. This produces a permanent displacement of test masses and has not yet been measured. We use general relativistic magnetohydrodynamic simulations, including neutrinos, with several representative viable equations of state, to quantify--for the first time--the effects of the neutron star magnetic field, neutrino emission, and the ejected mass on the linear and nonlinear displacement memory in binary neutron star mergers. We find that the additional contributions due to the emission of electromagnetic radiation, neutrinos and baryonic ejecta can be ~15% of the total memory for moderate magnetic fields and up to ~50% for extreme magnetic fields. The memory is most affected by changes in the equation of state, the binary mass, and the magnetic field. In particular, for moderate premerger field strengths, the dominant impact of the electromagnetic field is the change in the gravitational wave luminosity, and the associated gravitational wave null memory, due to the unstable growth of the magnetic field and the resulting redistribution of angular momentum it induces in the remnant. While the direct electromagnetic contribution to the null memory is additive, the change in the gravitational wave null memory can--in some cases--result in the total memory being smaller than that from the corresponding nonmagnetized binary. Furthermore, in contrast to binary black hole mergers, the growth of the memory in binary neutron star mergers is extended due to the long emission timescale of electromagnetic fields, neutrinos, and ejecta. These results necessitate the consideration of the magnetic field, as well as the equation of state, for accurate parameter estimation in future analyses of gravitational wave memory data.

Figures (5)

• Figure 1: The $h_+$ strain polarization observed in the equatorial plane ($\theta = 90^{\circ}$) showing the oscillatory wave (red), the total displacement memory (black dashed), and the combined signal (blue) for a BNS simulation with total gravitational mass $M=2.70M_\odot$, mass ratio $q=1$, ALF2 equation of state, and a pulsar-like magnetic field with $\vert B \vert_{\mathrm{max,ins}} = 1.4 \times 10^{16}$G (see main text for details). The memory includes the GW, EM, and ejecta contributions, with the blue rectangle showing the time over which 90% of the postmerger memory is accumulated.
• Figure 2: The dominant $l,m=2,0$ mode of the GW memory signal for the first set of simulations conducted with no neutrinos and with mass ratio $q=1$. For comparison, in the central panel we also show the memory from an equal mass BBH with the same ADM mass (gray line). The top two panels also show zoomed-in views of the last few ms for the smaller magnetic field lines.
• Figure 3: The dominant $l,m=2,0$ mode of the GW memory signal for the second set of simulations, which include interior-only EM fields and neutrino radiation. Note that all of these cases produce supramassive NSs Cook92b. Curves are labeled as in Fig. \ref{['fig:h_mem_20_mode']}.
• Figure 4: Characteristic strain curves for the oscillatory (dashed) and total memory (solid) parts of the signal for selected binaries with $M=2.57M_{\odot}$ and $q=1$ at a luminosity distance of $20$ Mpc (c.f., top two panels in Fig. \ref{['fig:h_mem_20_mode']}). We show zero magnetic field cases for the ALF2 and SLy EOS, and two nonzero (initially pulsar-like) magnetic field cases for ALF2.
• Figure 5: Magnitude of the time derivative of the $l=m=2$ mode of the complex strain for three different initial magnetic field strengths and pulsar-like initial magnetic field topology.