Numerical relativity simulations of the neutron star merger GW190425: microphysics and mass ratio effects

Abstract: GW190425 was the second gravitational wave (GW) signal compatible with a binary neutron star (BNS) merger detected by the Advanced LIGO and Advanced Virgo detectors. Since no electromagnetic counterpart was identified, whether the associated kilonova was too dim or the localisation area too broad is still an open question. We simulate 28 BNS mergers with the chirp mass of GW190425 and mass ratio $1 \leq q \leq 1.67$, using numerical-relativity simulations with finite temperature, composition dependent nuclear equation of state (EOS) and neutrino radiation. The energy emitted in GWs is $\lesssim 0.083 M_{\odot} c^2$ with peak luminosity of $1.1-2.4 \times 10^{58} {\rm erg~s^{-1}}/(1+q)^2$. Dynamical ejecta and disc mass range between $5 \times 10^{-6} - \sim 10^{-3}~M_{\odot}$ and $10^{-5} - 0.1~M_{\odot}$, respectively. Asymmetric mergers, especially with stiff EOS, unbind more matter and to form heavier discs compared to equal mass binaries. The angular momentum of the disc is $8-10 M_{\odot}~GM_{\rm disc}/c$ over three orders of magnitude in $M_{\rm disc}$. While the nucleosynthesis shows no peculiarity, the simulated kilonovae are relatively dim compared with the GW170817 event. For distances compatible with GW190425, AB magnitudes are always dimmer than $\sim20~{\rm mag}$ for the $B$, $r$ and $K$ bands, with brighter kilonovae associated to more asymmetric binaries and stiffer EOS. We suggest that, even assuming a good coverage of GW190425's sky location, the kilonova could hardly have been detected by present wide-field surveys and no firm constraints on the binary parameters or EOS can be argued from the lack of the detection.