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 ≤ q ≤ 1.67, using numerical-relativity simulations with finite-temperature, composition dependent equation of state (EOS) and neutrino radiation. The energy emitted in GWs is ≲ 0.083 M⊙c2 with peak luminosity of 1.1 - 2.4 × 1058/(1 + q)2 erg s−1. Dynamical ejecta and disc mass range between 5 × 10−6 - 10−3 and 10−5 - 0.1 M⊙, respectively. Asymmetric mergers, especially with stiff EOSs, unbind more matter and form heavier discs compared to equal mass binaries. The angular momentum of the disc is 8 - 10 M⊙ GMdisc/c over three orders of magnitude in Mdisc. While the nucleosynthesis shows no peculiarity, the simulated kilonovae are relatively dim compared with GW170817. For distances compatible with GW190425, AB magnitudes are always dimmer than ∼20 mag for the B, r and K bands, with brighter kilonovae associated to more asymmetric binaries and stiffer EOSs. 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.

Monte-Carlo Neutrino Transport in Neutron Star Merger Simulations

“…GW190425 has a higher total mass (3.4M ). There was no observed electromagnetic counterpart to that signal, a relatively unsurprising result considering the large uncertainty in the location of the source and the high likelihood that such a system did not eject a significant amount of matter [57][58][59][60]. At least two NSBH mergers were observed in 2020 [61], with more candidates also available in the latest gravitational wave catalogue [3].…”

“…All of the above features are found in simulations using leakage, moments, or Monte-Carlo transport, although disagreements between methods can be found on the exact neutrino luminosity and composition of the remnant (see below). Simulations that include reabsorption of neutrinos in the matter outflows also find that hot outflows originating from the colliding cores of two neutron stars or the hot corona of an accretion disk rapidly evolve to electron fractions Y e ∼ (0.2 − 0.4) [60,88,95,104,110,117,126], though the exact value of Y e can vary significantly depending on the chosen numerical algorithm (see below). Faster and colder outflows associated with the tidal disruption of a low-mass neutron star by a more massive companion (in either NSBH or BNS mergers) do not capture enough neutrinos to undergo sigificant changes of composition, and thus remain very neutron-rich (Y e 0.05), even in simulations that include neutrino absorption [25,60,88,126].…”

Second release of the CoRe database of binary neutron star merger waveforms

“…We presented a new set of BNS simulations for the second release of the CoRe database, expanding it to 254 different binary configurations covering a wide parameter space. The new data includes BNS consistent with the GW events GW170817 [57] and GW190425 [64,66]. Simulations were performed with a large number of EOSs, including several microphysical models [57,61,64].…”

“…The new data includes BNS consistent with the GW events GW170817 [57] and GW190425 [64,66]. Simulations were performed with a large number of EOSs, including several microphysical models [57,61,64]. Some simulations include the effects of neutrinos, either through the leakage scheme [104,127,190], or using the M0 approach [52,104].…”

“…The new data were computed in simulations presented in Refs. [52][53][54][55][56][57][58][59][60][61][62][63][64] and include BNS waveforms consistent with the GW events GW170817 [54,56,57,65] and GW190425 [64,66].…”