A More Stringent Constraint on the Mass Ratio of Binary Neutron Star Merger GW170817

Abstract

Recently, the LIGO-Virgo Collaborations reported their first detection of gravitational-wave (GW) signals from the low-mass compact binary merger GW170817, which is most likely due to a double neutron star (NS) merger. With the GW signals only, the chirp mass of the binary is precisely constrained to 1.188-0.002+0.004 M<inf>⊙</inf>, but the mass ratio is loosely constrained in the range 0.4-1, so that a very rough estimation of the individual NS masses (1.36M<inf>⊙</inf> <M<inf>1</inf> < 2.26M<inf>⊙</inf> and 0.86M<inf>⊙</inf> < M<inf>2</inf> < 1.36M<inf>⊙</inf>) was obtained. Here, we propose that if one can constrain the dynamical ejecta mass through performing kilonova modeling of the optical/IR data, by utilizing an empirical relation between the dynamical ejecta mass and the mass ratio of NS binaries, one may place a more stringent constraint on the mass ratio of the system. For instance, considering that the red "kilonova" component is powered by the dynamical ejecta, we reach a tight constraint on the mass ratio in the range of 0.46-0.59. Alternatively, if the blue "kilonova" component is powered by the dynamical ejecta, the mass ratio would be constrained in the range of 0.53-0.67. Overall, such a multi-messenger approach could narrow down the mass ratio of GW170817 system to the range of 0.46-0.67, which gives a more precise estimation of the individual NS mass than pure GW signal analysis, i.e., 1.61 M<inf>⊙</inf> < M<inf>1</inf> < 2.11 M<inf>⊙</inf> and 0.90 M<inf>⊙</inf> < M<inf>2</inf> < 1.16 M<inf>⊙</inf>.