Constraining common envelope evolution in binary neutron star formation with combined galactic and gravitational-wave observations

ABSTRACT Binary neutron stars (BNSs) are among the most interesting sources for multimessenger studies. A number of recently discovered BNSs in the Milky Way by radio telescopes have added new information to the parameter distribution of the Galactic BNSs. The scarcity of BNS mergers during the O4 run of the LIGO–Virgo–Kagra suggests a BNS local merger rate six times lower than the previous constraint obtained by O1–O3 runs. With these new multimessenger observations, in this letter, we adopt the compact binary population synthesis model and Bayesian analysis to constrain the formation and evolution of BNSs, especially the common envelope (CE) evolution. We find that the following are required: (1) a fraction ($f_{\rm HG}\sim 0.8$) but not all of the Hertzsprung gap donors merged with their companions in the CE stage, in order to simultaneously explain the low BNS merger rate density and the existence of the short-orbital-period ($\lesssim 1$ d) Galactic BNSs, different from either all ($f_{\rm HG}=1$) or none ($f_{\rm HG}=0$) adopted in previous studies; (2) a large CE ejection efficiency $\alpha$ ($\sim 5$), in order to explain the existence of the long-orbital-period ($\gtrsim 10$ d) Galactic BNSs.