Probing binary neutron star mergers in dense environments using afterglow counterparts

Understand

The only binary neutron star merger gravitational wave event with detected electromagnetic counterparts recorded to date is GRB170817A.

• This merger occurred in a rarefied medium with a density smaller than $10^{-3}-10^{-2}~{\rm cm}^{-3}$.
• Since kicks are imparted to neutron star binaries upon formation, and due to their long delay times before merger, such low-density circum-merger media are generally expected.
• However, there is some indirect evidence for fast-merging or low-kick binaries, which would coalesce in denser environments.