Host galaxies and electromagnetic counterparts to binary neutron star
  mergers across the cosmic time: Detectability of GW170817-like events
  The detection of electromagnetic radiation (EM) accompanying the
gravitational wave (GW) signal from the binary neutron star (BNS) merger
GW170817 has revealed that these systems constitute at least a fraction of the
progenitors of short gamma-ray bursts (SGRBs). As gravitational wave detectors
keep pushing their detection horizons, it is important to assess coupled GW/EM
probabilities, and how to maximize observational prospects. Here we perform
population synthesis calculations of BNS evolution with the code MOBSE, and
seed the binaries in galaxies at three representative redshifts (z=0.01,0.1,1)
of the Illustris TNG50 simulation. The binaries are evolved and their locations
numerically tracked in the host galactic potentials until merger. Adopting the
astrophysical parameters of GRB170817A as a prototype, we numerically compute
the broadband lightcurves of jets from BNS mergers, with the afterglow
brightness depending on the local medium density at the merger sites. We
perform Monte Carlo simulations of the resulting EM population assuming either
a random viewing angle with respect to the jet, or a jet aligned with the
orbital angular momentum of the binary, which biases the viewing angle
probability for GW-triggered events. We find that ~70-80% of BNSs from z=0.01
should be detectable in gamma-rays. The afterglow detection probabilities of
GW-triggered BNS mergers vary between ~0.3-0.7%, with higher values for jets
aligned with the BNS angular momentum, and are comparable across the high and
low-energy bands, unlike gamma-ray-triggered events (cosmological SGRBs) which
are significantly brighter at higher energies. We further quantify
observational biases with respect to host galaxy masses.
