2025ApJ...983...35M

The joint gravitational wave (GW) and electromagnetic observations of the binary neutron star (BNS) merger GW170817 marked a giant leap in multimessenger astrophysics. The extensive observation campaign of the associated gamma-ray burst (GRB) and its afterglow has strengthened the hypothesis associating GRBs with BNS mergers and provided insights into mass ejection, particularly the relativistic outflow launched in BNS mergers. In this paper, we investigate the joint detection probabilities of BNS mergers by GW detectors and the upcoming ground-based very-high-energy (VHE) γ-ray instrument, the Cherenkov Telescope Array (CTA). Using an empirical relation that constrains the distance-inclination angle plane, we simulated BNS mergers detectable in the O5 run of the LIGO/Virgo/Kagra network with a 300 Mpc BNS horizon. Assuming Gaussian structured jets and ignoring the large sky localization challenges of GW detectors, we estimated probability of detection of the VHE afterglow by CTA. We have explored the afterglow parameter space to identify conditions favourable for detecting synchrotron self-Compton emission by CTA. Our study reveals that events viewed at angles ≲3 times the jet core angle are detectable by CTA when the initial bulk Lorentz factor at the jet axis ranges between 100 and 800. We find high kinetic energy (E_k > 10⁵⁰ erg), ambient density (n₀ > 10^–1 cm^–3), and energy content in nonthermal electrons significantly enhance the likelihood of CTA detection within 300 Mpc. The joint detection rate varies significantly with afterglow parameter distributions, ranging from 0.003 to 0.5 per year.