Detectability of neutron star merger afterglows

VLBI and JVLA observations revealed that GW170817 involved a narrow jet (θj ≈ 4°) that dominated the afterglow peak at our viewing angle, θobs ≈ 20°. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at θobs ≫ θj, and it can be modelled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, θobs ≲ 20°, and only 20 per cent of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $70\,\rm{per\,cent}$ of the GW detections are expected to be followed by a detectable afterglow, typically at θobs ∼ 30°. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.