2018ApJ...856...90L

We study the first electromagnetic (EM) pulse after the gravitational-wave (GW) chirp signal, focusing on its profile and duration. It is found that the light curve, especially the steep decay (SD) phase, can be very different by adopting different viewing angles θ_view of the jet shell. For an on-axis jet with a power-law radiation spectrum, the observed flux in the SD is proportional to t_obs^-2-β^ with β being the spectral index and t_obs being the observer time. Here, t_obs = 0 is set at the time we observe the jet being ejected from the central engine. The SD may become steep by increasing θ_view. We also study the bolometric luminosity L from a jet shell with a non-power-law radiation spectrum. For an on-axis jet, L ∝ t_obs^–3 is found in the SD. However, the SD is steeper than L∝t_obs^–3 for radiation from an off-axis jet. The higher value of the θ_view is, the steeper SD would be. Then, we suggest that the SD phase can be used to discriminate an off-axis jet from an on-axis jet. The reason for the above behaviors is discussed. In addition, we find that the duration of first EM pulse is close to its peak time, especially for θ_view ∼ 20°. This result is consistent with that found in GW 170817/GRB 170817A. Thus, the jet corresponding to the prompt emission of GRB 170817A should be ejected immediately after the merger. Our results also reveal that the duration of the first EM pulse can provide information on the time to search for GWs.