tokens: an sGRB that can be observed along its axis. This successful jet with its cocoon, which is sometimes called the “structured jet,” is scenario E in Figure 2 of Mooley et al. ( 2018b ), and it is similar to the one considered by Lazzati et al. ( 2018 ) and Margutti et al. ( 2018 ). In the second simulation the jet is launched for a short duration and is choked before breaking out of the ejecta. In both cases the cocoon does not affect the UV/optical emission on a timescale of half a day, but we show that the shock breakout of the successful jet can potentially generate the γ -ray signal (Figure 4 ). In both simulations there are afterglow parameters (ISM density and microphysics) for which the observed radio and X-ray emission is produced as shown in Figure 3 . Figure 4. Shock breakout γ -ray light curve (top panel) and spectrum (bottom panel) from the successful jet simulation. The blue thin line is the spectrum during the peak, while the red thick line is the spectrum during the tail. 4. Was There a Relativistic Jet in GW170817? A relativistic jet is necessary to produce an sGRB. Therefore, the question whether a relativistic jet can successfully break out of a neutron star merger ejecta and in particular whether a jet broke out in this event is interesting. In GW170817 we consider the option of a successful jet as possible but less likely. First, it does not explain the early optical light. Second, and more importantly, to explain the observed radio emission, either the jet should be much weaker than the cocoon, so that its contribution is unimportant, or it should be so powerful that its contribution begins only at very late time, at least months after the merger. Without fine-tuning, the jet is not expected to have much less energy than the cocoon (Nakar Piran 2017 ), while a very powerful jet produces a very luminous sGRB, and these are known to be extremely rare (Nakar 2007 ; Wanderman Piran 2015 ). However, this is only circumstantial evidence. Unfortunately, a weak successful jet does not contribute to the observed emission, and therefore we may never know whether it existed or not. A powerful jet, however, might be detectable. If it is powerful enough, it may be detectable while being still off-axis. In that case the radio light curve should rise sharply and then decline as a power law, after a short plateau. This unique prediction of the jet signature is the easiest to identify. A less powerful jet may contribute to the radio light curve, but its contribution may not be easily identified (see, e.g., Figure 3 and Lazzati et al. 2018 ; Margutti et al. 2018 ), since a continuous rise to a peak followed by a decay can arise either from a successful or from a choked jet (see Figure 3 and Nakar Piran 2018 ). Additional information, especially in the radio near the time that the afterglow flux peaks, may provide the key to resolve this question. First, the radio image of the source is expected to be resolved soon. Figure 5 shows the images
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