**5. Gw170817 and GRB170817A: An Observational Test-Bed for Off-Axis Structured Jet Theory**

As noted in the introduction, the discovery of GW170817 and its electromagnetic counterparts [25,26] marks a discontinuity in the evolution of the interest of the astrophysical community in structured jets. The reason is that a structured jet observed off-axis provides the most satisfactory and self-consistent explanation for the behavior of the associated short gamma-ray burst GRB170817A [55,56] and of the non-thermal emission observed at the source position starting on the second week after the gravitational wave event in radio [63,66,67,275–280], X-rays [58,60,281–286] and optical around the peak (with the Hubble Space Telescope [287,288] and with the Large Binocular Telescope [67]). Striking evidence in favor of such a scenario came from the latter component: its initial light curve evolution, with an unprecedented shallow increase in flux as ∼ *t* 0.8 over three months (see Figure 11), sparked a debate within the community about its interpretation. The two main

competing scenarios attributed the emission to a mildly relativistic shock propagating into the interstellar medium. In one scenario, the shock was produced by an off-axis structured jet, e.g., [58,62] that successfully broke out of the merger ejecta. In the other, it was due to a quasi-spherical outflow with a velocity profile, with most energy in the slower ejecta (models of the radio surface brightness in the two scenarios are shown in Figure 12).

**Figure 11.** Observations and model of the afterglow of GRB170817A. Colored circles with error bars show flux densities measured in radio, X-ray and optical observations (data from [289]) at the position of GW170817, rescaled to a common frequency of 3 GHz assuming a power-law spectrum *F<sup>ν</sup>* ∝ *ν* <sup>−</sup>0.584 [60]. The data points are color-coded according to the color bar on the right in order to show the original observing frequency. The red solid line is the prediction of an off-axis structured jet model with a power-law profile of both the energy and the Lorentz factor, with the same form and similar parameters as that in [67], namely *E*(*θ*) = *E*(0)/(1 + (*θ*/*θ*c) *<sup>s</sup><sup>E</sup>* ) and <sup>Γ</sup>(*θ*) = <sup>1</sup> + (Γ(0) <sup>−</sup> 1)/(1 + (*θ*/*θ*c) *<sup>s</sup>*<sup>Γ</sup> ) with *θ*<sup>c</sup> = 3.2◦ , *<sup>E</sup>*(0) = <sup>4</sup> <sup>×</sup> <sup>10</sup><sup>52</sup> erg, *<sup>s</sup><sup>E</sup>* <sup>=</sup> 4.5, <sup>Γ</sup>(0) = 1000, *<sup>s</sup>*<sup>Γ</sup> <sup>=</sup> 3.3, *<sup>n</sup>* <sup>=</sup> <sup>10</sup>−<sup>3</sup> cm−<sup>3</sup> , *e*<sup>e</sup> = 0.1, *e<sup>B</sup>* = 10−<sup>4</sup> , *p* = 2.168 and *θ*v = 19◦ .

The latter outflow could have been either the result of the jet being present, but 'choked' [279,290] (i.e., the central engine turned off before the head was able to break out), or could have arisen from the rapid conversion of magnetic energy into kinetic energy soon after the merger, e.g., [291,292]. Unfortunately, Nakar and Piran [239] showed that it was impossible to tell apart the two scenarios solely from the light curve evolution before the peak, because a shallow power-law increase in the radio and X-ray flux density could be produced by ejecta with an appropriately chosen angular profile, or velocity profile (or an infinite family of combinations of the two), and in neither case the required parameters were unrealistic. The solution to the riddle was eventually provided by high-resolution VLBI observations [66,67] at 75, 207 and 230 days after the merger, which revealed an apparently superluminal motion of the radio source centroid and a very small projected size of the image [67] see Figure 13. Only the off-axis structured jet scenario has been demonstrated to provide a complete, self-consistent explanation of the light curves and centroid motion to date.

The off-axis viewing angle is most likely in the 15–25◦ range, see [66,293], which is in good agreement with the binary orbital plane inclination derived from the gravitational wave analysis [25], has been also identified as the culprit of the extremely low luminosity of the GRB170817A prompt emission approximately *<sup>L</sup>*iso <sup>∼</sup> <sup>10</sup><sup>47</sup> erg/s, see [26,55,56] when compared to the other known short GRBs with a redshift measurement, since the latter is instead observed within ∼2*θ*<sup>c</sup> [198,205]. However, the simple interpretation of GRB170817A as being regular short GRBs with a suppressed flux due to relativistic beaming e.g., [26,294] is not viable. Compactness limits [204,205] indicate that the GRB170817A emission was produced by material moving at a Lorentz factor Γ & few, but seen under a viewing angle *θ*<sup>e</sup> . 5 ◦ . Given the viewing angle *θ*<sup>v</sup> & 15◦ and the opening angle *θ*<sup>c</sup> . 5 ◦ , e.g., [293], this is not compatible with emission originating at the border of the jet core, for which *θ*<sup>e</sup> = (*θ*<sup>v</sup> − *θ*c) > 5 ◦ . The mechanism that produced the observed emission could still have been a similar one as that behind the known short GRB population, but operating well outside the jet core, e.g., [197], or a different mechanism, such as the cocoon shock breakout, e.g., [79,239,290] see Section 2.5.

**Figure 12.** Model images of an off-axis structured jet (A) and of three quasi-spherical explosions (B, C and D) with similar parameters, but differing opening angles *θ*c. The angle between the line of sight and the shock symmetry axis in all cases is *θ*v = 15◦ . The images show the surface brightness at 5 GHz, 207 days after the merger, as seen from a distance of 41 Mpc. In each panel, the merger is located at coordinates (0, 0), while the grey cross shows the image centroid and the full width at half the maximum of the image in two perpendicular directions. All models are compatible with the observations of the GW170817 non-thermal multi-wavelength counterpart (shown in Figure 11) before the peak. Adapted from [67].

**Figure 13.** Global VLBI image of the GRB170817A afterglow 207 days after the merger. The main panel (**A**) shows the cleaned surface brightness map (color-coded according to the color bar on the right) of a small region around the position of GW170817, observed 207 days after the merger. Red contours are lines of constant surface brightness corresponding to −20 (dashed), 20 and 40 (solid) µJy/beam. The root-mean-square of the image noise is 8 µJy/beam. The ellipse in the lower left corner of the plot encompasses the full width at half the maximum of the synthesized beam (i.e., the resolution element). The inset, panel (**B**), shows a zoom of the region close to the peak of the surface brightness distribution, with black error bars marking the best fit and one-sigma errors on the centroid position of the source at 75 and 230 days, as measured by [66], with the axes showing the displacement with respect to the position at 75 d. Reproduced from [67].
