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tokens: is γβ ≳ 1.3, i.e., β ≳ 0.8. So, this means that the fastest ejecta has a velocity v ≳ 0.8 c . For lower-density medium, the fastest velocity should be even higher. On the other hand, Figure 4 shows that the lower limit to the shock energy at t = 156.4 days is E sh ≳ 2 × 10 50 erg assuming B ≲ 10 −2 , implying that the total ejecta kinetic energy is E ej ≳ 2 × 10 50 erg. This constraint is insensitive to B and/or E 1 . 4. Conclusion and Discussion We have used a simple spherical shock model to explain the nonthermal afterglow emission of GW 170817. We consider only the emission from the forward shock. The shock dynamics derived is available not only to ultrarelativistic and Newtonian shocks, but also to mildly relativistic shocks. The model parameters, except for those assuming e = 10 −1 , are constrained by multiband observations, and the main results are as follows: 1. The merger ejecta profile has k ≈ 6.7 and E 1 ≳ 4 × 10 49 erg for n 10 −1 cm −3 . 2. The fastest ejecta has γ max β max ≳ 1.3, i.e., β max ≳ 0.8, for n 10 −1 cm −3 . The slowest ejecta has γ min β min ≳ 0.7, i.e., β min ≳ 0.6, for n 10 −1 cm −3 . 3. The total ejecta kinetic energy is E ej ≳ 2 × 10 50 erg, if B ≲ 10 −2 . Recently, there have been some works claiming that the rising light curve turns over around t turn ∼ 150 days (Alexander et al. 2018 ; Dobie et al. 2018 ; Nynka et al. 2018 ). This turnover implies that there is no longer any energy injection, and thus the light curve starts to decline. So, the lower panel (for t = 156 days) in Figure 4 shows us the constraint of the total kinetic energy of the ejecta. The H i observation of the host galaxy constrained n 0.04 cm −3 (Hallinan et al. 2017 ). However, since the location of GW 170817 appears to be within the effective radius of the host galaxy (Levan et al. 2017 ), the density of the medium at GW 170817's location may not be very low. So, a reasonable circumburst medium density for GW 170817 could be n ∼ 10 −2 cm −3 . Moreover, GRB afterglow modeling usually gives a postshock magnetic field with B ∼ 10 −5 –10 −3 (e.g., Lemoine et al. 2013 ; Barniol Duran 2014 ; Santana et al. 2014 ). Taking n ∼ 10 −2 cm −3 and B ∼ 10 −5 –10 −3 , we can obtain from the lower panel of Figure 4 that the total kinetic energy is E ej ∼ (0.3–5) × 10 51 erg and that the slowest ejecta has γ min β min ∼ 1–1.5, i.e., β min ∼ 0.7–0.8. For the same range of n and B values, the upper panel of Figure 4 implies that the fastest ejecta has a velocity γ max β max ≳ 1.8, i.e., γ max ≳ 2. The gamma-ray emission from GW 170817 may be produced by the fastest part of the ejecta. Its energy E fast ≈ E γ ≃6 × 10 46 erg (Abbott et al. 2017b ) is many orders of magnitude smaller, compared with the constrained kinetic energy of the ejecta. For n ∼ 10 −2 cm −3 and B ∼ 10 −5 –10 −3 , the lower panel of Figure 4 also shows that E 1 ∼ 10 50.5 –10 52.5 erg, with which we can estimate the velocity of the fastest ejecta, γ max β max = ( E 1 / E γ ) 1/ k ∼