  The radio and X-rays that followed GW170817 increased gradually over
$\sim~100$ days, resembling the radio flare predicted to arise from the
interaction of a binary neutron star merger outflow with the ISM
\citep{nakar2011}. Considering a blast wave moving with a Lorentz factor
$\Gamma$, we show that an off-axis observer, namely an observer at
$\theta_{obs}>1/\Gamma$, sees a light curve rising faster than $F_\nu\propto
t^3$. Therefore, the observed rise, $F_\nu\propto~t^{0.78}$, implies that at
all times we have seen an on-axis emission. Namely, the emitting matter was
within $\theta_{obs}<1/\Gamma$ at the time of observations (even if it was
off-axis beforehand). The observations tightly constrain the blast wave Lorentz
factor: $\Gamma\sim(1.5-7)(t/10~{\rm day})^{-0.21}$.The isotropic equivalent
energy in the observed region increases with time,
$E_{iso}\sim~10^{50}{\rm~erg} ~(t/150~{\rm day})^{1.3}$, implying that the
outflow was structured. The energy increase can arise from a slower material
moving behind the blast wave or from a matter moving at larger angles that has
slowed down. Such a structure could have different origins, however, the only
physically motivated one, proposed so far, is the interaction of a relativistic
jet with the ejecta and the resulting cocoon. The jet could have been choked or
successful. In the latter case, it has produced a short GRB pointing elsewhere
(this successful jet-cocoon system is sometimes called a "structured jet").
Although circumstantial evidence disfavors a successful jet, the jet fate
cannot be decisively determined from current observations. The light curve
alone may not be sufficient to resolve this question, since both chocked and
successful jets can lead to a gradual rise to a peak, followed by a decay.
Therefore, the recent turnover of the light curve does not necessitate a
successful jet.
