  We report deep Chandra, HST and VLA observations of the binary neutron star
event GW170817 at $t<160$ d after merger. These observations show that GW170817
has been steadily brightening with time and might have now reached its peak,
and constrain the emission process as non-thermal synchrotron emission where
the cooling frequency $\nu_c$ is above the X-ray band and the synchrotron
frequency $\nu_m$ is below the radio band. The very simple power-law spectrum
extending for eight orders of magnitude in frequency enables the most precise
measurement of the index $p$ of the distribution of non-thermal relativistic
electrons $N(\gamma)\propto \gamma^{-p}$ accelerated by a shock launched by a
NS-NS merger to date. We find $p=2.17\pm0.01$, which indicates that radiation
from ejecta with $\Gamma\sim3-10$ dominates the observed emission. While
constraining the nature of the emission process, these observations do
\emph{not} constrain the nature of the relativistic ejecta. We employ
simulations of explosive outflows launched in NS ejecta clouds to show that the
spectral and temporal evolution of the non-thermal emission from GW170817 is
consistent with both emission from radially stratified quasi-spherical ejecta
traveling at mildly relativistic speeds, \emph{and} emission from off-axis
collimated ejecta characterized by a narrow cone of ultra-relativistic material
with slower wings extending to larger angles. In the latter scenario, GW170817
harbored a normal SGRB directed away from our line of sight. Observations at
$t\le 200$ days are unlikely to settle the debate as in both scenarios the
observed emission is effectively dominated by radiation from mildly
relativistic material.
