  Recent long-term radio follow-up observations of GW 170817 reveals a simple
power-law rising light curve, with a slope of $t^{0.78}$, up to 93 days after
the merger. The latest X-ray detection at 109 days is also consistent with such
a temporal slope. Such a shallow rise behavior requires a mildly relativistic
outflow with a steep velocity gradient profile, so that slower material with
larger energy catches up with the decelerating ejecta and re-energizes it. It
has been suggested that this mildly relativistic outflow may represent a cocoon
of material. We suggest that the velocity gradient profile may form during the
stage that the cocoon is breaking out of the merger ejecta, resulted from shock
propagation down a density gradient. The cooling of the hot relativistic cocoon
material immediately after it breaks out should have produced soft X-ray to UV
radiation at tens of seconds to hours after the merger. The soft X-ray emission
has a luminosity of $L_{\rm X}\sim 10^{45}{\rm erg s^{-1}}$ over a period of
tens of seconds for a merger event like GW 170817. The UV emission shows a rise
initially and peaks at about a few hours with a luminosity of $L_{\rm UV}\sim
10^{42} {\rm erg s^{-1}}$. The soft X-ray transients could be detected by
future wide-angle X-ray detectors, such as the Chinese mission Einstein Probe.
This soft X-ray/UV emission would serve as one of the earliest electromagnetic
counterparts of gravitation waves from double neutron star mergers and could
provide the earliest localization of the sources.
