  The recent observations of GW170817 and its electromagnetic (EM) counterparts
show that double neutron star mergers could lead to rich and bright EM
emissions. Recent numerical simulations suggest that neutron star and neutron
star/black hole (NS-NS/BH) mergers would leave behind a central remnant
surrounded by a mildly isotropic ejecta. The central remnant could launch a
collimated jet and when the jet propagating through the ejecta, a mildly
relativistic cocoon would be formed and the interaction between the cocoon and
the ambient medium would accelerate electrons via external shock in a wide
angle. So that the merger-nova photons (i.e., thermal emission from the ejecta)
would be scattered into higher frequency via inverse compton (IC) process when
they propagating through the cocoon shocked region. We find that the IC
scattered component peaks at X-ray band and it will reach its peak luminosity
in order of days (simultaneously with the merger-nova emission). With current
X-ray detectors, such a late X-ray component could be detected out to 200 Mpc,
depending on the merger remnant properties. It could serve as an important
electromagnetic counterpart of gravitational wave signals from NS-NS/BH
mergers. Nevertheless, simultaneous detection of such a late X-ray signal and
the merger-nova signal could shed light on the cocoon properties and the
concrete structure of the jet.
