  A long-standing paradigm in astrophysics is that collisions- or mergers- of
two neutron stars (NSs) form highly relativistic and collimated outflows (jets)
powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the
observational support for this model is only indirect. A hitherto outstanding
prediction is that gravitational wave (GW) events from such mergers should be
associated with GRBs, and that a majority of these GRBs should be off-axis,
that is, they should point away from the Earth. Here we report the discovery of
the X-ray counterpart associated with the GW event GW170817. While the
electromagnetic counterpart at optical and infrared frequencies is dominated by
the radioactive glow from freshly synthesized r-process material in the merger
ejecta, known as kilonova, observations at X-ray and, later, radio frequencies
exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray
emission at a location coincident with the kilonova transient provides the
missing observational link between short GRBs and GWs from NS mergers, and
gives independent confirmation of the collimated nature of the GRB emission.
