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A supra-massive neutron star (NS) spinning extremely rapidly could survivefrom a merger of NS-NS binary. The spin-down of this remnant NS that is highlymagnetized would power the isotropic merger ejecta to produce a brightmergernova emission in ultraviolet/optical bands. Before the mergernova, theearly interaction between the NS wind and the ejecta can drive a forward shockpropagating outwards into the ejecta. As a result, a remarkable amount of heatcan be accumulated behind the shock front and the final escaping of this heatcan produce a shock breakout emission. We describe the dynamics and thermalemission of this shock with a semi-analytical model. It is found that sharp andluminous breakout emission appears mainly in soft X-rays with a luminosity of$\sim10^{45}~\rm erg~s^{-1}$ at a few hours after the merger, by leading themergernova emission as a precursor. Therefore, detection of such an X-rayprecursor would provide a smoking-gun evidence for identifying NS-poweredmergernovae and distinguishing them from the radioactive-powered ones (i.e.,kilonovae or macronovae). The discovery of NS-powered mergernovae would finallyhelp to confirm the gravitational wave signals due to the mergers and theexistence of supra-massive NSs.

Shock breakout driven by the remnant of a neutron star binary merger: An X-ray precursor of mergernova emission