  Mergers of neutron star-neutron star (NS-NS) or neutron star-black hole
(NS-BH) binaries are candidate sources of gravitational waves (GWs). At least a
fraction of the merger remnant should be a stellar mass BH with a
sub-relativistic ejecta. A collimated jet is launched via Blandford-Znajek
mechanism from the central BH to trigger a short gamma-ray burst (SGRB). At the
same time, a near-isotropic wind can be driven by the Blandford-Payne mechanism
(BP). In previous work, additional energy injection to the ejecta from the BP
mechanism was ignored, and radioactive decay has long been thought as the main
source of energy for "kilonova" in the BH scenario. In this letter, we propose
that the wind driven by BP mechanism from the new-born BH-disk can heat up and
push the ejecta during the prompt emission phase or even at late time when
there is mass fall-back. Such a BP-powered merger-nova could be bright at
optical band even for a low-luminosity SGRB. The detection of a GW event with a
merger product of BH, and accompanied by a bright merger-nova, would be a
robust test of our model.
