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Magnetically powered outbursts from white dwarf mergers

Merger of a white dwarf binary creates a differentially rotating object which is expected to generate strong magnetic fields. Kinetic energy stored in differential rotation is partially dissipated in the magnetically dominated corona, which forms a hot variable outflow with ejection velocity comparable to $10^9$ cm s$^{-1}$. The outflow should carry significant mass and energy for hours to days, creating an expanding fireball with the following features. (i) The fireball is initially opaque and its internal energy is dominated by the trapped thermal radiation. The stored heat is partially converted to kinetic energy of the flow (through adiabatic cooling) and partially radiated away. (ii) Internal shocks develop in the fireball and increase its radiative output. (iii) A significant fraction of the emitted energy is in the optical band. As a result, a bright optical transient with luminosity $L\sim 10^{41}-10^{42}$ erg s$^{-1}$ and a characteristic peak duration comparable to 1 day may be expected from the merger. In contrast to classical novae or supernovae, the transient does not involve nuclear energy. The decay after its peak reflects the damping of differential rotation in the merger remnant. Such outbursts may be detected in the local Universe with current and upcoming optical surveys.