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The coalescence of binary neutron stars (NSs) may in some cases produce astable massive NS remnant rather than a black hole. Due to the substantialangular momentum from the binary, such a remnant is born rapidly rotating andlikely acquires a strong magnetic field (a `millisecond magnetar'). Magneticspin-down deposits a large fraction of the rotational energy from the magnetarbehind the small quantity of mass ejected during the merger. This has thepotential for creating a bright transient that could be useful for determiningwhether a NS or black hole was formed in the merger. We investigate theexpected signature of such an event, including for the first time the importantimpact of electron/positron pairs injected by the millisecond magnetar into thesurrounding nebula. These pairs cool via synchrotron and inverse Comptonemission, producing a pair cascade and hard X-ray spectrum. A fraction of theseX-rays are absorbed by the ejecta walls and re-emitted as thermal radiation,leading to an optical/UV transient peaking at a luminosity of ~1e43-1e44 erg/son a timescale of several hours to days. This is dimmer than predicted bysimpler analytic models because the large optical depth of electron/positronpairs across the nebula suppresses the efficiency with which the magnetar spindown luminosity is thermalized. Nevertheless, the optical/UV emission is morethan two orders of magnitude brighter than a radioactively powered `kilonova.'In some cases nebular X-rays are sufficiently luminous to re-ionize the ejecta,in which case non-thermal X-rays escape the ejecta unattenuated with a similarpeak luminosity and timescale as the optical radiation. We discuss theimplications of our results for the temporally extended X-ray emission that isobserved to follow some short gamma-ray bursts (GRBs), including the kilonovacandidates GRB 080503 and GRB 130603B.

Optical and X-ray emission from stable millisecond magnetars formed from the merger of binary neutron stars