  There is accumulating evidence that at least a fraction of binary neutron
star mergers result in rapidly spinning magnetars, with subrelativistic
neutron-rich ejecta as massive as a small fraction of solar mass. The ejecta
could be heated continuously by the Poynting flux emanated from the central
magnetars. Such Poynting flux could become lepton-dominated so that a reverse
shock develops. It was demonstrated that such a picture is capable of
accounting for the optical transient PTF11agg (Wang & Dai 2013b). In this paper
we investigate the X-ray and ultraviolet (UV) radiation as well as the optical
and radio radiation studied by Wang & Dai (2013b). UV emission is particularly
important because it has the right energy to ionize the hot ejecta at times
$t\lesssim 600$ s. It is thought that the ejecta of binary neutron star mergers
are a remarkably pure sample of r-process material, about which our
understanding is still incomplete. In this paper we evaluate the possibility of
observationally determining the bound-bound and bound-free opacities of the
r-process material by timing the X-ray, UV, and optical radiation. It is found
that these timings depend on the opacities weakly and therefore only loose
constraints on the opacities can be obtained.
