Early X-ray and optical afterglow of gravitational wave bursts from mergers of binary neutron stars

Double neutron star mergers are strong sources of gravitational waves. The upcoming advanced gravitational wave detectors are expected to make the first detection of gravitational wave bursts (GWBs) associated with these sources. Proposed electromagnetic counterparts of a GWB include a short GRB, an optical macronova, and a long-lasting radio afterglow. Here we suggest that at least some GWBs could be followed by an early afterglow lasting for thousands of seconds, if the post-merger product is a short-lived massive neutron star rather than a black hole. This afterglow is powered by dissipation of a proto-magnetar wind. The X-ray flux is estimated to be as bright as 10^{-8}-10^{-7} erg/s/cm^2. The optical flux is subject to large uncertainties but could be as bright as 17th magnitude in R-band. We provide observational hints of such a scenario, and discuss the challenge and strategy to detect these signals.