A kilonova associated with GRB 070809

GRB 070809 shows an unusual optical radiation component at t approximate to 0.47 d, at odds with a forward shock afterglow. If interpreted as a kilonova, the possibility arises of using short gamma-ray bursts to identify neutron star mergers beyond LIGO's reach. For on-axis typical short gamma-ray bursts (sGRBs), the forward shock emission is usually so bright(1,2) that it renders the identification of kilonovae (also known as macronovae)(3-6) in the early afterglow (t < 0.5 d) phase rather challenging. This is why previously no thermal-like kilonova component has been identified at such an early time(7-13) except in the off-axis dim GRB 170817A (refs. (14-19)) associated with GW170817 (ref. (20)). Here we report the identification of an unusual optical radiation component in GRB 070809 at t ~ 0.47 d, thanks plausibly to the very-weak/subdominant forward shock emission. The optical emission with a very red spectrum is well in excess of the extrapolation of the X-ray emission that is distinguished by an unusually hard spectrum, which is at odds with the forward shock afterglow prediction but can be naturally interpreted as a kilonova. Our finding supports the speculation that kilonovae are ubiquitous(11), and demonstrates the possibility of revealing the neutron star merger origin with the early afterglow data of some typical sGRBs that take place well beyond the sensitive radius of the advanced gravitational wave detectors(21,22) and hence the opportunity of organizing dedicated follow-up observations for events of interest.