Magnetically driven baryon winds from binary neutron star merger remnants and the blue kilonova of August 2017

The observation of a radioactively-powered kilonova associated with the first binary neutron star (BNS) merger detected in gravitational waves proved that these events are ideal sites for the production of heavy r-process elements. However, the physical origin of the ejected material responsible for the early ("blue") and late ("red") components of this kilonova is still debated. Here, we investigate the possibility that the early/blue kilonova originated from the magnetically driven baryon wind launched after merger by the metastable neutron star remnant. Exploiting a magnetized BNS merger simulation with over 250 ms of post-merger evolution, we can follow for the first time the full mass ejection process up to its final decline. We find that the baryon wind carries $\simeq\!0.010\!-\!0.028\,M_\odot$ of unbound material, proving that the high mass estimated for the blue kilonova can be achieved. We also find expansion velocities of up to $\sim\!0.2\,c$, consistent with the lower end of the observational estimates, and we discuss possible effects neglected here that could further increase the final ejecta velocity. Overall, our results show that the magnetically driven baryon wind represents a viable channel to explain the blue kilonova.