Magnetically Driven Baryon Winds from Binary Neutron Star Merger Remnants and the Blue Kilonova of 2017 August

Abstract

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 ≃0.010–0.028 M ⊙ of unbound material, proving that the high mass estimated for the blue kilonova can be achieved. We also find expansion velocities of up to ∼0.2c, 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.

Turbulent magnetic field amplification in binary neutron star mergers

“…In Ref. [84], the authors studied the magnetically driven baryon-loaded wind launched in the post-merger for a similar BNS model (although with much lower resolution), finding a massive outflow emerging around the time of saturation of the magnetic energy growth (≈ 50 ms) and lasting up to 200 ms. We also note that the mass outflow may be partially hampered by the braking effect of the floor density in the atmosphere. To assess the error caused by extraction at finite radii, we compare the total ejected mass for three different radii.…”

Binary Neutron Star Mergers After GW170817

“…The former still raises doubts over a dynamical ejection, while the latter represents a potential problem for post-merger baryon-loaded winds. The magnetically driven wind from the (meta)stable NS remnant offers a viable solution (Ciolfi and Kalinani, 2020), thanks to the enhanced mass ejection and the simultaneous acceleration due to the magnetic field (as previously suggested, e.g., in Metzger et al, 2018). In this case, neutrino irradiation would also be fundamental for raising the Y e of the material, limiting the r-process nucleosynthesis, and thus maintaining a low opacity (Metzger et al, 2018).…”

Electromagnetic counterparts of compact binary mergers

“…Dessart et al 2009;Perego et al 2014;Martin et al 2015) and/or magnetically driven (e.g. Siegel, Ciolfi & Rezzolla 2014;Ciolfi & Kalinani 2020) baryon-loaded winds, with a potentially high electron fraction due to the effect of neutrino irradiation from the NS itself, a mass of up to a few ∼10 −2 M , and a velocity of ∼0.1-0.2 c (where the higher end can be achieved in the polar region in presence of a strong magnetic field; Ciolfi & Kalinani 2020).…”

“…Moreover, the (meta)stable massive NS resulting from the merger can launch an additional baryon-loaded wind up to its collapse to a BH, if any (e.g. Ciolfi & Kalinani 2020). More details on these different ejecta components are given in § § 4.3 and 4.4.…”