Dynamics of baryon ejection in magnetar giant flares: implications for radio afterglows, r-process nucleosynthesis, and fast radio bursts

Abstract

ABSTRACT We explore the impact of a magnetar giant flare (GF) on the neutron star (NS) crust, and the associated baryon mass ejection. We consider that sudden magnetic energy dissipation creates a thin high-pressure shell above a portion of the NS surface, which drives a relativistic shockwave into the crust, heating a fraction of these layers sufficiently to become unbound along directions unconfined by the magnetic field. We explore this process using spherically symmetric relativistic hydrodynamical simulations. For an initial shell pressure PGF we find the total unbound ejecta mass roughly obeys the relation $M_{\rm {ej}}\sim 4\!-\!9\times 10^{24}\, \rm {g}\, (P_{\rm GF}/10^{30}\, \rm {erg}\, \rm {cm}^{-3})^{1.43}$. For $P_{\rm {GF}}\sim 10^{30}\!-\!10^{31}\, \rm {erg}\, \rm {cm}^{-3}$ corresponding to the dissipation of a magnetic field of strength $\sim 10^{15.5}\!-\!10^{16}\, \rm {G}$, we find $M_{\rm {ej}}\sim 10^{25}\!-\!10^{26}\, \rm {g}$ with asymptotic velocities vej/c ∼ 0.3–0.6 compatible with the ejecta properties inferred from the afterglow of the 2004 December GF from SGR 1806-20. Because the flare excavates crustal material to a depth characterized by an electron fraction Ye ≈ 0.40–0.46, and is ejected with high entropy and rapid expansion time-scale, the conditions are met for heavy element r-process nucleosynthesis via the alpha-rich freeze-out mechanism. Given an energetic GF rate of roughly once per century in the Milky Way, we find that magnetar GFs could be an appreciable heavy r-process source that tracks star formation. We predict that GFs are accompanied by short ∼minutes long, luminous $\sim 10^{39}\, \rm {erg}\, \rm {s}^{-1}$ optical transients powered by r-process decay (nova brevis), akin to scaled-down kilonovae. Our findings also have implications for the synchrotron nebulae surrounding some repeating fast radio burst sources.