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

Jakub Cehula, Todd A. Thompson, Brian D. Metzger

Published 2023-11-09Version 1

We explore the impact of a magnetar giant flare (GF) on the neutron star (NS) crust, and the associated potential 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 to sufficiently high energies to become unbound along directions unconfined by the magnetic field. We explore this process by means of spherically-symmetric relativistic hydrodynamical simulations. For an initial shell pressure $P_{\rm GF}$ we find that the total unbound ejecta mass roughly obeys the relation $M_{\rm ej}\sim4-9\times 10^{24}$ g $(P_{\rm GF}/10^{30}$ ergs cm$^{-3})^{1.43}$. For $P_{\rm GF}\sim10^{30}-10^{31}$ ergs cm$^{-3}$ corresponding to the dissipation of a magnetic field of strength $\sim10^{15.5}-10^{16}$ G, we find $M_{\rm ej}\sim10^{25}-10^{26}$ g with asymptotic velocities $v_{\rm ej}/c\sim 0.3-0.6$ compatible with the ejecta properties inferred from the radio afterglow of the GF from SGR 1806-20. Because the flare excavates crustal material to a depth characterized by an electron fraction $Y_e \approx 0.40-0.46$, and is ejected with high entropy and rapid expansion timescale, 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 GFs could contribute an appreciable heavy $r$-process source that tracks star formation. We predict that GFs are accompanied by short minutes long, luminous $\sim 10^{39}$ ergs s$^{-1}$ optical transients powered by $r$-process decay ("nova brevis"), akin to scaled-down kilonovae. Our findings also have implications for FRBs from repeating magnetar flares, particularly the high rotation measures of the synchrotron nebulae surrounding these sources.