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

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 $P_{\mathrm{GF}}$ we find the total unbound ejecta mass roughly obeys the relation $M_{\mathrm{ej}}\sim 4-9\times 10^{24}\mathrm{g}({\mathrm{P}}_{\mathrm{GF}}/10^{30}\mathrm{ergs}{\mathrm{cm}}^{-3}{)}^{1.43}$. For $P_{\mathrm{GF}}\sim 10^{30}-10^{31}\mathrm{ergs}{\mathrm{cm}}^{-3}$ corresponding to the dissipation of a magnetic field of strength $\sim 10^{15.5}-10^{16}\mathrm{G}$, we find $M_{\mathrm{ej}}\sim 10^{25}-10^{26}\mathrm{g}$ with asymptotic velocities $v_{\mathrm{ej}}/c\sim 0.3-0.6$ compatible with the ejecta properties inferred from the afterglow of the December 2004 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 magnetar GFs could be an appreciable heavy $r$-process source that tracks star formation. We predict that GFs are accompanied by short $\sim$minutes long, luminous $\sim 10^{39}\mathrm{ergs}{\mathrm{s}}^{-1}$ 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.

arXiv comment24 pages, 12 figures, 2 tables. v2: matches the accepted version in MNRAS (only minor edits were made). Videos showing our Fiducial simulation are available at https://dx.doi.org/10.5281/zenodo.10593900 and at https://www.youtube.com/playlist?list=PLtfS1xXqUmtC2A48QhRvGXxbeMK1vkNnC

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• From Rare Events to a Population: Discovering Overlooked Extragalactic Magnetar Giant Flare Candidates in Archival Fermi Gamma-ray Burst Monitor Data