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Abstract We present nucleosynthesis and light-curve predictions for a new site of the rapid neutron capture process (r-process) from magnetar giant flares (GFs). Motivated by observations indicating baryon ejecta from GFs, J. Cehula et al. proposed that mass ejection occurs after a shock is driven into the magnetar crust during the GF. We confirm using nuclear reaction network calculations that these ejecta synthesize moderate yields of third-peakr-process nuclei and more substantial yields of lighterr-nuclei, while leaving a sizable abundance of free neutrons in the outermost fastest expanding ejecta layers. The finalr-process mass fraction and distribution are sensitive to the relative efficiencies ofα-capture andn-capture freeze-outs. We use our nucleosynthesis output in a semianalytic model to predict the light curves of novae breves, the transients following GFs powered by radioactive decay. For a baryonic ejecta mass similar to that inferred of the 2004 Galactic GF from SGR 1806-20, we predict a peak UV/optical luminosity of ∼10³⁹–10⁴⁰erg s⁻¹at ∼10–15 minutes, rendering such events potentially detectable to several Mpc following a gamma-ray trigger by wide-field transient monitors such as ULTRASAT/UVEX. The peak luminosity and timescale of the transient increase with the GF strength due to the larger ejecta mass. Although GFs likely contribute 1%–10% of the total Galacticr-process budget, their short delay-times relative to star formation make them an attractive source to enrich the earliest generations of stars. 

Abstract The origin of heavy elements synthesized through the rapid neutron capture process (r-process) has been an enduring mystery for over half a century. J. Cehula et al. recently showed that magnetar giant flares, among the brightest transients ever observed, can shock heat and eject neutron star crustal material at high velocity, achieving the requisite conditions for anr-process. A. Patel et al. confirmed anr-process in these ejecta using detailed nucleosynthesis calculations. Radioactive decay of the freshly synthesized nuclei releases a forest of gamma-ray lines, Doppler broadened by the high ejecta velocitiesv ≳ 0.1cinto a quasi-continuous spectrum peaking around 1 MeV. Here, we show that the predicted emission properties (light curve, fluence, and spectrum) match a previously unexplained hard gamma-ray signal seen in the aftermath of the famous 2004 December giant flare from the magnetar SGR 1806–20. This MeV emission component, rising to peak around 10 minutes after the initial spike before decaying away over the next few hours, is direct observational evidence for the synthesis of ∼10⁻⁶M_⊙ofr-process elements. The discovery of magnetar giant flares as confirmedr-process sites, contributing at least ∼1%–10% of the total Galactic abundances, has implications for the Galactic chemical evolution, especially at the earliest epochs probed by low-metallicity stars. It also implicates magnetars as potentially dominant sources of heavy cosmic rays. Characterization of ther-process emission from giant flares by resolving decay line features offers a compelling science case for NASA’s forthcoming COSI nuclear spectrometer, as well as next-generation MeV telescope missions.