r-process Nucleosynthesis and Radioactively Powered Transients from Magnetar Giant Flares

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-peak r-process nuclei and more substantial yields of lighter r-nuclei, while leaving a sizable abundance of free neutrons in the outermost fastest expanding ejecta layers. The final r-process mass fraction and distribution are sensitive to the relative efficiencies of α-capture and n-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 ∼1039–1040 erg s−1 at ∼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 Galactic r-process budget, their short delay-times relative to star formation make them an attractive source to enrich the earliest generations of stars.