<i>r</i>-process Nucleosynthesis and Radioactively Powered Transients from Magnetar Giant Flares
Anirudh Patel, Brian D. Metzger, Jared A. Goldberg, Jakub Cehula, Todd A. Thompson, Mathieu Renzo
Abstract
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-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 ∼10 39 –10 40 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.