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

TL;DR

This paper investigates the nucleosynthesis and observable transients resulting from magnetar giant flares, revealing their potential role in producing r-process elements and predicting detectable UV/optical signals shortly after such events.

Contribution

It introduces a new model for r-process nucleosynthesis in magnetar giant flares and predicts associated transient light curves, expanding understanding of their astrophysical significance.

Findings

Magnetar giant flares synthesize light r-process nuclei and some heavier ones.

Predicted UV/optical transients peak at 10-15 minutes with luminosities of 10^{39}-10^{40} erg/s.

GFs could contribute 1-10% to the Galactic r-process element budget.

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, Cehula et al. (2024) proposed 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 $\alpha$-capture and $n$-capture freeze-outs. We use our nucleosynthesis output in a semi-analytic 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 $\sim 10^{39}$-$10^{40}\,\rm erg\,s^{-1}$ at $\sim 10$-$15$ minutes, rendering such events potentially detectable following a gamma-ray trigger by wide-field transient monitors such as ULTRASAT/UVEX to several Mpc. 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.