Gamma-ray Signatures of r-Process Radioactivity from the Collapse of Magnetized White Dwarfs

Abstract

We predict the gamma-ray line emission from $r$-process nuclei synthesized in the ejecta of the accretion-induced collapse (AIC) of a magnetized, rapidly rotating white dwarf. Using ejecta from a two-dimensional general-relativistic neutrino-magnetohydrodynamic simulation, further evolved with a radiation-hydrodynamics code coupled to an in-situ nuclear reaction network, we construct angle-dependent gamma-ray spectra in the $0.01$-$10\mathrm{MeV}$ band via composition-dependent ray-tracing through the ejecta. The emission between $\sim$1 and $10$d is dominated by ${}^{132}$I ($t_{1/2}=2.3$h), continuously replenished by the decay of its parent ${}^{132}$Te ($t_{1/2}=3.2$d), with additional contributions from ${}^{131}$I, ${}^{133}$Xe, and ${}^{132}$Te. At $t\gtrsim 20$ d, ${}^{56}$Co (from ${}^{56}$Ni decay) becomes the primary emitter. The simultaneous presence of $r$-process and iron-peak gamma-ray lines is distinctive of AIC ejecta and absent in binary neutron star mergers, where iron-peak nuclei are generally not synthesized. Comparing with the $3\sigma$ continuum sensitivities of planned MeV gamma-ray telescopes (COSI, AMEGO-X, e-ASTROGAM, GRAMS, GammaTPC), we find the brightest $r$-process lines detectable to $\sim 10\mathrm{Mpc}$ by GammaTPC and GRAMS, with the signal approaching their sensitivity threshold at $30\mathrm{Mpc}$. The $r$-process spectral features survive time integration over $\sim 30$ d exposures, demonstrating robustness against the long observation times required by gamma-ray detectors.

Comments: 13 pages, 9 figures. Accepted for publication in PRD; added Figs. 2 and 5 and clarifications, conclusions unchanged