Identifying observable MeV lines from the decays of weak and main $r$-process isotopes in mergers

TL;DR

This paper predicts MeV gamma-ray spectra from neutron star mergers, identifying spectral lines from isotope decays to distinguish nuclear processes and models over days to years.

Contribution

It develops a search algorithm for spectral peaks, predicts emission spectra considering various nuclear models, and tabulates observable decay lines for different isotopes.

Findings

Rh-106 emission spectrum can distinguish between main and weak r-process.

Certain isotopes like Hf-181 and Ta-182 can differentiate nuclear models.

Tl-208 line remains observable over years, aiding detection efforts.

Abstract

We consider predictions for the MeV gamma-ray spectrum emitted by the $\beta$ decays of freshly synthesized isotopes from a neutron star merger at timescales of relevance for post-merger (days) and remnant (years) emission. We develop a search algorithm to identify observable spectral peaks and then determine if a specific isotope has a dominant emission line producing the spectral feature. We predict emission spectra using nucleosynthesis calculations which consider nuclear models with distinct masses, $\beta$-decays, and fission properties as well as variations on main ($A>130$) and weak ($A<130$) $r$-process astrophysical conditions. We tabulate all lines from decaying isotopes that our procedure identifies and provide the predicted range in time over which each line could be visible. We find that Rh-106 presents a unique opportunity to distinguish between main and weak $r$-process emission, as our calculated spectrum above $\sim 1$ MeV for an event dominated by the weak $r$ process is identical to the Rh-106 emission spectrum from $\sim$ 0.2 to $\sim$17 years. We further find emission from species such as Hf-181, Ta-182, Ta-184, and Re-188 offers the potential to be able to distinguish between nuclear models. We investigate whether the 2.6 MeV strong gamma-ray line from Tl-208 is predicted to be robustly observable across calculation variations on both timescale of days and years. We find Tl-208 to consistently shine through on the order of years, though it can face competition from Ga-72 and La-140 at early times ($\sim$ days). We additionally highlight numerous isotopes of interest for observation and nuclear experiment.