Thallium-208: A Beacon of In Situ Neutron Capture Nucleosynthesis

We demonstrate that the well-known 2.6 MeV gamma-ray emission line from thallium-208 could serve as a real-time indicator of astrophysical heavy element production, with both rapid ($r$) and intermediate ($i$) neutron capture processes capable of its synthesis. We consider the $r$ process in a Galactic neutron star merger and show Tl-208 to be detectable from $\sim 12$ hours to $\sim \mathrm{ten}$ days, and again $\sim 1–20\mathrm{years}$ postevent. Detection of Tl-208 represents the only identified prospect for a direct signal of lead production (implying gold synthesis), arguing for the importance of future MeV telescope missions which aim to detect Galactic events but may also be able to reach some nearby galaxies in the Local Group.

Figure 1

Left: snapshot of $r$-process abundances for a merger ejecta simulation at 6.7 sec postmerger during which the decaying nuclei approach species along the Th-232 decay chain. Right: snapshot of $i$-process abundances 383 min after building from iron seeds in a one-zone model with a neutron density reaching $5\times {10}^{15}{\mathrm{cm}}^{-3}$.

Figure 2

The total $\beta$-decay gamma-ray spectrum from a Galactic neutron star merger at 10 kpc compared to the Tl-208 spectrum alone and the total without Tl-208 during the 12 hour to ten day period (top) after Tl is populated by Bi-212 and Ra-224 decay, followed by emission 1–20 years postevent (bottom) due to Ra-228 decay. Also shown is the estimated sensitivity limit for next-generation MeV detectors like MeVGRO with approximately one day exposure time.

Figure 3

The light curve (flux integrated) over the $2.5\le E_{\gamma }\le 2.8\mathrm{MeV}$ range with and without Tl-208 as compared to the contribution from Tl-208 alone (all before radiation transfer). Also shown are other species whose $\beta$-decay gammas dominate at these energies, as well as the prompt gammas from Cf-254 spontaneous fission (sf) and the subsequent gammas from the $\beta$ decay of its fragments.

Figure 4

Top: abundance patterns (compared to solar [54]) for merger dynamical ejecta [23] with $Y_e\sim 0.01$ and a parametrized disk wind (entropy of $s=30$, dynamical timescale of $\tau =70\mathrm{ms}$, with $Y_e=0.3$). Bottom: comparison of the light curve (in the $2.5\le E_{\gamma }\le 2.8\mathrm{MeV}$ range) for the disk versus the dynamical (with Tl-208 and without Tl-208), along with the MeV detector sensitivity.