We examine the detectability of $\gamma$-ray emission originating from the radioactive decays of unstable nuclei that are synthesized in relativistic outflows launched in magnetorotational core-collapse supernovae. The observed lines have enhanced energies due to the Lorentz boosted nuclei and can also be seen until later times due to time dilation of the rest-frame half-lives. We find that instruments like e-ASTROGAM and INTEGRAL/SPI are sensitive to these boosted line emissions from hundreds of keV to tens of MeV at a distance of 10 kpc over timescales of tens of days. For favorable viewing angles, these decays can be detected to extragalactic distances for rapidly spinning protomagnetar models. On the other hand, detection for off-axis jets is challenging, even for a supernova at the Galactic Center. Measuring multiple decay lines in addition to the integrated luminosity over $\sim 10\mathrm{days}$ postbounce would allow for the ability to distinguish between models and shed light on central engine properties like magnetic field and spin.

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• Gamma ray astronomy
• Novae & supernovae
• r process