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A magnetar is a neutron star whose surface magnetic field is typically stronger than $10^{13}\,{\rm G}$. Its large magnetic energy powers outbursts in the X-ray to $\gamma$-ray bands. The most energetic class of such bursts, with released energies of $10^{44}$--$10^{47}\,{\rm erg}$, is called a giant flare (GF). Radio afterglows detected from two Galactic GFs suggest the presence of massive, fast ejecta with masses of $(10^{-8}$--$10^{-6})\,M_\odot$ and velocities exceeding $0.1c$, where $c$ is the speed of light.
For the Galactic magnetar SGR 1806--20, an enhancement in the MeV $\gamma$-ray band was observed approximately $1000\,{\rm s}$ after the GF. Patel et al. (2025) showed that the observed MeV $\gamma$-ray light curve and spectra can be approximately reproduced by nuclear $\gamma$-ray emission from the radioactive decay of nuclei synthesized in mildly neutron-rich ejecta, assuming an ejecta mass, velocity, and electron fraction of $10^{-6}\,M_\odot$, $0.1c$, and $Y_e=0.4$, respectively.
In this work, we calculate X-ray and $\gamma$-ray light curves and spectra by taking into account nuclear $\gamma$-ray emission and self-absorption in the ejecta, as in Patel et al. (2025). In addition, we include decays through nuclear isomeric states, internal-conversion X-rays (ICX-rays), and composition-dependent opacities, none of which were considered in Patel et al. (2025). Using a spherical, adiabatically expanding, constant-velocity ejecta model coupled to a nuclear reaction network, similar to that of Patel et al. (2025), we evaluate the time evolution of the density, temperature, and abundances of ejecta with a mass of $10^{-6}\,M_\odot$ and a velocity of $0.1c$. We refer to this case as the P25 model. We also consider a heavier and faster ejecta model with a mass of $2\times10^{-5}\,M_\odot$ and a velocity of $0.3c$, motivated by the MHD simulations of Bransgrove et al. (2026). We refer to this case as the B26 model.
We find that, in the P25 model, the light curves and spectra above $200\,{\rm keV}$ are similar to those obtained by Patel et al. (2025). Below $200\,{\rm keV}$, the fluxes are enhanced relative to those of Patel et al. (2025) owing to the effects of nuclear isomers and ICX-rays. Even with the forthcoming COSI mission, the detection of nuclear $\gamma$-rays from a GF in the Large Magellanic Cloud (LMC) at $50\,{\rm kpc}$ would be challenging. ICX-rays in the $20$--$30\,{\rm keV}$ band could be detected with NuSTAR for a nearby Galactic GF within $3\,{\rm kpc}$. In the B26 model, nuclear $\gamma$-rays above $200\,{\rm keV}$ could be detected with COSI even for a GF in the LMC. However, identifying individual nuclear $\gamma$-ray lines would be difficult because of Doppler broadening. For a Galactic GF at $10\,{\rm kpc}$, nuclear $\gamma$-rays and ICX-rays could be detected with NuSTAR in the $30$--$80\,{\rm keV}$, $10$--$30\,{\rm keV}$, and $\lesssim10\,{\rm keV}$ bands at epochs of a few $10^3\,{\rm s}$, $10^4\,{\rm s}$, and a few $10^4\,{\rm s}$ after the GF, respectively.
| Category | Theory |
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