Relativistic \(^{56}\text{Ni}\) Decay Lines in GRB 221009A

Abstract

Long Gamma Ray Bursts are thought to originate from the core collapse of massive stars that give rise to energetic broad-lined Type Ic supernovae. The brightest burst ever recorded, GRB 221009A, has been linked to a broad-lined Type Ic supernova through late-time observations by the James Webb Space Telescope. An emission line evolving from $\sim$37 to $\sim$6~MeV is detected during the prompt phase. We propose that this time-evolving line is consistent with Doppler-boosted radioactive decay of nickel synthesized in the associated supernova and entrained in the relativistic jet, corresponding to the boosted 158~keV decay branch. We also report evidence for an additional higher-energy excess near $\sim$24~MeV at 290--300~s, detected at moderate statistical significance and consistent with the boosted 270~keV decay branch. The observed kinematics and flux evolution are compatible with expectations from radioactive decay, providing direct spectroscopic evidence linking prompt emission to supernova nucleosynthesis.

Figures (6)

• Figure 1: Predicted observable spectrum of Nickel lines. This includes $\gamma\gamma$ annihilation assuming the internal shock occurs at $R=10^{14}$ cm at time 290-300 s. The plot shows the $E^2 \mathrm{d}N/\mathrm{d}E$ spectrum after applying the attenuation to each individual $^{56}\mathrm{Ni}$ line component due to $\gamma\gamma$ collisions. The total attenuated spectrum (thick solid orange line) is compared to the intrinsic spectrum (dotted grey line). Individual attenuated line components are also shown (colored solid lines). Spectra are normalized such that the attenuated 158 keV (observed 12 MeV) line peak aligns with the observed peak. Hatched and dotted regions indicate the sensitive ranges of Fermi-GBM and Fermi-LAT, respectively. Note the significant suppression of the 1562 keV line (attenuation factor $\sim 0.007$). If the internal shock occurs at a smaller radius, the attenuation, especially for higher energy lines, will be stronger.
• Figure 2: The $E^2 dN/dE$ spectrum of GRB 221009A observed by Fermi-GBM (NaI7+BGO1). The spectrum is integrated from 290 s to 300 s and the data are represented by the black points. Error bars represent 1 $\sigma$ errors. This time interval (290--300 s) is when the second line significance reaches maximal, resulting in smaller statistical errors than in the broader, data-driven time intervals selected by Zhang et al 2024SCPMA..6789511Z, and Ravasio et al. (2024) 2024Sci...385..452R. The solid blue line shows a model comprising a Band function plus two Gaussian components centered at 12.28 MeV (dotted) and at 24.24 MeV (dashed). The lower panel displays the residuals (data - model) in units of standard deviation ($\sigma$).
• Figure 3: Parameter constraints for the GRB jet-Nickel model. The region enclosed by the solid black boundary curve and marked with a dotted fill pattern shows viable combinations of radiative efficiency $\eta$ and half-opening angle $\theta_{\rm j}$ that satisfy the observed isotropic-equivalent luminosity (246–256 s) within $1\sigma$ errors, for a nickel mass fraction $\mathcal{R}=40\%$. Our analysis yields $0.2\% \lesssim \eta \lesssim 0.75\%$ and $0.016\,\mathrm{rad} \lesssim \theta_{\rm j} \lesssim 0.04\,\mathrm{rad}$, consistent with: (i) isotropic-equivalent kinetic energies $E_{\rm kinetic,\, iso}=9.8\times10^{56}$–$7.1\times10^{57}\,\rm erg$2023ApJ...944L..34R, (ii) independent jet angle estimates 2024ApJ...962..115R2025arXiv250317765G, and (iii) an initial jet mass $<3\times10^{-2}M_\odot$. The upper bound on $\eta$ derives from the $\theta_{\rm j}\lesssim0.04\,\mathrm{rad}$ constraint.
• Figure 4: Line Luminosity Evolution and Inferred Jet–Nickel Mass Growth in GRB 221009A. (a): Red data points represent the luminosity from Ravasio et al. (2024) 2024Sci...385..452R (Table. \ref{['tab:Lgauss']}), while blue points are from Zhang et al. (2024) 2024SCPMA..6789511Z (Table. \ref{['tab:MeV_Lines']}). Black line shows the luminosity evolution corresponding to the best-fit $E_{\rm line}(t)$ model within $1\sigma$ dispersion region defined by $\sigma_{\mathrm{int}}$ (see Methods), for a jet opening angle of $\theta_{\rm j} = 0.02$ rad and efficiency $\eta = 0.2\%$. The blue line shows the best-fit joint luminosity evolution, obtained by combining the luminosty datapoints from Ravasio et al. (2024) and Zhang et al. (2024) (see Time-dependent Luminosity in Methods). (b): Time evolution of the total jet mass and entrained Nickel mass $M_{\rm Ni}(t) \propto (t_{\rm obs}-243)^{1.6}$, both in solar mass units, within the $1\sigma$ confidence interval derived from the observational best-fit of line's energy vs. time. The initial jet and Nickel masses are approximately $5 \times 10^{-3}\,M_\odot$ and $2 \times 10^{-3}\,M_\odot$, respectively. The total jet mass grows significantly, reaching $\sim 0.1\,M_\odot$ at late times. The model prediction for the luminosity, the total jet mass and the entrained Nickel mass accounts only for the 158 keV decay branch.
• Figure 5: Radius-Dependent $\gamma\gamma$ Opacity and Survival of Doppler-Boosted Nickel Lines. (a): Gamma-gamma annihilation optical depth within the GRB jet at time 290-300 s. Calculated optical depth, $\tau_{\gamma\gamma}$, for radioactive photons traversing the prompt emission field. Curves represent different assumed emission radii ($R$) from the central engine, ranging from $10^{12}$ cm to $10^{18}$ cm. The target photon field is modelled using the observed Band function component of the GRB spectrum transformed into the comoving frame of energy $E'$. Circles mark the calculated optical depths for the comoving frame centre energies of the $^{56}\mathrm{Ni}$ decay lines. The dashed horizontal line indicates $\tau_{\gamma\gamma}=1$. (b): Attenuation factor for $^{56}\mathrm{Ni}$ lines due to $\gamma\gamma$ annihilation at time 290-300 s. The plot shows the attenuation factor (survival probability $e^{-\tau_{\gamma\gamma}}$) for photons originating from specific $^{56}\mathrm{Ni}$ decay lines identified by their comoving energy (boosted energy in the parenthesis) as a function of the emission radius ($R$) within the jet. The optical depth $\tau_{\gamma\gamma}$ for each line's comoving energy is calculated using the comoving Band spectrum as the target field (see Methods and panel (a)). Higher energy lines experience significant attenuation at smaller radii.