OBSERVATIONS OF GAMMA-RAY LINE-PROFILES FROM SN-1987A

We report observations of fully resolved gamma-ray lines from the decay of 56Co in SN 1987A. Data are from the first two balloon flights of the Gamma-Ray Imaging Spectrometer (GRIS), the first of a new generation of high-resolution gamma-ray spectrometers for astrophysics. On day 433 (after the initial optical sighting) the 847 and 1238 keV lines from 56Co were observed at 2.3 and 4.3 σ significance. On day 613, the lines at 847, 1238, and 2599 keV were observed at 4.6, 3.4, and 1.9 σ, respectively. The combined significance for the three-line complex in both flights is 7.8 σ. Gaussian profiles yield acceptable least-squares fits to the lines. The line profiles are centered on the red side of the rest energy with typical velocity dispersions of ∼3500 km s-1 FWHM, consistent with an optically thin source, but the line intensities are ≲30% of those produced by the 0.075 M⊙ of 56Co determined from the bolometric light curve. Although the line fluxes derived from these fits are consistent with spherically symmetric mixed models of the supernova shell, the line profiles are not. These models predict lines blueshifted from the observed lines and narrower. The observed line profiles could potentially be explained by models that include fragmentation or nonspherical geometries.