A prominent feature in the infrared spectra of SN 1987A, which grew in strength from 1987 May to July, has been identified as He I 1.083 {micron}. The lower level of this transition is populated by recombination of He II, and therefore the strengthening of the line requires an increasing rate of ionization in the supernova envelope. At this time the supernova luminosity was due to radioactive ^56^Co. It is possible that the ionization is due to ^56^Co decay γ-rays. The observed γ-ray emission from the supernova in 1987 November implies a flux of energetic photons through the supernova envelope which can cause enough He ionization to account for the presence of the He I line. The dramatic strengthening of He I 1.083 microns from 1987 May through July may be due to a rapid rise in the flux of γ-rays through the envelope and signals the onset of γ-ray escape from SN 1987A.
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James Richard Graham (1988) studied this question.