Observation of an Extra Emission around 2 MeV in the Spectrum of an Intense Gamma‐Ray Burst

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Abstract
We present a time-averaged spectral analysis (0.1-100 MeV) of two intense γ-ray bursts detected by the PHEBUS experiment. The νfν spectra of these two events are observed to peak between 1 and 2 MeV. For the least intense of the two bursts, a single maximum in νfν is seen. For the other, there is a suggestion of two peaks, with similar strengths, at ~1 and ~2 MeV. The most commonly used photon models, with four or five parameters, provide inadequate fits to the latter spectrum. We have tested two seven-parameter models that satisfactorily fit the observations by the two brightest detectors of the stronger event. Both of them are built from and compared with the standard four-parameter γ-ray burst (GRB) function. The first model assumes an additional emission described by a Gaussian expression. The second one supposes a further spectral break at higher energy. For the two models, we find an extra emission around 2 MeV relative to the standard GRB model. In addition, the second seven-parameter model supports a sudden change in the photon spectral index from about -2 to -3.6 over a restricted energy range near 2.4 MeV. The probability of rejecting, as due to chance, these two models relative to the standard four-parameter expression does not exceed 6.8 × 10-8. Although its origin is unclear, the 2 MeV emission excess may provide important constraints on the physical conditions and radiation mechanisms at the emission site of γ-ray bursts.