Observation of an extra emission around 2 MeV in the spectrum of an intense gamma-ray burst

We present a time-averaged spectral analysis (0.1-100 MeV) of two intense g amma-ray bursts detected by the PHEBUS experiment. The vf(v) spectra of the se two events are observed to peak between 1 and 2 MeV. For the least inten se of the two bursts, a single maximum in vf(v) is seen. For the other, the re is a suggestion of two peaks, with similar strengths, at similar to 1 an d similar to 2 MeV. The most commonly used photon models, with four or five parameters, provide inadequate frits to the latter spectrum. We have teste d two seven-parameter models that satisfactorily fit the observations by th e two brightest detectors of the stronger event. Both of them are built fro m and compared with the standard four-parameter gamma-ray burst (GRB) funct ion. The first model assumes an additional emission described by a Gaussian expression. The second one supposes a further spectral break at higher ene rgy. 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 supp orts a sudden change in the photon spectral index from about -2 to -3.6 ove r 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 exp ression does not exceed 6.8 x 10(-8). Although its origin is unclear, the 2 MeV emission excess may provide important constraints on the physical cond itions and radiation mechanisms at the emission site of gamma-ray bursts.