Analysis of the SN 1987A neutrinos with a flexible spectral shape

We analyze the neutrino events from the supernova (SN) 1987A detected by the Kamiokande-II (KII) and Irvine-Michigan-Brookhaven (IMB) experiments. For the time-integrated flux we assume a quasithermal spectrum of the form (E/E0)αe-(α+1)E/E0 where α plays the role of a spectral index. This simple representation not only allows one to fit the total energy Etot emitted in νe and the average energy , but also accommodates a wide range of shapes, notably antipinched spectra that are broader than a thermal distribution. We find that the pile-up of low-energy events near threshold in KII forces the best-fit value for α to the lowest value of any assumed prior range. This applies to the KII events alone as well as to a common analysis of the two data sets. The preference of the data for an 'unphysical' spectral shape implies that one can extract meaningful values for and Etot only if one fixes a prior value for α. The tension between the KII and IMB data sets and theoretical expectations for is not resolved by an antipinched spectrum