New analysis of the SN 1987A neutrinos with a flexible spectral shape

Abstract

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 quasi-thermal spectrum of the form $(E/E_0)^αe^{-(α+1)E/E_0}$ where $α$ plays the role of a spectral index. This simple representation not only allows one to fit the total energy $E_{\rm tot}$ emitted in $\barν_e$ and the average energy $ $, but also accommodates a wide range of shapes, notably anti-pinched 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 $E_{\rm tot}$ 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 anti-pinched spectrum.