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 $(E/E_0{)}^{\alpha }{e}^{-(\alpha +1)E/E_0}$ where $\alpha$ plays the role of a spectral index. This simple representation not only allows one to fit the total energy $E_{\mathrm{tot}}$ emitted in ${\bar{\nu }}_e$ and the average energy $<E_{{\bar{\nu }}_e}>$, 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 $\alpha$ 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 $<E_{{\bar{\nu }}_e}>$ and $E_{\mathrm{tot}}$ only if one fixes a prior value for $\alpha$. The tension between the KII and IMB data sets and theoretical expectations for $<E_{{\bar{\nu }}_e}>$ is not resolved by an anti-pinched spectrum.

Comments: to appear in PRD (6 pages, 6 eps figures)