Following the supernova explosion 1987A at a distance d = 1.5 10 21 m, a burst of e interactions was observed in a

Following the supernova explosion 1987A at a distance d = 1.5 × 10²¹ m, a burst of ν̅_e interactions was observed in a terrestrial detector. The interactions occurred over a time interval of 7 seconds and the anti neutrinos had a range of energies between 7 MeV and 11 MeV. Use this to estimate an upper limit on the ν̅_e mass.