Final answer:
The first signal from supernova SN1987A was in the form of neutrinos, detected by neutrino telescopes prior to any visual confirmation.
Step-by-step explanation:
The first signal received from the 1987 supernova SN1987A came in the form of neutrinos. These neutrinos were detected almost a full day before the visible light from the supernova was identified by Canadian astronomer Ian Shelton. The detection was made by two separate neutrino telescopes—one located in a deep mine in Japan and the other under Lake Erie. These instruments detected the elusive neutrinos as they interacted with purified water, causing positrons and electrons to emit deep blue light. This discovery was significant because neutrinos escape an exploding star more easily than light, marking the first time such particles from a supernova had been observed on Earth.