The first time that astronomers observed both gravitational waves and electro-magnetic waves from the same event, what they were observing was:

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The observed event was the spiraling toward each other of two neutron stars, which produced both gravitational waves and electromagnetic waves (namely, gamma-ray bursts). This event, called GW170817, was observed on August 17, 2017, and it marked the first time that multi-messenger astronomy (the observation of different types of signals from the same astronomical event) was achieved.

The First Time That Astronomers Observed Both Gravitational Waves and Electro-Magnetic