Gravitational-wave astronomy
Scientists announce that the LIGO and VIRGO gravitational wave observatories detected for the first time gravitational waves from the merger of two neutron stars. The event coincided with a short gamma ray burst GRB 170817A detected by the Fermi Gamma-ray Space Telescope, leading to SSS17a, the immediate follow-up observations by the Hubble Space Telescope and numerous ground-based telescopes. This confirms the theory according to which neutron star mergers are the cause of at least some short gamma-ray bursts.
Sources: LIGO press release The Verge