Neutron Stars Observed Colliding

GW170817 was a gravitational wave signal observed by the LIGO/Virgo collaboration on 17 August 2017, and was the first gravitational wave event that was observed to have a simultaneous electromagnetic signal. The gravitational wave signal, which had a duration of about 100 seconds, is the first gravitational wave detection of the merger of two neutron stars, and was associated with a soft short gamma-ray burst GRB 170817A, and an optical transient that was found in the galaxy NGC 4993. No neutrino candidates consistent with the source were found in follow-up searches.

Announcement of the Event

The event was officially announced on 16 October 2017 at press conferences at the National Press Club in Washington, D.C. and at ESO's headquarter in Garching, Germany.

The first public information about the event was tweeted by astronomer J. Craig Wheeler of the University of Texas at Austin on 18 August 2017. He later deleted the tweet and apologized for scooping the official announcement protocol. Other people followed up on the rumour, and reported that the public logs of several major telescopes listed priority interrupts in order to observe NGC 4993, a galaxy 40 Mpc (130 Mly) away in the Hydra constellation. The collaboration had earlier declined to comment on the rumors, not adding to a previous announcement that there were several triggers under analysis.

Gravitational Wave Detection

The gravitational wave signal lasted for about 100 seconds and covered about 3000 cycles, with the gravitational wave frequency increasing to a few hundred hertz (cycles per second). It arrived first at the Virgo detector in Italy, then 22 milliseconds later at the LIGO-Livingston detector in Louisiana, U.S., and another 3 milliseconds later at the LIGO-Hanford detector in Washington State, U.S. The three detections localized the source to an area of 28 degrees squared in the Southern sky with a 90% probability.

Scientific Impact

This event is the strongest evidence to confirm the hypothesis that mergers of binary stars are linked to short gamma-ray bursts. The event also provides a limit on the difference between the speed of light and that of gravity. Assuming the first photons were emitted between 0 and 10 seconds after peak gravitational wave emission constrains the difference between the speeds of gravitational and electromagnetic waves, v_GW - v_EM, to between -3×10⁻¹⁵ and +7×10⁻¹⁶ times the speed of light. In addition, it allows investigation of Lorentz invariance. The limits of possible violations of Lorentz invariance (values of 'gravity sector coefficients') are reduced by the new observations, in some cases by ten orders of magnitude.

Gravitational wave signals such as GW170817 can be used as a standard siren to provide an independent measurement of the Hubble constant.

Electromagnetic observations helped to support the theory that the mergers of neutron stars contribute to r-process nucleosynthesis.

https://en.wikipedia.org/wiki/GW170817