First cosmic discovery in both gravitational waves and light

A ground­break­ing announce­ment was made on 16^th of Octo­ber by the LIGO Sci­en­tific Col­lab­o­ra­tion, Virgo Col­lab­o­ra­tion, and its part­ners. For the very first time, both grav­i­ta­tional waves and elec­tro­mag­netic waves have been observed from the same cos­mic event, the inspi­ral and col­li­sion of two neu­tron stars. This marks also the first direct detec­tion of grav­i­ta­tional waves from a neu­tron star binary sys­tem at all.

The event was observed on August 17, 2017 in a dis­tance of about 130 mil­lion light-​years from Earth in the galaxy NGC 4993, in the con­stel­la­tion Hydra, using the Laser Inter­fer­om­e­ter Gravitational-​Wave Obser­va­tory (LIGO), includ­ing the twin LIGO detec­tors located in Han­ford, Wash­ing­ton, and Liv­ingston, Louisiana, together with the Virgo gravitational-​wave detec­tor in Italy, and about 70 earth and space-​based obser­va­to­ries and tele­scopes.

The dis­cov­ery marks a break­through in many ways.
Involv­ing thou­sands of researchers work­ing at more than 70 lab­o­ra­to­ries and tele­scopes on every con­ti­nent, the detec­tion her­alds in a new era in space research, in “multi-​messenger astro­physics”.

As France A. Cór­dova, direc­tor of the National Sci­ence Foun­da­tion (NSF), which funds LIGO, put it:

“It is tremen­dously excit­ing to expe­ri­ence a rare event that trans­forms our under­stand­ing of the work­ings of the uni­verse. This dis­cov­ery real­izes a long-​standing goal many of us have had, that is, to simul­ta­ne­ously observe rare cos­mic events using both tra­di­tional as well as gravitational-​wave obser­va­to­ries. Only through NSF’s four-​decade invest­ment in gravitational-​wave obser­va­to­ries, cou­pled with tele­scopes that observe from radio to gamma-​ray wave­lengths, are we able to expand our oppor­tu­ni­ties to detect new cos­mic phe­nom­ena and piece together a fresh nar­ra­tive of the physics of stars in their death throes.”

This result is a great exam­ple of the effec­tive­ness of team­work, of the impor­tance of coor­di­nat­ing, and of the value of sci­en­tific col­lab­o­ra­tion.- EGO-​Virgo direc­tor Fed­erico Fer­rini

After grav­i­ta­tional waves had been pre­dicted a cen­tury ago by Albert Ein­stein, they were directly detected for the very first time in 2015, which was rec­og­nized with this year’s Nobel Prize in physics ear­lier this month. Three more gravitational-​wave dis­cov­er­ies have been made since, though from the merg­ing of black holes. This event, observed only three days after the first joint LIGO-​Virgo detec­tion of a binary black hole merger, rep­re­sents the first direct detec­tion of grav­i­ta­tional waves from a binary sys­tem of neu­tron stars.

It also pro­vides the first mea­sure­ment of the speed of grav­i­ta­tional waves, con­firm­ing that grav­i­ta­tional waves are actu­ally prop­a­gat­ing with the speed of light, as pre­dicted by Ein­stein.

Neu­tron star bina­ries and grav­i­ta­tional waves

Neu­tron stars rep­re­sent one of the end-​points of stel­lar evo­lu­tion, formed in super­nova explo­sions when mas­sive stars have run out of nuclear fuel after burn­ing for mil­lions of years and undergo grav­i­ta­tional col­lapse.
At the very high pres­sures involved in this col­lapse, pro­tons and elec­trons are com­bined to form neu­trons plus neu­tri­nos (in the so-​called beta decay). While the neu­tri­nos are help­ing the super­nova to hap­pen, the neu­trons set­tle down to become a neu­tron star, with neu­tron degen­er­acy man­ag­ing to oppose grav­ity. With masses of about 1.4 times the mass of our sun, but only radii of 10 to 20 km, neu­tron stars are extremely dense, with den­si­ties com­pa­ra­ble to that inside atomic nuclei.

Neu­tron stars can also be found in pairs orbit­ing each other. Such binary star sys­tems com­posed of two neu­tron stars were long assumed to be among the lead­ing poten­tial sources for the detec­tion by gravitational-​wave obser­va­to­ries.

The first binary neu­tron star sys­tem to be dis­cov­ered was PSR B1913+16 by Rus­sell Hulse and Joseph Tay­lor in 1974, in which a radio pul­sar was found to be in close orbit around another neu­tron star. Since its dis­cov­ery, the decay of the orbit of PSR B1913+16 at exactly the rate pre­dicted by Einstein’s gen­eral the­ory of rel­a­tiv­ity had pro­vided strong indi­rect evi­dence that grav­i­ta­tional radi­a­tion exists.

Now, the obser­va­tions by the LIGO-​Virgo detec­tor net­work pro­vide the first direct detec­tion of grav­i­ta­tional waves com­ing from the inspi­ral of two low-​mass com­pact objects con­sis­tent with a binary neu­tron star merger.
The inspi­ral­ing objects were esti­mated to be in a range from around 1.1 to 1.6 times the mass of the sun, in the mass range of neu­tron stars.