Gravitational waves, a Gamma-Ray Burst and a kilonova
The gravitational-wave signal, named GW170817, was first detected on Aug. 17 at 8:41 a.m. Eastern Daylight Time, with the LIGO and Virgo detectors.
For about 100 seconds, the gravitational waves emitted by the two neutron stars when spiraling together were detectable on earth. Before the neutron stars merged, they were separated by roughly 400 kilometer and completed about 12 orbits every second, while inspiraling and moving closer to each other, moving faster and accelerating the process, in which gravitational waves were emitted, until the stars merged and formed a single remnant.
Animation revealing the LIGO and Fermi-GBM data synchronised to the merger time of the neutron stars. The GRB occurred 1.7 seconds after the merger. The audio is the "chirp" sound of the gravitational wave signal, followed by a "ding" at the GRB time.
Credit: NASA GSFC & Caltech/MIT/LIGO Lab
Just 1.7 seconds after the gravitational-wave signal was detected, a burst of short gamma rays, later named GRB 170817 A, was observed by two gamma-ray observatories orbiting the Earth, NASA’s Fermi space telescope (or the Fermi Gamma-ray Burst Monitor GBM, resp.) and INTEGRAL, ESA's INTErnational Gamma-Ray Astrophysics Laboratory.
The gravitational-wave and gamma-ray signals sparked ambitious observations of dozens of ground-based telescopes around the world and space-based observatories that resulted in many detections, in the following days and weeks, of the light from the event across the electromagnetic spectrum, from X-ray, ultraviolet, optical, infrared, up to radio waves.
For a long time, scientists had predicted that when neutron stars collide, they should give off gravitational waves and give rise to gamma- ray bursts, along with powerful jets emitting light across the electromagnetic spectrum.
Credit: NASA's Goddard Space Flight Center/CI Lab
Now, the detection of the gravitational-wave signal GW170817 together with the gamma-ray burst GRB 170817A provides the first direct evidence that colliding neutron stars can indeed produce short gamma-ray bursts.
Moreover, the follow-up observations revealed that the detected electromagnetic counterpart due to the outburst after the merging of the two neutron stars was a so-called kilonova, a phenomenon that had long been theorized though never conclusively observed before. The name stems from the prediction that a kilonova would be a thousand times brighter than a nova, though dimmer than a supernova.
Kilonovae are thought to be the primary source of all the elements heavier than iron in the universe. For example, most of the gold on Earth may have been created in a kilonova. Thus, the observation brings us closer to solving the puzzle of where all the heavier elements in the universe come from. The electromagnetic radiation captured from GW170817 now confirms that the elements heavier than iron are synthesized in the aftermath of neutron star collisions.
Implications for cosmology and alternative gravity models
Observing an event in both gravitational waves and across the electromagnetic spectrum offers new possibilities for precise distance measurements in the universe, which could mark the start of a new era in cosmology.
The event extensively observed also by telescopes across the entire electromagnetic spectrum allowed scientists to clearly identify the host-galaxy of the gravitational-wave source. It is the closest ever observed gravitational-wave source, with a determined distance of about 40 Mpc or 130 million light years.
This event is just so rich. It is a gift that will keep on giving.- David Shoemaker
“This event has the most precise sky localization of all detected gravitational waves so far. This record precision enabled astronomers to perform follow-up observations that led to a plethora of breathtaking results,” as Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration, put it.
Moreover, by combining the galaxy distance measured from the gravitational-wave data with the radial velocity measured from electromagnetic data, scientists were able to make an entirely independent determination of the expansion rate of the universe, the Hubble constant.
Besides the benefit for astrophysics and cosmology in general, the new observations have also far reaching implications for alternative theories of gravity, like modified gravity alternatives to dark matter and dark energy, the two mysterious phenomena which should make up about 95% of the total mass and energy of our universe. The simultaneous detection of gravitational and electromagnetic waves, for example, should rule out a class of modified gravity theories, which go without the need for dark matter.
As David Shoemaker, spokesperson for the LIGO Scientific Collaboration, emphasized the huge importance of the new discovery:
“From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving.”
Watch the press conference of October 16 with the announcement of the spectacular discoveries again:
If you want to learn more about gravitational waves, from their theoretical prediction to the first detection and beyond, stay tuned!
Our next ScienceQuest edition will be about "The Mystery of Gravitational Waves".