There was some big astronomy news this week, as astronomers announced the first direct observation of gravitational waves produced by the collision, or merging, of two neutron stars. This collision even produced some heavy elements, such as gold. It sounds like science fiction, but is very real. Gravitational waves have been seen before, but those ones were caused by the collision of two black holes. This was also the first time that such an event (known as GW170817 in this case) had been detected in both visible light and gravitational waves.
Neutron stars are the extremely dense leftover cores of massive stars which exploded as supernovas. These two neutron stars were no larger than Washington, D.C. but had masses from 10-60 times greater than that of the Sun.
The observations were made primarily by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo Interferometer in Italy. Only about two seconds later, a short gamma-ray burst was then detected by NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL). The light from the collision was also witnessed by NASA's Swift, Hubble, Chandra and Spitzer space telescopes.
"This is the one we've all been waiting for," said David Reitze, executive director of the LIGO Laboratory at Caltech in Pasadena, California. "Neutron star mergers produce a wide variety of light because the objects form a maelstrom of hot debris when they collide. Merging black holes - the types of events LIGO and its European counterpart, Virgo, have previously seen - very likely consume any matter around them long before they crash, so we don't expect the same kind of light show."
"This is extremely exciting science," said Paul Hertz, director of NASA's Astrophysics Division at the agency's headquarters in Washington. "Now, for the first time, we've seen light and gravitational waves produced by the same event. The detection of a gravitational-wave source's light has revealed details of the event that cannot be determined from gravitational waves alone. The multiplier effect of study with many observatories is incredible."
Hubble Observes the Neutron Star Collision
In Chile, several other ground telescopes also took part, including ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA), the VLT Survey Telescope (VST) at the Paranal Observatory, the Italian Rapid Eye Mount (REM) telescope at ESO’s La Silla Observatory, the LCO 0.4-meter telescope at Las Cumbres Observatory, and the American DECam at Cerro Tololo Inter-American Observatory. Later, the Pan-STARRS and Subaru telescopes in Hawaii also observed the historic event.
Gravitational waves were first detected in 2015 by LIGO. They are ripples in spacetime created by moving masses, like ripples on water. In this case, they were produced as the two neutron stars whirled around each other hundreds of times a second before the collision.
“There are rare occasions when a scientist has the chance to witness a new era at its beginning,” said Elena Pian, an astronomer with INAF, Italy, and lead author of one of the new Nature papers. “This is one such time!”
The event was seen on Aug. 17, 2017, shortly after 5:41 a.m. PDT (8:41 a.m. EDT) in the lenticular galaxy NGC 4993, about 130 million light-years away.
“Once I saw that there had been a trigger from LIGO and Virgo at the same time as a gamma-ray burst I was blown away,” noted Andrew Levan of the University of Warwick, who led the Hubble team. “When I realised that it looked like neutron stars were involved, I was even more amazed. We’ve been waiting a long time for an opportunity like this!”
The Hubble Space Telescope was able to observe the event, known as a kilonova, in both visible and infrared light. A kilonova is 1,000 times brighter than a typical nova.
“It was surprising just how closely the behaviour of the kilonova matched the predictions,” said Nial Tanvir, a professor at the University of Leicester and leader of another Hubble observing team. “It looked nothing like known supernovae, which this object could have been, and so confidence was soon very high that this was the real deal.”
Doomed Neutron Stars Create Blast of Light and Gravitations Waves
Neutron Star Collision Produces Heavy Elements
One of the most interesting aspects of these observations was the detection of heavy elements created by the explosion, such as platinum and gold. These results fitted what theoretical models had predicted.
“The spectrum of the kilonova looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear,” said Levan. “It ties this object to the gravitational wave source beyond all reasonable doubt.”
This also helps answer the question of where these kinds of heavy elements first come from - apparently, from the collisions of neutron stars.
Surprisingly, however, no X-rays were detected, perhaps due to the viewing angle and the time it took for the jet directed toward Earth to expand into our line of sight.
"The detection of X-rays demonstrates that neutron star mergers can form powerful jets streaming out at near light speed," said Eleonora Troja at Goddard, who led one of the Chandra teams and found the X-ray emission. "We had to wait for nine days to detect it because we viewed it from the side, unlike anything we had seen before."
Such phenomena as neutron stars and gravitational waves can be difficult to comprehend, just like black holes, but this discovery is another reminder that the Universe can be stranger than we often imagine.