For the first time, astronomers have detected evidence of a cocoon of material blasting out from a pair of merging neutron stars. Such mergers may be the source of many of the universe's heaviest elements.
In August, astronomers witnessed the never-before-seen phenomenon of two neutron stars merging. Neutron stars are corpses of large stars that perished in catastrophic explosions known as supernovas. Although neutron stars are usually small, with diameters of about 12 miles (19 kilometers) or so, they are extremely dense. A neutron star's mass may be about the same as that of the sun; a teaspoon of neutron-star material has a mass of about a billion tons, making neutron stars the universe's densest objects besides black holes.
The August discovery was made when scientists detected ripples in the fabric of space and time known as gravitational waves, which emanated from a crash between a pair of neutron stars located about 130 million light-years away from Earth, a merger dubbed GW170817. Astronomers quickly followed up this find with observations from conventional telescopes, marking the first time both gravitational waves and electromagnetic waves were seen from an astronomical event.
The wavelengths of light released by the material ejected from GW170817 revealed that this matter was loaded with newly synthesized elements. These findings confirmed 70 years of research suggesting that neutron-star mergers are powerful enough to synthesize heavy elements such as gold, platinum and lead.
Scientists had already known where lighter elements were synthesized; most hydrogen and helium came from the Big Bang, and elements up to iron on the periodic table are mostly forged in the cores of stars. GW170817 provided the first concrete evidence that neutron-star mergers are the birthplaces of half of the universe's elements that are heavier than iron.
The emissions of near-infrared, visible and ultraviolet light that the researchers saw can be explained by the radioactive decay of heavy elements such as uranium and gold that were spewed out during GW170817. However, the emissions of radio waves, X-rays and gamma-rays that the scientists detected posed a mystery, researchers on the new study said in a statement.
According to one model of neutron-star collisions, these puzzling emissions were the result of powerful, narrow jets of radiation given off during GW170817 and directed "off axis," or away from the line of sight of Earth. This model also suggests that neutron-star mergers are the primary sources of short gamma-ray bursts, which are among the most powerful explosions in the universe.
However, researchers monitoring the merger with radio telescopes found that radio emissions from GW170817 steadily gained strength over time. This is not consistent with the off-axis jet model, which suggests the radio emissions would slowly weaken over time.
Instead, the researchers now suggest that the merging neutron stars gave birth to a cocoon of material. "This is the first time a structure like this has been seen," study lead author Kunal Mooley, an astrophysicist at the California Institute of Technology in Pasadena, told Space.com. The researchers suspect that a large fraction of neutron-star mergers may generate such cocoons, which implies "there is a whole new population of as-yet-unidentified transient astrophysical events that we need to now start searching for," Mooley said.
Both models suggest that when neutron stars merge, it results in an explosion called a kilonova that gives off a spherical expanding shell of debris. However, whereas one model suggests that the merger also emits a pair of tight jets of radiation that punch through this shell, the cocoon model suggests that a merger can emit a pair of much broader cones of radiation that essentially blasts a wide cocoon of matter outward "at 90 percent of the speed of light in the case of GW170817," Mooley said. Analyzing the cocoons from neutron-star mergers may shed light on the origin of many of universe's heaviest elements. "We want to find out what the frequency of such cocoon events is and the dynamics of the chemically enriched material that neutron star mergers have produced over the universe's history," Mooley said.
He added that such mergers could also be the origin of cosmic rays, which are made of atomic nuclei that zip through outer space with extraordinarily high amounts of energy.
The scientists detailed their findings online Dec. 20 in the journal Nature.
Radio Observations Point to Likely Explanation for Neutron-Star Merger
Three months of observations with the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have allowed astronomers to zero in on the most likely explanation for what happened in the aftermath of the violent collision of a pair of neutron stars in a galaxy 130 million light-years from Earth. What they learned means that astronomers will be able to see and study many more such collisions.
On August 17, 2017, the LIGO and VIRGO gravitational-wave observatories combined to locate the faint ripples in spacetime caused by the merger of two superdense neutron stars. It was the first confirmed detection of such a merger and only the fifth direct detection ever of gravitational waves, predicted more than a century ago by Albert Einstein.
The gravitational waves were followed by outbursts of gamma rays, X-rays, and visible light from the event. The VLA detected the first radio waves coming from the event on September 2. This was the first time any astronomical object had been seen with both gravitational waves and electromagnetic waves.
The timing and strength of the electromagnetic radiation at different wavelengths provided scientists with clues about the nature of the phenomena created by the initial neutron-star collision. Prior to the August event, theorists had proposed several ideas — theoretical models — about these phenomena. As the first such collision to be positively identified, the August event provided the first opportunity to compare predictions of the models to actual observations.
Astronomers using the VLA, along with the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India, regularly observed the object from September onward. The radio telescopes showed the radio emission steadily gaining strength. Based on this, the astronomers identified the most likely scenario for the merger’s aftermath.
“The gradual brightening of the radio signal indicates we are seeing a wide-angle outflow of material, traveling at speeds comparable to the speed of light, from the neutron star merger,” said Kunal Mooley, now a National Radio Astronomy Observatory (NRAO) Jansky Postdoctoral Fellow hosted by Caltech.
The observed measurements are helping the astronomers figure out the sequence of events triggered by the collision of the neutron stars.
The initial merger of the two superdense objects caused an explosion, called a kilonova, that propelled a spherical shell of debris outward. The neutron stars collapsed into a remnant, possibly a black hole, whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated a pair of narrow, superfast jets of material flowing outward from its poles.
If one of the jets were pointed directly toward Earth, we would have seen a short-duration gamma-ray burst, like many seen before, the scientists said.
“That clearly was not the case,” Mooley said.
Some of the early measurements of the August event suggested instead that one of the jets may have been pointed slightly away from Earth. This model would explain the fact that the radio and X-ray emission were seen only some time after the collision.
“That simple model — of a jet with no structure (a so-called top-hat jet) seen off-axis — would have the radio and X-ray emission slowly getting weaker. As we watched the radio emission strengthening, we realized that the explanation required a different model,” said Alessandra Corsi, of Texas Tech University.
The astronomers looked to a model published in October by Mansi Kasliwal of Caltech, and colleagues, and further developed by Ore Gottlieb, of Tel Aviv University, and his colleagues. In that model, the jet does not make its way out of the sphere of explosion debris. Instead, it gathers up surrounding material as it moves outward, producing a broad “cocoon” that absorbs the jet’s energy.
The astronomers favored this scenario based on the information they gathered from using the radio telescopes. Soon after the initial observations of the merger site, the Earth’s annual trip around the Sun placed the object too close to the Sun in the sky for X-ray and visible-light telescopes to observe. For weeks, the radio telescopes were the only way to continue gathering data about the event.
“If the radio waves and X-rays both are coming from an expanding cocoon, we realized that our radio measurements meant that, when NASA’s Chandra X-ray Observatory could observe once again, it would find the X-rays, like the radio waves, had increased in strength,” Corsi said.
Mooley and his colleagues posted a paper with their radio measurements, their favored scenario for the event, and this prediction online on November 30. Chandra was scheduled to observe the object on December 2 and 6.
“On December 7, the Chandra results came out, and the X-ray emission had brightened just as we predicted,” said Gregg Hallinan, of Caltech.
“The agreement between the radio and X-ray data suggests that the X-rays are originating from the same outflow that’s producing the radio waves,” Mooley said.
“It was very exciting to see our prediction confirmed,” Hallinan said. He added, “An important implication of the cocoon model is that we should be able to see many more of these collisions by detecting their electromagnetic, not just their gravitational, waves.”
Mooley, Hallinan, Corsi, and their colleagues reported their findings in the scientific journal Nature.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.