On August 17, 2017, researchers observed a neutron star collision for the first time with both gravitational waves and “regular” light. The discovery immediately led to the confirmation of several phenomena, but for other mysteries it took a bit longer.
Among the latter, there’s the mystery of what the merger actually looked like. This has now been solved thanks to the radio observations of the event. The cosmic collision, also known as a kilonova, became visible to radio telescopes on September 2 and became brighter and brighter as time went by.
This behavior allowed researchers to better understand the merger. The simplest model of the event sees the merger creating jets of material that shoot into space, but not pointing exactly at Earth (thus the delay in detecting radio waves). However, this model requires the radio emissions to get weaker, when in fact the opposite is true.
As reported in Nature, the team needed a different model. One such model that was proposed in a different paper in October saw the jets pushing through a shell of debris. As it moves this away from the merger, the energy of the jet is transferred to a broad cocoon that emits it in radio waves and X-rays.
“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,” lead author Kunal Mooley, now a National Radio Astronomy Observatory (NRAO) Jansky Postdoctoral Fellow hosted by Caltech, said in a statement.
Once the researchers began to consider this the most likely model, they looked at a way to confirm it. If the cocoon emits both radio waves and X-rays, it should see a similar brightening in both types of light. The team used NASA’s X-ray observatory Chandra and found that the brightening of the X-ray emissions are indeed in line with the radio ones.
“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 stated.
“It was very exciting to see our prediction confirmed,” co-author Gregg Hallinan, also at Caltech, 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.”
The gravitational wave observatories, LIGO and Virgo, are currently undergoing a minor upgrade and will be back in operation in late 2018. It is expected that they will detect many more neutron star collisions.