Last August astronomers witnessed the long-awaited merger of two neutron stars. The event was among the most significant in the history of astronomy, studied using everything from gravitational wave observatories to gamma-ray telescopes, and the subsequent papers had 45,000 authors – a third of working astronomers. However, the event was no “one and done”. We are now seeing radiation produced by material spat out by the merger, but only after the pesky Sun stopped blocking our view.
Neutron stars are the remnants of supernova explosions and billions of times denser than the Earth. Stars large enough to achieve this state are often formed together, so neutron stars frequently orbit each other. These orbits gradually decay, causing the two objects to eventually merge. For a long time astronomers have speculated such mergers could explain some of the strange events we witness in the universe, such as gamma-ray bursts, but it was only when the two stars dubbed GW170817 collided we were able to verify these theories.
Theoretical physicists have produced many different models of the aftermath of such mergers, and some of these proposed we would be able to detect an afterglow. Unfortunately however, 10 days after the merger, the Earth's movement put GW170817 too close in the sky to the Sun for us to see anything, leaving the University of Warwick's Professor Andrew Levan and many others anxiously waiting for another chance. Exactly 110 days after the merger, the Hubble Space Telescope was again trained on the galaxy NGC 4993, where the event occurred.
The resulting observations, reported in Nature Astronomy, did indeed reveal the anticipated radiation, although it was brighter and bluer than predicted by models that anticipate neutron stars expel material in all directions. The paper proposes the light we are seeing is the result of being side-on to a jet of material beamed out from the merged stars traveling at relativistic speeds (that is, close enough to the speed of light for special relativity to have an effect).
The authors believe the merger produced a beam of radiation. "If we'd looked straight down this beam we'd have seen a really powerful burst of gamma-ray,” Levan said in a statement. Instead, our partially side-on position exposed us to visible light as the beam broadened to a cone.
The authors note that even though what we have seen rules out a number of theories, several remaining models of neutron star mergers all fit with what we have seen. They express confidence the “future evolution of the afterglow” will reveal which of these models best describes what is occurring. Fortunately, by the time GW170817 again appears from our vantage point to pass behind the Sun, we will have 265 days of data to analyze.