It has been more than three years since history was made with the first ever detection of colliding neutron stars. From 130 million light-years away, astronomers observed a brilliant flash of gamma radiation, heralded by rippling gravitational waves, as the two dead stars merged.
Since then, astronomers have been keeping an eye on the corner of space where the collision occurred, to see what happens in the aftermath of such a violent event. And, surprisingly, they found that it continued to glow in the X-ray spectrum long after the models predicted that that brightness would cease.
“We are entering a new phase in our understanding of neutron stars,” said astronomer Eleonora Troja of the University of Maryland.
The collision event, named GW170817, was first detected on August 17, 2017 as gravitational waves emanating from a section of the sky in the constellation Hydra, thanks to the LIGO-Virgo gravitational wave detectors.
Then, just 1.7 seconds later, two space observatories, NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory, detected an intense gamma-ray burst – the brightest and most energetic events on Earth. Universe – from the same sky area.
Nine days later, astronomers detected a glow spanning the electromagnetic spectrum from radio waves to X-rays. It was something new, never seen after a gamma-ray burst. Previously, all gamma-ray bursts had completely faded in minutes, as this glow challenged our understanding of the consequences of the gamma-ray burst.
This new afterglow emission has been interpreted as the result of a relativistic jet – that is, a jet moving at a significant percentage of the speed of light – from the kilonova’s explosion. As this jet expands into space, it generates its own shock wave, which emits light across the spectrum, from radio waves to X-rays.
The glow continued to grow in brightness, peaking at 160 days and then rapidly fading, but the X-radiation remained. It was last detected in March of this year by the Chandra X-ray Observatory, two and a half years after the first collision was detected; in subsequent May observations using the Australian Telescope Compact Array, the glow was below the detection threshold.
Troja and his team mapped the X-ray glow and found that the prolonged emission is still consistent with a relativistic jet, but they are not entirely sure what allowed it to continue for so long after the collision.
As GW170817 is the first event of its kind that we have been able to observe, there are likely things we don’t understand about how gamma-ray bursts and neutron star collisions happen.
“Having a collision so close to us that it is visible opens a window into the whole process that we rarely have access to,” Troja said. “There may be physical processes that we haven’t included in our models because they aren’t relevant in the early stages we’re most familiar with when forming jets.”
It is also possible that it is not the jet itself that causes the prolonged emission, but an expanding cloud of gas from the kilonova that followed, creating its own shockwave. If multiple shock waves occur at different times and behave differently, this could explain the differences in how the different wavelengths are washed out.
Or the X-rays may have been prolonged by what the researchers called “continuous injection of energy by a long-lasting central engine” – that whatever was left behind by the collision continued to emit X radiation.
We currently don’t have enough data to figure out which of these scenarios caused the continuous glow, but some things are clear. First, we don’t fully understand neutron star mergers. Something is missing in our models and only continuous observation and analysis will help to understand what it is.
Second, since this glow has only been identified in relation to a neutron star collision, it could be a signature we can use to identify other neutron star collisions that we may have missed. Its features could be used to search for similar emissions in X-ray data archives to discover these missed events.
More observations of the GW170817 patch of sky will begin in December of this year and astronomers aren’t sure what they’ll find. Either way, it will help us limit our understanding of the event.
“This could be the last breath of a historical source or the beginning of a new story, where the signal comes alive again in the future and could remain visible for decades or even centuries,” Troja said. “Whatever happens, this event is changing what we know about neutron star mergers and rewriting our models.”
The search should appear in the Royal Astronomical Society Monthly Notices, and is available on arXiv.