Neutron star collision: what happened next

Astronomers are slowly deciphering the aftermath of August’s big star crash. Andrew Masterson reports.

Different scenarios for the aftermath of the collision of two neutron stars: top-hat jets and cocoons.
Different scenarios for the aftermath of the collision of two neutron stars: top-hat jets and cocoons.

The detection earlier this year of a collision between two superdense neutron stars 130 million light-years from Earth has been justifiably called the science story of the year by many outlets.

In the best astronomical showbiz traditions, however, the collision and its immediate aftermath, detected and measured by the LIGO and VIRGO gravitational-wave observatories as well as several telescope arrays, captivated researchers and the public around the world – leaving unanswered a single, highly pertinent question: what happened next?

Now, observations from NASA's Chandra X-ray Observatory combined with others from the National Science Foundation's Karl G. Jansky Very Large Array (VLA), Australia’s Telescope Compact Array and the Giant Metrewave Radio Telescope in India provide at least some partial answers.

In a paper published in the journal Nature, a team led by Kunal Mooley from the National Radio Astronomy Observatory (NRAO) combine measurements of gamma rays, radio waves, X-rays and visible light from observations made since the event to narrow the range of possibilities.

In particular, they have been able to discount one of the leading theories arising from modelling work done before the collision was recorded.

Standard models agree that when the neutron stars merged, they would have collapsed, possibly transforming into a black hole. The collision would also have resulted in an immense explosion known as a kilonova, creating an enormous spherical shell of debris moving rapidly outwards.

Gravity produced by the black hole, however, would eventually balance the outwards thrust, resulting in a rapidly spinning disc of debris and dust.

So far, so uncontroversial, but analysis by Mooley and colleagues has confounded the next part of the modelling, which predicted that the spinning disc would produce a pair of narrow superfast jets of material spinning out from its poles.

Early measurements, taken days after the event was recorded in August, suggested that the jets were created as predicted. In particular, a time lag between radio and X-ray detections stemming from the event was possible evidence that one of the jets was pointed slightly away from earth.

However, under this model the jets, called “top hats”, would have to lack structure, and decrease in intensity quite quickly. Mooley’s team found that wasn’t happening.

“As we watched the radio emission strengthening, we realised that the explanation required a different model," says co-author Alessandra Corsi, of Texas Tech University in the US.

To make sense of the data, the researchers reached for a model published by researchers at Caltech in the US and Tel Aviv University in Israel.

In this set of calculations, the jets produced by the spinning disc never make it out of the expanding sphere created by the kilonova, but instead gather in material, producing a “cocoon” that absorbs the jets’ energy.

The cocoon model predicted that over time both radio waves and X-rays emanating from the collision site should increase in strength.

Ironically enough, as the researchers began taking readings across the spectrum to confirm or confound their theory, the Earth’s orbit around the sun meant that X-ray and visible light telescopes were temporarily unable to make observations.

Radio wave observations during the interregnum, however, revealed a strengthening signal. Based on this, the scientists made an online prediction that when the Chandra X-ray Observatory was next able to observe the collision aftermath, the results would be in agreement.

"On December 7, the Chandra results came out, and the X-ray emission had brightened just as we predicted," says co-author Gregg Hallinan, of Caltech. “It was very exciting to see our prediction confirmed.”

Mooley’s team was not the only group of astronomers awaiting the Chandra results. On the day they were announced, a second group, led by John Ruan of the McGill Space Institute and Department of Physics in Quebec, Canada, posted a paper on the pre-print server Arxiv.

The paper found that X-ray data indicated that “the outflow in these models may be either a cocoon shocked by the jet or dynamical ejecta from the merger”.

Ruan and colleagues, like Mooley’s team, found the results discounted the presence of “simple top-hat jet models”. However, they added that other “more advanced models of structured jets” could not yet be ruled out.

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Andrew Masterson is news editor of Cosmos.
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