Bizarre merger of neutron stars might be ‘cocooning’ itself in hot gas

19 Jan 2018

Image: Jurik Peter/Shutterstock

The merging of two massive neutron stars has understandably created bright emissions, but why it keeps getting brighter is a total mystery.

Logic should conclude that after the merging of two neutron stars, the reaction would cause a blinding flash that would then gradually decrease over time.

But, for one such instance 138m light years away, the complete opposite is happening: it’s getting brighter.

In a paper published to Astrophysical Journal Letters, a team of researchers using NASA’s orbiting Chandra X-ray Observatory revealed the puzzling findings after its gravitational waves, which were created after the merger, were sent rippling through the universe.

According to McGill University astrophysicist Daryl Haggard, a typical gamma-ray burst results in a jet emission that gets bright for a short time as it smashes into the surrounding space, but then fades as the system stops injecting energy into the outflow.

However, she added: “This one is different; it’s definitely not a simple, plain-Jane narrow jet.”

Neutron star collisions

This graphic shows the x-ray counterpart to the gravitational wave source GW170817, produced by the merger of two neutron stars. The left image is the sum of observations with NASA’s Chandra X-ray Observatory taken in late August and early September 2017, and the right image is the sum of Chandra observations taken in early December 2017. Image: NASA/CXC/McGill/J Ruan et al

So, what could be causing such an astronomical puzzle?

One possible answer sounds as strange as the event itself: the merger could have launched a jet that shock-heated the surrounding gaseous debris, creating a hot ‘cocoon’ around the jet that has glowed in x-rays and radio light for many months.

This would appear to agree with radio-wave data recorded last month by another team of scientists, which also found that the emissions from the collision continued to brighten over time.

‘It’s a gift that seems to keep on giving’

Frustratingly, for all of the astrophysicists involved, the merger entered ‘radio silence’ for three whole months because its point in the sky was too close to the sun during that period.

When it re-emerged, researcher John Ruan was more than surprised: “When the source emerged from that blind spot in the sky in early December, our Chandra team jumped at the chance to see what was going on.

“Sure enough, the afterglow turned out to be brighter in the x-ray wavelengths, just as it was in the radio.”

With a true scientific mystery on their hands, astrophysicists are now anxious to find a definite answer for the brightness. It could even lead to the dawn of a new era in astronomy as it marked the first observation of a cosmic event with both light waves and gravitational waves.

The study’s co-author, Melania Nynka, summed it up by saying: “For astrophysicists, it’s a gift that seems to keep on giving.”

Updated, 3.30pm, 19 January 2018: This article has been amended to reflect that Daryl Haggard was incorrectly referred to as he, instead of she.

Colm Gorey was a senior journalist with Silicon Republic