Neutron stars’ afterglow brightens

In August 2017, scientists detected gravitational waves generated by the merging of two neutron stars. The afterglow from that stellar collision has continued to brighten over time.

Illustration of two merging neutron stars. Credit: NSF/LIGO/Sonoma State University/A. Simonnet

The afterglow from the merging of two neutron stars that generated gravitational waves in August 2017 has continued to brighten over time.

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Observations using NASA’s orbiting Chandra X-ray Observatory reveal the gamma ray burst generated by the collision is more complex than originally imagined.

Gravitational waves produced by the event were detected in August and announced the following October by the LIGO-Virgo observatory network.

It marked the first time visible light from the source event of a gravitational wave had been detected.

Gravitational waves are ripples in space time caused by moving masses, and have only recently been detected.

The merging of two neutron stars in August also generated a flash of radiation known as a gamma ray burst, and this is what astronomers have continued to observe.

“Usually when we see a short gamma-ray burst, the jet emission generated gets bright for a short time as it smashes into the surrounding medium – then fades as the system stops injecting energy into the outflow,” says McGill University astrophysicist Daryl Haggard, whose team led the study.

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

This image shows the X-ray counterpart to the gravitational wave source GW170817, produced by the merger of two neutron stars. Left shows how it appeared to the Chandra X-ray Observatory late August and early September 2017, and right, early December 2017.Credit: NASA/CXC/McGill/J.Ruan et al.
This image shows the X-ray counterpart to the gravitational wave source GW170817, produced by the merger of two neutron stars. Left shows how it appeared to the Chandra X-ray Observatory late August and early September 2017, and right, early December 2017. Credit: NASA/CXC/McGill/J.Ruan et al.

One possibility is that the merger generated a jet that heated the surrounding debris of gas, creating a bubble of heat that has glowed in X-rays and radio light for months afterwards.

A different team of scientists also reported radio wave emissions continuing to brighten over time.

“This neutron-star merger is unlike anything we’ve seen before,” says Melania Nynka, another McGill postdoctoral researcher.

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“For astrophysicists, it’s a gift that seems to keep on giving.”