The Afterglow Of The First Neutron Star Collision Opens A Brand New Window On The Event

The host galaxy. The square indicates the location of the neutron star collision. Wen-fai Fong/Northwestern University

In October 2017, researchers announced the first discovery of a collision between two neutron stars. The momentous detection was possible thanks to the combined power of the LIGO and Virgo gravitational observatories, as well as a large number of observatories across all wavelengths of light.

It was not just about catching the event in the act, but continuing observations for months after the merger. The immediate bright emission and afterglow revealed much about the nature of the event. Researchers will now publish the full analysis of the afterglow in visible light this month in The Astrophysical Journal Letters.

The merger, called GW170817, saw two neutron stars colliding, creating a black hole and throwing a huge amount of heavy elements and powerful electromagnetic radiation into space. The collision was a kilonova, 1,000 times brighter than a classical nova. The kilonova itself shined for over three months, transitioning afterward to a weaker afterglow.

The collision released a jet of material moving at almost the speed of light. This hit the surrounding interstellar medium, making it glow – the afterglow this research focused on. Such observations required the power and sensitivity of the Hubble Space Telescope.

“This is the deepest exposure we have ever taken of this event in visible light,” lead author Wen-fai Fong, assistant professor at Northwestern University, said in a statement. “The deeper the image, the more information we can obtain.”

The team had to work hard to remove every source of contamination, especially the one coming from the host galaxy. The work, however, paid off. The analysis showed that these two neutron stars were quite isolated from other objects in the galaxy.

“Previous studies have suggested that neutron star pairs can form and merge within the dense environment of a globular cluster,” Fong said. “Our observations show that’s definitely not the case for this neutron star merger.”

While the event produced plenty of data linking neutron star collisions to short gamma-ray bursts, there were also some unexpected findings, possibly due to the angle of view. Usually, we see gamma-ray bursts down the “barrel of the gun”, but this one was seen at a 30-degree angle.

“GW170817 is the first time we have been able to see the jet ‘off-axis,’” Fong said. “The new time-series indicates that the main difference between GW170817 and distant short gamma-ray bursts is the viewing angle.”

The incredible series of observations were taken over 10 sessions for a total of 7.5 hours. The analysis is in agreement with what has been seen with other wavelengths and predicted with theoretical models.

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