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X-Rays From Neutron Star Merger Still Persist 1,000 Days After Collision

KEY POINTS

  • In 2017, scientists detected X-rays following the collision of two neutron stars
  • It was the first time that X-rays were observed following a gamma ray burst
  • The X-rays were stil observable even 2 1/2 years after the collision
  • Scientists offer possible explanations for the X-ray emission’s strange behavior

A team of researchers can still detect lingering X-rays from a neutron star collision that happened 1,000 days prior. The prolonged X-ray emission continues to puzzle scientists.

It was on Aug. 17, 2017, when the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo first detected gravitational waves from the  merger of two neutron stars. Dubbed GW 170817, the event was observed by various telescopes from all over the world within hours of the first detection.

The initial burst was followed by a short-duration gamma ray-burst (GRB) and a slower kilonova. Nine days later, scientists detected an afterglow that was visible in the electromagnetic spectrum including X-rays, something that was never observed before.

Apart from the fact that it was the first time for X-rays to be detected following a GRB, the event continued to surprise scientists by emitting X-rays for longer than expected. In the case of GW 170817, the afterglow peaked after 160 days then rapidly faded away. But even after the afterglow faded, the X-ray emissions persist even 2 1/2 years after the merger.

“This behavior is markedly different from the garden-variety GRB afterglows, observed to fade within a few minutes since the burst,” the researchers of a new study wrote.

In the study published in the Monthly Notices of the Royal Astronomical Society, a team of researchers offered possible explanations for why the X-rays lasted for as long as it did.

It’s possible, the researchers say, that it was a new feature. Because GW 170817 was relatively close, it allowed scientists to observe this feature.

It’s also possible that the kilonova that followed the jet of gamma rays had its own afterglow. And because GW 170817 is close enough, the instruments captured and detected it as well.

“We saw the kilonova, so we know this gas cloud is there, and the X-rays from its shock wave may just be reaching us,” study co-author Geoffrey Ryan of the University of Maryland (UMD) Department of Astronomy said in the UMD news release. “But we need more data to understand if that’s what we’re seeing. If it is, it may give us a new tool, a signature of these events that we haven’t recognized before. That may help us find neutron star collisions in previous records of X-ray radiation.”

That said, exactly what is causing the persistent X-ray emissions remains unclear but further observations could help determine which of these possibilities are more likely to be true and,  perhaps even help detect other such neutron star mergers. 

“We are entering a new phase in our understanding of neutron stars, ” study lead Eleonora Troja of UMD said in the news release. “It may take years to find out the

Explosive neutron star collision is still emitting X-rays, puzzling astronomers

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Two neutron stars colliding, generating gravitational waves and a huge, bright jet.


Caltech/LIGO

When two neutron stars smashed into each other, about 130 million light-years from Earth, the universe lit up. The collision, between some of the densest objects in the cosmos, produced gravitational waves and a spattering of fireworks on Aug. 17, 2017. Dozens of telescopes on Earth captured the rare merger across different wavelengths of the electromagnetic spectrum. First, there came a burst of highly energetic gamma rays, followed by bursts of light and UV, radio and infrared signals.

About nine days after the collision, NASA’s Chandra observatory picked up an X-ray signal. According to our understanding of neutron stars, it should have faded away by now. 

But in a new study, published Monday in the journal Monthly Notices of the Royal Astronomical Society, researchers have studied the neutron-star-on-neutron-star impact, designated GW170817, and discovered that 1,000 days later, the X-ray signal was still detectable.

“We really don’t know what to expect from this point forward, because all our models were predicting no X-rays,” said Eleonora Troja, an astrophysicist at NASA’s Goddard Space Flight Center and lead author on the study, in a press release.

GW170817 is the first neutron star merger detected by the three gravitational wave observatories stationed on Earth. The triad of observatories were able to triangulate the position of the merger moments after it happened, allowing researchers to turn their telescopes to space and get a good look at the event. And it’s a violent one.

See also: These telescopes work with your phone to show exactly what’s in the sky 

Because we haven’t seen many neutron star collisions (only two have been recorded and confirmed so far), scientists have had to rely on models to predict the aftermath. For the most part, the models lined up with what was detected with GW170817. When two neutron stars collide, they release a jet of gamma rays and a huge blast of bright gas, known as a “kilonova.” Those events are transient — we see them for a few days or weeks and then they disappear. That was the case with GW170817.

But Chandra, NASA’s X-ray observatory, was still detecting X-rays at the location when it focused on the merger in February, two and a half years after it flared to life. The latest measurements show the signal has faded, but the specter of an X-ray burst is still visible and it’s a little brighter than models predicted. Why are these X-rays still visible? That’s a puzzle the researchers are trying to solve.

It may be there’s an additional component of the neutron star mergers models have not previously accounted for. Or perhaps the dynamics of the energy released in the aftermath of the collision are a little different to what we expect. An exciting possibility is that the remains of the merger represent an X-ray-emitting neutron star — though much

Neutron star collision continues to emit X-rays, puzzling astronomers

When two neutron stars smashed into each other, about 130 million light-years from Earth, the universe lit up. The collision, between some of the densest objects in the cosmos, produced gravitational waves and a spattering of fireworks on Aug. 17, 2017. Dozens of telescopes on Earth captured the rare merger across different wavelengths of the electromagnetic spectrum. First, there came a burst of highly energetic gamma rays, followed by bursts of light and UV, radio and infrared signals.



Two neutron stars colliding, generating gravitational waves and a huge, bright jet. Caltech/LIGO


© Provided by CNET
Two neutron stars colliding, generating gravitational waves and a huge, bright jet. Caltech/LIGO



Two neutron stars colliding, generating gravitational waves and a huge, bright jet.


© Caltech/LIGO

Two neutron stars colliding, generating gravitational waves and a huge, bright jet.


About nine days after the collision, NASA’s Chandra observatory picked up an X-ray signal. According to our understanding of neutron stars, it should have faded away by now. 

But in a new study, published Monday in the journal Monthly Notices of the Royal Astronomical Society, researchers have studied the neutron-star-on-neutron-star impact, designated GW170817, and discovered that 1,000 days later, the X-ray signal was still detectable.

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“We really don’t know what to expect from this point forward, because all our models were predicting no X-rays,” said Eleonora Troja, an astrophysicist at NASA’s Goddard Space Flight Center and lead author on the study, in a press release.

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GW170817 is the first neutron star merger detected by the three gravitational wave observatories stationed on Earth. The triad of observatories were able to triangulate the position of the merger moments after it happened, allowing researchers to turn their telescopes to space and get a good look at the event. And it’s a violent one.

See also: These telescopes work with your phone to show exactly what’s in the sky 

Because we haven’t seen many neutron star collisions (only two have been recorded and confirmed so far), scientists have had to rely on models to predict the aftermath. For the most part, the models lined up with what was detected with GW170817. When two neutron stars collide, they release a jet of gamma rays and a huge blast of bright gas, known as a “kilonova.” Those events are transient — we see them for a few days or weeks and then they disappear. That was the case with GW170817.

But Chandra, NASA’s X-ray observatory, was still detecting X-rays at the location when it focused on the merger in February, two and a half years after it flared to life. The latest measurements show the signal has faded, but the specter of an X-ray burst is still visible and it’s a little brighter than models predicted. Why are these X-rays still visible? That’s a puzzle the researchers are trying to solve.

It may be there’s an additional component of the neutron star mergers models have not previously accounted

Astronomers find x-rays lingering years after landmark neutron star collision

UMD astronomers find x-rays lingering years after landmark neutron star collision
Researchers have continuously monitored the radiation emanating from the first (and so far only) cosmic event detected in both gravitational waves and the entire spectrum of light. The neutron star collision detected on August 17, 2017, is seen in this image emanating from galaxy NGC 4993. New analysis provides possible explanations for X-rays that continued to radiate from the collision long after other radiation had faded and way past model predictions. Credit: E. Troja

It’s been three years since the landmark detection of a neutron star merger from gravitational waves. And since that day, an international team of researchers led by University of Maryland astronomer Eleonora Troja has been continuously monitoring the subsequent radiation emissions to provide the most complete picture of such an event.


Their analysis provides possible explanations for X-rays that continued to radiate from the collision long after models predicted they would stop. The study also reveals that current models of neutron stars and compact body collisions are missing important information. The research was published on October 12, 2020, in the journal Monthly Notices of the Royal Astronomical Society.

“We are entering a new phase in our understanding of neutron stars,” said Troja, an associate research scientist in UMD’s Department of Astronomy and lead author of the paper. “We really don’t know what to expect from this point forward, because all our models were predicting no X-rays and we were surprised to see them 1,000 days after the collision event was detected. It may take years to find out the answer to what is going on, but our research opens the door to many possibilities.

The neutron star merger that Troja’s team studied—GW170817—was first identified from gravitational waves detected by the Laser Interferometer Gravitational-wave Observatory and its counterpart Virgo on August 17, 2017. Within hours, telescopes around the world began observing electromagnetic radiation, including gamma rays and light emitted from the explosion. It was the first and only time astronomers were able to observe the radiation associated with gravity waves, although they long knew such radiation occurs. All other gravity waves observed to date have originated from events too weak and too far away for the radiation to be detected from Earth.

Seconds after GW170817 was detected, scientists recorded the initial jet of energy, known as a gamma ray burst, then the slower kilonova, a cloud of gas which burst forth behind the initial jet. Light from the kilonova lasted about three weeks and then faded. Meanwhile, nine days after the gravity wave was first detected, the telescopes observed something they’d not seen before: X-rays. Scientific models based on known astrophysics predicted that as the initial jet from a neutron star collision moves through interstellar space, it creates its own shockwave, which emits X-rays, radio waves and light. This is known as the afterglow. But such an afterglow had never been observed before. In this case, the afterglow peaked around 160 days after the gravity waves were detected and then rapidly faded away. But the X-rays