In 2017, LIGO (Laser Interferometer Gravitational Wave Observatory) and Virgo discovered gravitational waves resulting from the fusion of two neutron stars. They called the signal GW170817. Two seconds after detection, the NASA Fermi satellite detected a Gamma Ray Burst (GRB) named GRB170817A. Within minutes, telescopes and observatories around the world joined the event.
The Hubble Space Telescope played a role in this historic discovery of the fusion of two neutron stars. From December 2017, Hubble discovered the visible light of this merger and turned the strong mirror more than ten times in the same place over the next year and a half. The result?
The deepest picture of the afterglow of this event and a wealth of scientific details.
"This is the deepest exposure we've ever made of this event in the visible light," said Northwestern's Wen- Fai Fong, who led the research. "The deeper the image, the more information we can get."
Hubble not only revealed a deep picture of the afterglow of the merger, but also some unexpected secrets of the merger itself, the jet he created, and also some details of the kind of short gamma-ray bursts.
For many scientists, GW170817 is the most important discovery of LIGO so far. The discovery was honored in 2017 by the journal Science with the Breakthrough of the Year Award. Although much talked about collisions or mergers between two neutron stars, this was the first time astrophysicists could observe one. Since they also observed it in electromagnetic light and in gravitational waves, it was also the first "multi-messenger observation between these two forms of radiation," as stated in a press release.
This is partly due to circumstances. GW170817 is astronomically pretty close to Earth: only 140 million light-years away in the elliptical galaxy NGC 4993. It was bright and easy to find.
The collision of the two neutron stars caused a kilonova. They arise when two neutron stars merge in this way or when a neutron star and a black hole merge. A Kilonova is about 1000 times brighter than a classic Nova, which occurs in a binary system when a white dwarf and his companion come together. The extreme brightness of a Kilonova is caused by the heavy elements that form after the merger, including gold.
The merger produced a stream of material that moved at near the speed of light and made it hard to see the afterglow. Although the jet impacting the surround material made the fusion so bright and easy to see, it also darkened the afterglow of the event. To see the afterglow, astrophysicists had to be patient.
"So that we could see the afterglow, the Kilonova had to stay out of the way," said Fong. "About 100 days after the merger, the Kilonova had certainly been forgotten, and the afterglow took over. However, the afterglow was so weak, leaving the most sensitive telescopes to catch it.
This is where the Hubble Space Telescope came into play. In December 2017, Hubble saw the visible light from the afterglow of the merger. From then until March 2019, Hubble visited afterglow 10 more times. The final image was the deepest ever, with the venerable oscilloscope at the point where the merger took place for 7.5 hours. From this image, the astrophysicists knew that the visible light had finally disappeared 584 days after the fusion of the two neutron stars.
The afterglow of the event was decisive and weak. To see and study it, the team behind the study had to remove the light from the surrounding galaxy, NGC 4993. The galactic light is complicated and would effectively "infect" the afterglow and affect the results.
"To accurately measure the light of the afterglow, you have to take away all other light," said Peter Blanchard, postdoctoral fellow at CIERA and second author of the study. "The biggest culprit is the slight contamination of the galaxy, whose structure is extremely complicated."
Now, however, they were able to work with 10 Hubble images of afterglow. In these pictures the Kilonova had disappeared and only the afterglow was visible. In the last picture, the afterglow disappeared. They superimposed the other 10 images of afterglow with the final image and meticulously used an algorithm to remove all light from the previous Hubble images that showed afterglow. Pixel by pixel.
In the end, they had a series of images over time that showed only afterglow without any contamination by the galaxy. The picture agreed with the modeled predictions and is also the most accurate time series of pictures of afterglow of the event.
"The brightness development fits in perfectly with our theoretical models of jets," said Fong. "It also perfectly matches what the radio and X-rays tell us."
So, what did you find in these pictures? Something that earlier studies had predicted should be the case.
"Earlier studies have shown that neutron star pairs can form and fuse in the dense environment of a globular cluster," Fong said. "Our observations show that this is definitely not the case with this neutron star fusion."
Fong also believes that this work has shed some light on gamma-ray bursts. She thinks these distant explosions are in fact neutron star fusions like GW170817. They all produce relativistic jets, according to Fong, it's just that they are viewed from different angles.
Astrophysicists typically see these rays from a different angle than GW170817, usually head-on. But GW170817 was seen from an angle of 30 degrees. This had never been seen before with optical light.
"GW170817 is the first time we've been able to see the jet off-axis," Fong said. "The new time series shows that the main difference between GW170817 and distant short gamma-ray bursts is the viewing angle." It is entitled "The Optical Afterglow of GW170817: An Off-axis Patterned Beam and Deep Boundary Conditions for a Globular Cluster Origin." Like this Loading …