In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo discovered gravitational waves coming from the confluence of two neutron stars. They named this signal GW170817. Two seconds after it was detected, a NASA Fermi satellite detected a gamma ray burst (GRB) called GRB170817A. In just a few minutes, telescopes and observatories around the world have become entrenched in the event.
The Hubble Space Telescope played a role in this historic discovery of a fusion of two neutron stars. Since December 2017, Hubble has detected the visible light from this merger and in the next year and a half has turned its powerful mirror in the same place more than 10 times. The result?
The deepest image of the subsequent radiance of this event and one full of scientific details.
"This is the deepest exposure we have ever taken from this event in visible light," says Wen-Northwestern Fai Fong, who is leading the research. "The deeper the image, the more information we can get."
In addition to providing a deep picture of the subsequent fusion of the merger, Hubble also revealed some unexpected secrets of the merger itself, the jet created by it and also some details about the nature of the short bursts of gamma rays.
For many scientists, GW170817 is LIGO's most important discovery to date. The discovery won the Breakthrough of the Year award in 2017 from Science magazine. Although much is said about collisions or mergers between two neutron stars, this is the first time astrophysicists have been able to observe one. Because they also observe it in both electromagnetic light and gravitational waves, it was also the first "multidirectional observation between these two forms of radiation," as the press release states.
Part of the reason was this. GW170817 is pretty close to Earth in astronomical terms: only 140 million light-years away in the elliptical galaxy NGC 4993. It was bright and easy to find.
The collision of two neutron stars triggered a kilo. They are caused when two neutron stars merge in this way or when a neutron star and a black hole merge. Kilonova is about 1000 times brighter than the classic new one, which appears in a binary star system when a white dwarf and its satellite merge. The exceptional brightness of a kilo is caused by the heavy elements formed after the merger, including gold.
The merger created a jet of material moving at near light speed, which made it difficult to see. Although the jet is being rammed into the surround material, it is what made the merger so bright and easy to see, it also overshadowed the event's subsequent radiance. To see the subsequent glow, astrophysicists had to be patient.
"In order to see the ensuing glow, a kilo must be moved off the road," Fong said. "Certainly, about 100 days after the merger, the kilowatt disappeared into oblivion and the subsequent glow took over. However, the subsequent glow was so weak that it was left to the most sensitive telescopes to capture. "
That's where the Hubble Space Telescope came from. In December 2017, Hubble saw visible light from the following merger light. Since March 2019, Hubble has visited the glow 10 more times. The final image was the deepest ever, with the scope of the noble space staring at the point where the merger occurred in 7.5 hours. From this image, astrophysicists knew that the visible light had finally disappeared, 584 days after the merging of the two neutron stars.
The subsequent illumination of the event was key and it faded. To see and study it, the team behind the study had to remove light from the surrounding galaxy, NGC 4993. Galactic light is complex and, by speaking, would "contaminate" the subsequent glow and worsen the results.
"To accurately measure the light from a subsequent glow, you have to take all the other light," says Peter Blanchard, a doctoral student at CIERA and the second author of the study. "The biggest culprit is light pollution from the galaxy, which is extremely complex in structure."
But now they had 10 Hubble images of the subsequent glow to work with. There was no kinone in these images and only the subsequent glow remains. In the final image, the subsequent glow also disappeared. They superimposed the final image on the other 10 after-light images and, using an algorithm, carefully removed all light from Hubble's earlier after-light images. Pixel by pixel.
In the end, they had a series of images over time showing only the subsequent glow without any pollution from the galaxy. The image agrees with the modeled forecasts, and is also the most accurate time series of images of the event's subsequent illumination.
"The evolution of brightness is completely in line with our theoretical jet models," Fong said. "He also completely agrees with what radio and X-rays tell us."
And what did they find in these images?
In the first place, the region where the neutron stars merge is not densely populated with clusters, ie.
"Previous studies have suggested that pairs of neutron stars can form and merge into the dense environment of a globular cluster," says Fong. "Our observations show that this is definitely not the case for this fusion of neutron stars."
Fong also believes that this work shed some light on the gamma ray bursts. She believes these distant explosions are actually fusions of neutron stars like GW170817. They all produce relativistic jets, according to Fong, just viewed from different angles.
Astrophysicists typically see these streams of gamma ray bursts from a different angle than GW170817, usually pointing. But the GW170817 was visible from a 30-degree angle. This has never before been seen in optical light.
"The GW170817 is the first time we have been able to see the jet off-axis," said Fong. "The new date range shows that the main difference between GW170817 and distant short-range beams is the viewing angle."
A document outlining these results will be published in a letter to the Astrophysical Journal this month. It is entitled "Optical Glare of GW170817: Out-of-Axis Structured and Deep Limitations for the Origin of a Ball Cluster."