In 2017, LIGO (Observatory of gravitational waves with laser interferometer) and Virgo detected gravitational waves coming from the fusion of two neutron stars. They named this signal GW170817. Two seconds after detecting it, NASA's Fermi satellite detected a GRB (GRB) surge that is known as GRB170817A. In a few minutes, telescopes and observatories from around the world participated in the event.
The Hubble Space Telescope played a role in this historical detection of the fusion of two neutron stars. As of December 2017, Hubble detected the visible light from this merger and, over the next 18 months, turned its powerful mirror in the same spot more than 10 times. The result?
The most profound picture of the glow of this event and a host of scientific details.
"This is the deepest exposure of this event to the visible light we have ever had," said Wen-fai Fong, of the Northwest, who led the research. "The deeper the image, the more we can get information."
In addition to providing a deep image of the afterglow of the fusion, Hubble also revealed unexpected secrets about the fusion itself, the jet she created, and some details about the nature of short gamma-ray bursts.
For many scientists, the GW170817 is LIGO's most important discovery to date. This discovery won the 2017 Breakthrough Award in the journal Science. Although much has been said about collisions or mergers between two neutron stars, it was the first time that astrophysicists could observe one. As they also observed both in electromagnetic light and in gravitational waves, it was also the first "multi-messenger observation between these two forms of radiation", as it is said in A press release.
That's partly because of that. GW170817 is quite close to the Earth in astronomical terms: just 140 million light-years away from the elliptical galaxy NGC 4993. It was bright and easy to find.
The collision of the two neutron stars caused a kilonova. They are caused by the fusion of two neutron stars, or by the fusion of a neutron star and a black hole. A kilonova is about 1000 times brighter than a classic nova, which occurs in a binary star system when a white dwarf and his mate merge. The extreme brightness of a kilonova is caused by heavy elements formed after melting, including gold.
The fusion created a jet of material moving at a speed close to light that made the post-glow difficult to see. Although the bursting jet in the surround material was what made the fusion so brilliant and easy to see, it also masked the glow of the event. To see the rest of the light, the astrophysicists had to be patient.
"In order for us to see the reverb, the kilonova had to move away from the path," Fong said. "Quite surely, about 100 days after the merger, the kilonova fell into oblivion and posterity took over. The reverberation was so weak, however, leaving the most sensitive telescopes to capture it. "
This is where the Hubble Space Telescope came into play. In December 2017, Hubble saw the visible light of the fusion burst. From that point until March 2019, Hubble visited 10 times more remanence. The final image was the deepest to date, with the venerable "scope" looking at where the fusion took place for 7.5 hours. According to this image, astrophysicists knew that visible light had finally disappeared 584 days after the fusion of the two neutron stars.
The reverberation of the event was the key and she fainted. To see and study it, the study team had to remove the light from the surrounding galaxy, NGC 4993. The galactic light is complicated and, in a way, it would "infect" the aeration and would harm results. .
"To accurately measure the light of the subsequent light, you have to eliminate any other light," said Peter Blanchard, postdoctoral researcher at CIERA and second author of the study. "The main culprit is the luminous contamination of the galaxy, whose structure is extremely complicated."
But now they had 10 images of Hubble's post-light to work with. In these images, the kilonova had disappeared and only the glow remained. In the final image, the post-glow had also disappeared. They superimposed the final image on the remaining 10 images of the remanent and, with the help of an algorithm, they meticulously removed all light from previous Hubble images showing the remanence. Pixel by pixel.
In the end, they had a series of images in time, showing only the remanence without any contamination of the galaxy. The image matched the modeled predictions and was also the most accurate time series of the event's afterglow.
"The change in brightness perfectly matches our theoretical models of jets," Fong said. "It also perfectly matches what radio and X-rays tell us."
So what did they find in these pictures?
First, the region where the neutron stars were merged was not densely populated in clusters, which had already been predicted by previous studies.
"Previous studies have suggested that neutron star pairs can form and blend in with the dense environment of a globular cluster," Fong said. "Our observations show that this is certainly not the case for this fusion of neutron stars."
Fong also believes that this work has helped to better understand gamma-ray bursts. She thinks these far-off explosions are actually fusions of neutron stars like GW170817. They all produce relativistic jets, according to Fong, it's just that they are seen from different angles.
Astrophysicists generally see these jets from gamma ray bursts from a different angle than that of GW170817, usually from the front. But GW170817 was seen at an angle of 30 degrees. This had never been seen before in optical light.
"GW170817 is the first time we see the jet off-axis," said Fong. "The new time series indicates that the viewing angle is the main difference between GW170817 and short distant gamma-ray bursts."
An article exposing these results will be published in the journal Astrophysical Journal Letters this month. The title "GW170817 Optical Reverb: An Off-Axis Structured Stream and Deep Constraints on a Globular Cluster Origin" can be seen at the link above at arxiv.org.