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Hubble Looks at the 2017 Kilonova Explosion About a dozen Times, Watching It Slowly



In 2017, LIGO (Laser-Interferometer Gravitational Wave Observatory) and Virgo observed gravitational waves coming from the merging of two neutron stars. They named the signal GW170817. Two seconds after it was detected, NASA's Fermi satellite detected a gamma ray burst (GRB) named GRB170817A. Within minutes, telescopes and observatories around the world honored the event.

The Hubble Space Telescope plays a role in the historical discovery of two neutron stars. Beginning in December 2017, Hubble saw the visible light from this integration, and over the next year and a half it turned off strong glass in the same location 10 times. The result?

The deepest image of this event, and a chock full of scientific detail.

"This is the deepest exposure we have had to this event in visible light," said Northwestern's Wen-fai Fong, who led the research. "The deeper the image, the more information we can get."

On August 17, 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer both observed gravitational waves from collisions between two neutron stars. Within 12 hours observers identified the source of the event inside the lenticular galaxy NGC 4993, shown in this image gathered with the NASA / ESA Hubble Space Telescope. The associated stellar flare, a kilonova, is clearly seen in Hubble observations. This is the first time the optical counterpart has been seen in a gravitational wave event. Hubble saw the kilonova gradually fade over six days, as shown in these observations taken between 22 and 28 August (insets). Credit: NASA and ESA. Acknowledgments: A.J. Levan (U. Warwick), N.R. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI).

In addition to providing an in-depth image of the integration sequence, Hubble also revealed some unexpected secrets of integration, the jet it created, and also some details of the nature of short gamma ray explosion.

To many scientists, GW170817 has been the most important discovery of LIGO to date. Discovery won the Breakthrough of the Year Award in 2017 from the journal Science. Although collisions or fusion between two neutron stars have been discussed, this is the first time one of the astrophysicists has observed it. Because they also observed it in both electromagnetic light and in gravitational waves, it was also the first "multi-messenger observation between two forms of radiation," as stated in a press release.

The Laser Interferometer Gravitational-Wave Observatory consists of two detectors, this one in Livingston, La., And one near Hanford, Wash. Detectors use giant weapons in the shape of an "L" to measure small ripples on the fabric of the universe. Credit: Caltech / MIT / LIGO Lab

Part of this event occurred. GW170817 is relatively close to Earth in astronomical terms: 140 million light years away from the elliptical galaxy NGC 4993. It is bright and easy to find.

The collision of two neutron stars caused a kilonova. They are caused when two neutron stars combine like this, or when a neutron star and a black hole merge. A kilonova is approximately 1000 times brighter than a classical nova, which occurs in a binary star system when a white dwarf and merge it. The intense brightness of a kilonova is due to the heavy elements that are formed after fusion, including gold.

The fusion created a jet of material traveling near the speed of light that made it difficult to see in succession. Although the jet collapsed around the material is what made an incorporation apparent, and easy to see, it also noticed the sequel. To see successively, astrophysicists need to be patient.

The observations of kilonova. Credit: P.K. Blanchard / E. Berger / Pan-STARRS / DECam.

"For us to see successively, the kilonova had to leave the road," Fong said. "Certainly enough, about 100 days after the merger, the kilonova disappeared, and the collapse was taken. Only then was it weakened, leaving it with the most sensitive telescope to capture it."

Where to enter the Hubble Space Telescope. In December 2017, Hubble saw the visible light from after the merger. From then until March 2019 Hubble revisited the afterglow 10 more times. The final image is the deepest, with a respectable coverage of the space where the integration took place within 7.5 hours. From this image, astrophysicists know that the visible light has finally disappeared, 584 days after the two neutron stars were merged.

The blow to the event is key, and it is weak. To see this and study it, the team behind the study must remove the light from the surrounding galaxy, NGC 4993. The galactic light is complex, and in a way it is said to "affect" the stopping and avoiding results.

"To accurately measure light from succession, you have to do everything else," says Peter Blanchard, a fellow postdoctoral at CIERA and second author of the study. "The biggest culprit is the contamination of light from space, which is extremely complex in structure."

But they have 10 Hubble pictures in succession. In these pictures, the kilonova was lost and only the dwellings remained. In the final image, the afterglow is also lost. They overlaid the final image with the other 10 sequential images, and using an algorithm they began removing all the light from the previous Hubble images showing the sequence. Pixel by pixel.

  The square box where the afterglow is followed by neutron star integration. After 584 days, it was gone. Image Credit: Fong et al, 2019.
The square box where the afterglow is followed by neutron star integration. After 584 days, it was gone. Image Image: Fong et al, 2019.

Finally they have a series of images over time, showing only the losses without any contamination from space. The image agrees with model predictions, and is also the most accurate time series of post-event images.

"Light evolution fits perfectly with our theoretical model of jets," says Fong. "It also agrees perfectly with what radio and X-rays tell us."

So what do they find in these pictures?

First of all the places where neutron stars were assembled were not filled with clusters. something that previous studies have predicted should happen.

"Previous studies have suggested that neutron star pairs can form and aggregate within the dense environment of a globular cluster," Fong says. "Our observations show that is not really the case for this neutron star merger."

Fong also thought that this work had mitigated gamma ray explosions. He thinks that distant explosions are actually neutron star mergers like GW170817. They all make relativistic jets, according to Fong, only seeing them from different angles.

Gamma-ray (GRB) explosions are powerful flashing intense gamma-rays lasting from less than a second to several minutes. They released an enormous amount of energy this short time making them the most powerful event in the Universe. During the explosion, two jets of very fast moving material passed, as illustrated in this illustration by the artist. If a jet happens to be aimed at Earth, we see a brief but powerful gamma-ray explosion. Credit: ESO / A. Roquette

Astrophysicists typically see these jets from gamma rays exploding from a different angle than GW170817, usually headed. But GW170817 was seen from an angle of 30 degrees. That has never been seen in optical light.

"GW170817 is the first time we have seen the jet & # 39; off-axis, & # 39;" said Fong. "The new time series indicate that the main difference between GW170817 and far-short gamma-ray bursts is the viewing angle."

A paper summarizing the results will be published in the Astrophysical Journal Letters this month. It is titled "The optical afterglow of GW170817: An off-axis structured jet and deep constraints on a globular cluster structure." This can be found in the above link at arxiv.org.

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