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Hubble captures the deepest optical image of the first neutron star collision



  Neutron star merger rendering

An artistic representation of the fusion of two neutron stars. Credit: NSF / LIGO / Sonoma State / A. Simonnet

Afterglow provides insight into the nature and origin of neutron star collisions, as researchers use Hubble to capture the deepest optical image of the first neutron star fusion.

The last chapter of the historical discovery of the strong fusion of two neutron stars in 2017 was officially written. After the extremely bright eruption finally faded to black, an international team led by Northwestern University constructed its afterglow – the last piece of the famous event's life cycle.

The resulting image is not only the deepest image of the Neutron The previous afterglow of the Star Collision also reveals secrets about the origins of fusion, the jet it produced, and the nature of the shorter gamma-ray bursts.

visible light, "said Wen-fai Fong of Northwestern, who led the research. "The deeper the picture, the more information we can get."

The study will be published this month in The Astrophysical Journal Letters. Fong is an Assistant Professor of Physics and Astronomy at the Weinberg College of Arts and Sciences in the Northwest and a member of the CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), a spiked research center in the Northwestern United States, which focuses on supporting studies with a focus on on interdisciplinary connections.

Many scientists consider the 2017 neutron-star fusion, called GW170817, to be the most important discovery of LIGO (Laser Interferometer Gravitational-Wave Observatory). It was the first time that astrophysicists intercepted two colliding neutron stars. It was detected in gravitational waves as well as in electromagnetic light and was the first multi-messenger observation between these two forms of radiation.

The light from GW170817 was partially detected because it was nearby. makes it very bright and relatively easy to find. When the neutron stars collided, they emitted a kilonova that was 1000 times brighter than a classic nova because heavy elements formed after the fusion. But it was this brightness that made the afterglow – created by a beam that moved near the speed of light and destroyed the environment – so difficult to measure.

"In order for us to see the afterglow, the Kilonova had to move out of the way," Fong said. "About 100 days after the merger, the Kilonova had certainly been forgotten, and the afterglow took over. However, the afterglow was so weak that it was left to the most sensitive telescopes to capture.

Hubble to the rescue

As of December 2017, NASA's Hubble Space Telescope discovered the apparent aftermath of the merger and visited the merger's position ten times over a year and a half.

  Afterglow against galaxy brightness

The box indicates where the fusion of neutron stars (GW170817) and its afterglow took place. The brightness of the event is overwhelmed by the brightness of the galaxy (center). Credit: Wen-fai Fong / Northwestern University

At the end of March 2019, Fong's team used the Hubble to get the final picture and the deepest observation so far. Within seven and a half hours, the telescope recorded a picture of the sky from which the neutron star collision took place. The resulting image showed – 584 days after the neutron-star fusion – that the visible light emanating from the fusion had finally disappeared.

Next, Fong's team had to remove the brightness of the surrounding galaxy to extremely isolate the event's faint afterglow.

"To accurately measure the light from 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."

Fong, Blanchard, and their staff tackled the challenge by using all 10 images in which the Kilonova had disappeared, and the afterglow remained the final, deep Hubble image with no trace of collision. The team overlaid each of the 10 afterglow images with its deep Hubble image. Then, using an algorithm, they meticulously-pixel by pixel-subtracted all light from the Hubble image from the previous afterglow images.

The result: A final timeline of images showing the faint afterglow without light pollution from the background galaxy. Fully matched to model predictions, this is the most accurate imaging time series of GW170817's visible afterglow produced so far.

"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."

Enlightening Information

Hubble's space image gave Fong and her colleagues new insights into the home galaxy of GW170817. Most striking was that the area around the merger was not densely populated with star clusters.

"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."

According to the new picture, Fong also believes that distant, cosmic explosions known as short gamma-ray bursts are actually neutron star fusions – angles seen only from a different perspective. Both produce relativistic jets, which are like a fire hose made of material that moves near the speed of light. Astrophysicists usually see gamma-rays when they are directly aimed, as if they are staring directly into the fire hose. However, GW170817 was considered from a 30 degree angle never before performed at the optical wavelength.

"GW170817 is the first time we have been able to see the beam 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."

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The study "The Optical Afterglow of GW170817: An Off-Axial Structure of Jet and Deep Constraints for a Globular Cluster Origin" was first and foremost approved by the National Science Foundation (award number AST-1814782 and AST-1909358) and NASA (HST-GO-15606.001-A and SAO-G09-20058A).

DOI: 10.17909 / t9-6qez-fw41


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