Hubble captures the deepest optical image of the first collision between neutrons and stars

Afterglow highlights the nature, the origin of the neutron star collisions, while the researchers use Hubble to capture the deepest optical image of the first neutron star fusion .

The last chapter of the historical detection of the powerful fusion of two neutron stars in 2017 has been officially written. After the hugely brilliant gust of wind has finally gone black, an international team led by Northwestern University has painstakingly built its posterity, the last part of the life cycle of this famous event.

The resulting image is not only the deepest image of the neutron star After the collision to date, she also reveals secrets about the origins of the fusion, the jet she created and the nature of the shorter gamma rays.

"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."

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 North West, and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), a research center with specialists North West focused on advancing studies that emphasize interdisciplinary relationships.

Many scientists consider the 2017 neutron star fusion, dubbed GW170817, as LIGOThe gravitational wave laser interferometer observatory) the most important discovery to date. It was the first time that astrophysicists captured a collision of two neutron stars. Detected in both gravitational waves and the electromagnetic light, it was also the very first multi-messenger observation between these two forms of radiation.

The light from GW170817 was detected, in part, because it was nearby, which made it very bright and relatively easy to find. When neutron stars collided, they emitted kilonova light – 1,000 times greater than that of a conventional nova, resulting from the formation of heavy elements after fusion. But it is precisely this brightness that has made its reverberation – formed by a jet moving near the speed of light, striking the surrounding environment – so difficult to measure.

"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. "

Hubble to the rescue

As of December 2017, NASA's Hubble Space Telescope has detected the visible light of fusion and has revisited the location of the fusion ten more times in a year and a half.

At the end of March 2019, Fong's team used the Hubble to obtain the final image and the most profound observation to date. During the seven and a half hours, the telescope recorded an image of the sky from which the neutron star collision occurred. The resulting image showed – 584 days after the neutron star melting – that the visible light emanating from the fusion had finally disappeared.

Then, Fong's team needed to suppress the brightness of the surrounding galaxy in order to isolate the extremely pale light of this event.

"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."

Fong, Blanchard and their collaborators took up the challenge using the 10 images, in which the kilonova had disappeared and the post-glow persisting, as well as the last deep image of Hubble, without traces of the collision. The team superimposed their deep Hubble image on each of the 10 remanence images. Then, using an algorithm, they carefully subtracted – pixel by pixel – all the light from the Hubble image to previous remanence images.

The result: a last time series of images, showing the low remanence without light contamination of the galaxy in the background. Completely aligned with model predictions, this is the most accurate time series of images produced by GW170817's visible light.

"The change in brightness perfectly matches our theoretical models of jets," Fong said. "It also perfectly matches what radio and X-rays tell us."

Informative information

Thanks to the image of Hubble's Deep Space, Fong and his collaborators have collected new information on the birthplace of GW170817. Perhaps most strikingly, they noticed that the area around the merger was not densely populated with star clusters.

"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."

According to the new image, Fong also believes that far-off cosmic explosions known as "short bursts of gamma rays" are actually mergers of neutron stars viewed from a different angle. Both produce relativistic jets, which are like a fire hose material that travels at a speed close to that of light. Astrophysicists generally see gamma ray bursts when they are directed directly, as they directly fix the fire hose. But GW170817 was viewed at an angle of 30 degrees, which had never been done before in the optical wavelength.

"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."

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The study, "Optical Reverberation of GW170817: Structured Off-axis Spray and Deep Constraints on a Globular Cluster", was mainly funded by the National Science Foundation (price numbers AST-1814782 and AST-1909358) and NASA (numbers HST-GO-15606.001-A and SAO-G09-20058A).

DOI: 10.17909 / t9-6qez-fw41