Afterglow sheds light on the nature, origin of neutron star collisions, as researchers use Hubble to capture the deepest optical image of a first neutron star fusion.
The last chapter of the historic discovery of the powerful merger of two neutron stars in 2017 has been officially written. After an extremely bright outburst, finally faded to black, an international team led by Northwestern University painstakingly constructs its next glow – the last short of the life-cycle of a famous event.
Not only the image obtained is the deepest image of a neutron star after the collision of the collision to date, it also reveals secrets about the origin of the merger, the jet it created and the nature of the shorter bursts of gamma rays.
"This is the deepest exposure we have taken from this event through visible light," said Wen-phi Fong of the Northwest, who leads the research. "The deeper the image, 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 and a member of CIERA (Center for Interdisciplinary Research and Astrophysics Research), a gifted research center in the Northwest, focused on advancing research with a focus on interdisciplinarians.
Many scientists consider the 2017 fusion of neutron stars, called GW170817, as the LIGO (Laser Interferometer Gravity Wave Observatory) as the most important discovery to date. For the first time, astrophysicists shot two neutron stars in a collision. Found both in gravitational waves and in electromagnetic light, it was also the first in the history to observe many messages between these two forms of radiation.
The light from GW170817 was discovered in part because it was nearby, i.e. which makes it very bright and relatively easy to find. When the neutron stars collided, they emitted a kilo – light 1000 times brighter than the classic new one, resulting in the formation of heavy elements after the merger. But it was this brightness that made the subsequent glow – formed by a jet traveling close to the speed of light inflating the environment – so difficult to measure.
"In order to see the ensuing glow, the kilo must have gone like this," said Fong. "Certainly, about 100 days after the merger, the kilowatt disappeared into oblivion and the subsequent glow took over. The subsequent glow was so weak that it left it on the most sensitive telescopes to capture it. "
Since December 2017, NASA's Hubble Space Telescope has detected visible light after fusion and revisited the location of the merger 10 more times in a year and a half.
At the end of March 2019, Fong's team used Hubble to get the final image and deepest observation to date. For seven and a half hours, the telescope recorded an image of the sky where the neutron star collided. The resulting image showed – 584 days after the fusion of a neutron star – the visible light emitted by the fusion finally disappeared.
The Fong team then had to remove the brightness of the surrounding galaxy to isolate the extremely faint afterlife event.
"To accurately measure the light from a subsequent glow, you must 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."
Fong, Blanchard and their associates tackle the challenge, using all 10 images in which the kinone has disappeared and the subsequent glow is left, as well as the final, deep image of Hubble without trace of the collision. The team superimposes their deep Hubble image on each of the 10 after-world images. Then, using an algorithm, they subtly extract – pixel by pixel – all the light from the Hubble image from the earlier images after illumination.
The result: a finite time series of images showing low afterglow without polluting light from a galaxy background. Fully aligned with the model's predictions, this is the most accurate time series of GW170817 images of the following light of light produced so far.
"The evolution of brightness perfectly matches our theoretical jet models," Fong said. "In addition, he fully agrees with what radio and X-rays tell us."
With the image of the Hubble outer space, Fong and her associates were gathering new insights into the home galaxy of GW170817. Perhaps most strikingly, they noticed that the confluence area was not densely populated with star clusters.
"Previous studies have shown that neutron star pairs can form and merge in 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."
According to the new image, Fong also believes that distant cosmic explosions, known as short bursts of gamma, are actually fusions of neutron stars – simply viewed from a different angle. Both produce relativistic jets, which are like a fire hose of material that travels close to the speed of light. Typically, astrophysicists see jets of gamma ray burst when directed directly by looking directly into the fire hose. But the GW170817 is viewed from a 30-degree angle, which has never before been done in optical wavelength.
"The GW170817 is the first time we have been able to see the jet off-axis," said Fong. "The new time period shows that the main difference between the GW170817 and the distant short gamma rays is the viewing angle."
The study, "Optical after-light of GW170817: Structured off-axis jet and deep constraints origin of the globular cluster, ”was supported primarily by the National Science Foundation (award numbers AST-1814782 and AST-1909358) and NASA (award numbers HST-GO-15606.001-A and SAO-G09-20058A).
doi: 10.17909 / t9-6qez-fw41