After sky light came from nature, the source of neutron star collisions, as researchers used Hubble to capture the deepest optical images of first neutron star merger.
The last chapter of the historical discovery of the strong coupling of two neutron stars. in 2017 officially written. After the highly apparent eruption finally faded into the black, an international team led by Northwestern University relentlessly built its afterglow – the last piece of the event's life-cycle of desire. star after the collision, it also revealed secrets about the origin of the merger, the jet it created and the nature of the shorter gamma ray explosion.
"This is the deepest exposure we have ever received in this event in visible light," said Wen-fai Fong of Northwestern, who led 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 at Northwestern and a member of the CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), an endowed research center at Northwestern dedicated to advancing astrology studies with an emphasis on interdisciplinary connections.
Many scientists have considered the 2017 neutron-star merger, dubbed GW170817, as the LIGO & # 39; (Laser Interferometer Gravitational-Wave Observatory) the most important discovery to date. This is the first time astrophysicists have been able to detect two neutron stars colliding. Seen in both gravitational waves and electromagnetic light, it is also the first multi-messenger observation between these two forms of radiation.
The light from GW170817 is detected, partly, because it is close, making it brighter and easier to find. When the neutron stars collided, they released a kilonova – light 1,000 times brighter than a classical nova, resulting from the formation of heavy elements after fusion. But it is precisely this brightness that makes its afterglow – formed from a jet traveling near the speed of light, which excites the surrounding environment – very difficult to measure.
"For us to see successively, the kilonova had to move out of the way," Fong said. "Certainly enough, about 100 days after the merger, the kilonova disappeared, and the collapse took off. So did the decay, however, that it was left to the most sensitive telescope to capture it."
Hubble to rescue
Beginning in December 2017, NASA's Hubble Space Telescope detected visible light after merging and redefined the merger location 10 more times over a year and a half.
by March 2019, Fong's team used Hubble to capture the final image and the deepest observations to date. Over seven and a half hours, telescopes recorded a sky image from where the neutron-star collision took place. The resulting image – 584 days after neutron-star aggregation – was shown that the visible light coming from the merge had finally disappeared.
Next, Fong's team needed to remove the brightness of the surrounding galaxy, to isolate the extreme event. fuzzy afterglow.
"To accurately measure light from succession, you have to remove all other light," 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 very complex in structure."
Fong, Blanchard and their collaborators approached the challenge by using all 10 images, in which the kilonova disappeared and the afterglow remained. the final, deep Hubble image with no trace of the collision. The team overlaid their deep Hubble image with each of the 10 afterglow images. Then, using an algorithm, they completely subtracted – pixel by pixel – all the light from the Hubble image from the previous afterglow images.
The result: a final time series of images, showing the faint flow without light contamination from the background galaxy. Fully in line with model predictions, this is the most accurate imaging time of the GW170817 visible-light afterglow made to date.
"Light evolution fits perfectly with our theoretical models of jets," Fong says. "It also agrees perfectly with what radio and X-rays tell us."
Using Hubble's deep imagery of space, Fong and his collaborators have announced new insights about the GW170817 home. Perhaps most notably, they noticed that the area around the merger was not populated with star clusters.
"Previous studies have suggested that neutron star pairs can form and integrate within the dense environment of a globular cluster," Fong says. "Our observations show that is certainly not the case for this neutron star merger."
According to the new image, Fong also believes that distant, cosmic eruptions known as short gamma ray bursts are actually neutron star mergers – viewed only from a different angle Both make relativistic jets, which are like a fire hose of material traveling near the speed of light. Astrophysicists typically see jets from gamma rays exploding when they aim straight, such as staring directly at the fire hose. But the GW170817 is viewed from a 30-degree angle, which has never been done at optical wavelengths.
"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."
The study, "The optical conformation of GW170817: An off-axis jet structure and deep constraints on a globular cluster of origins," is primarily supported by National Science Foundation (grant 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