The ray (in red) sways inside the collapse before exiting the photosphere. Credit: Ore Gottlieb / Northwestern University
A team of astrophysicists led by Northwestern University has developed the first complete 3D simulation of an entire evolution of a jet formed by a sinking star, or a “collapse.”
Because these jets generate gamma-ray bursts (GRBs), the most energetic and luminous events in the universe since the Big Bang, the simulations have illuminated these peculiar and intense bursts of light. His new findings include an explanation of the long-standing question of why GRBs are mysteriously marked by quiet moments: blinking between powerful emissions and a strangely silent stillness. The new simulation also shows that GRBs are even rarer than previously thought.
The new study will be published on June 29 at Letters from astrophysical journals. It marks the first full 3D simulation of the entire evolution of a jet, from its birth near the black hole to its emission after escaping the collapsing star. The new model is also the highest resolution simulation of a large-scale jet.
3D visualization of the lightning propagation and a close-up view of the tilt of the collapse disk, causing the jets to falter. Credit: Ore Gottlieb / Northwestern University
“These jets are the most powerful events in the universe,” said Ore Gottlieb of Northwestern, who led the study. “Previous studies have tried to understand how they work, but these studies were limited by computational power and had to include many assumptions. We were able to model the entire evolution of the jet from the beginning, from its birth through a black hole … without assuming anything about the structure of the jet. We have followed the jet from the black hole to the emission site and found processes that have been overlooked in previous studies. “
Gottlieb is a member of Rothschild at the Northwestern Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He co-authored the article with Sasha Tchekhovskoy, a member of CIERA, an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences.
After being released from the collapse, the beam generates a bright gamma-ray burst (GRB). Credit: Ore Gottlieb / Northwestern University
Strange balance
GRBs, the brightest phenomenon in the universe, arise when the core of a massive star collapses under its own gravity to form a black hole. When the gas falls into the rotating black hole, it is energized, throwing a ray at the collapsing star. The lightning strikes the star until it finally escapes from it, accelerating to speeds close to the speed of light. After being released from the star, the beam generates a bright GRB.
“The jet generates a GRB when it reaches about 30 times the size of the star, or a million times the size of the black hole,” Gottlieb said. “In other words, if the black hole is the size of a beach ball, the jet must expand to the full size of France before it can produce a GRB.”
Due to the enormity of this scale, previous simulations have not been able to model the complete evolution of the birth and the subsequent journey of the jet. Using assumptions, all previous studies found that the beam propagates along an axis and never deviates from that axis.
But Gottlieb’s simulation showed something very different. When the star sinks into a black hole, the material of that star falls on the magnetized gas disk that revolves around the black hole. The falling material causes the disk to tilt, which in turn tilts the jet. As the jet struggles to align with its original trajectory, it staggers within the collapse.
A close-up view of the disc (in orange) tilting, causing the jets (purple) to flutter. Credit: Ore Gottlieb / Northwestern University
This wobble provides a new explanation for why GRBs flicker. During quiet moments, the ray does not stop: its emission moves away from the Earth, so telescopes simply cannot observe it.
“The issuance of GRBs is always erratic,” Gottlieb said. “We see peaks in the broadcast and then a quince time that lasts a few seconds or more. The entire duration of a GRB is about a minute, so these quince times are a not insignificant fraction of the total duration. The previous models were not able to explain where these times of stillness came from.This hesitation naturally gives an explanation for this phenomenon.We observe the ray when it points at us.But when the ray swings to point us away from we can’t see its broadcast. That’s part of Einstein’s theory of relativity. “
Rare becomes rarer
These wobbly jets also provide new insights into the speed and nature of GRBs. Although previous studies estimated that about 1% of collapses produce GRBs, Gottlieb believes GRBs are actually much rarer.
If the lightning were forced to move along an axis, it would only cover a thin part of the sky, limiting the likelihood of observing it. But the faltering nature of the jet means that astrophysicists can observe GRBs in different orientations, increasing the likelihood of detecting them. According to Gottlieb’s calculations, GRBs are 10 times more observable than previously thought, meaning astrophysicists are missing 10 times less GRB than previously thought.
“The idea is that we observe GRBs in the sky at a certain rate and we want to know the true rate of GRBs in the universe,” Gottlieb explained. “The observed and actual rates are different because we can only see the GRBs pointing at us. That means we have to assume something about the angle these jets cover in the sky, to infer the actual GRB rate. That’s a say, what fraction of GRB do we lose. Hesitation increases the number of detectable GRBs, so the correction of the observed rate to true is less. If we lose less GRB, then there is less GRB in general in the sky. ” .
If this is true, says Gottlieb, then most planes do not launch at all or never manage to escape collapse to produce a GRB. Instead, they remain buried inside.
Mixed energy
The new simulations also revealed that part of the magnetic energy from the jets is partially converted into thermal energy. This suggests that lightning has a hybrid composition of magnetic and thermal energies, which produce GRB. In a major step forward in understanding the mechanisms that feed GRBs, this is the first time that researchers have deduced the ray composition of GRBs at the time of emission.
“Studying jets allows us to‘ see ’what happens inside the star when it collapses,” Gottlieb said. “Otherwise, it’s hard to learn what’s going on in a collapsed star because light can’t escape the stellar interior. But we can learn from the emission of lightning: the history of lightning and the information it carries from the star.” systems that launch them “.
The great advancement of the new simulation lies in part in its computational power. Using the code “H-AMR” on the supercomputers of the Oak Ridge Leadership Computing Facility in Oak Ridge, Tennessee, the researchers developed the new simulation, which uses graphics processing units (GPUs) instead of central processing units ( CPU). Extremely efficient for manipulating computer graphics and image processing, GPUs accelerate the creation of images on a screen.
The cocoons of dying stars could explain the fast blue optical transients. More information: Ore Gottlieb et al, Black Hole to Photosphere: 3D GRMHD Simulations of Collapsars Reveal Wobbling and Hybrid Composition Jets, The Astrophysical Journal Letters (2022). DOI: 10.3847 / 2041-8213 / ac7530 Provided by Northwestern University
Quote: Falling star dust, faltering jets explain the flickering gamma-ray bursts (2022, June 29) recovered on June 29, 2022
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