Falling stardust, wobbling rays explain blinking gamma-ray bursts

A team of astrophysicists led by Northwestern University has developed the first-ever full 3D simulation of a full evolution of a jet formed by a collapsing star or a ‘collapsar’.

Because these jets generate gamma-ray bursts (GRBs) — the most energetic and luminous events in the universe since the Big Bang — the simulations shed light on these peculiar, intense bursts of light. Their new findings provide an explanation for the long-standing question of why GRBs are mysteriously interrupted by quiet moments — blinking between powerful emissions and an eerily quiet silence. The new simulation also shows that GRBs are even rarer than previously thought.

The new study will be published June 29 in Astrophysical Journal Letters. It marks the first full 3D simulation of a jet’s full evolution – from its birth near the black hole to its emission after its escape from the collapsing star. The new model is also the highest-resolution simulation ever of a large-scale jet.

“These jets are the most powerful events in the universe,” said Northwestern’s Ore Gottlieb, who led the investigation. “Previous studies have tried to understand how they work, but those studies were limited by computing power and had to make many assumptions. We were able to model the entire evolution of the jet from the very beginning – from birth through a black hole – without assuming anything about the structure of the jet. We tracked the jet from the black hole to the emission site and found processes that had been overlooked in previous studies.”

Gottlieb is a Rothschild Fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He co-authored the article with CIERA member Sasha Tchekhovskoyan assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences

Strange wobbling

The most luminous phenomenon in the universe, GRBs form when the core of a massive star collapses under its own gravity, forming a black hole. As gas falls into the rotating black hole, it is energized and a jet is launched into the collapsing star. The jet strikes the star until it finally escapes and accelerates at speeds close to the speed of light. After breaking away from the star, the beam generates a bright GRB.

“The jet generates a GRB when it is 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 has to expand over all of France before it can produce a GRB.”

Due to the sheer size of this scale, previous simulations were unable to model the full evolution of the jet’s birth and subsequent journey. Using assumptions, all previous studies found that the jet propagates along one axis and never deviates from that axis.

But Gottlieb’s simulation showed something quite different. As the star collapses into a black hole, material from that star falls onto the disc of magnetized gas that swirls around the black hole. The falling material causes the disc to tilt, which in turn tilts the beam. As the jet struggles to realign with its original trajectory, it wobbles in the collapsesar.

This wobbling offers a new explanation for why GRBs blink. During the quiet moments, the jet doesn’t stop – its emission beams radiate away from Earth, so telescopes simply can’t observe it.

“Emission of GRBs is always erratic,” Gottlieb said. “We see peaks in emission and then a rest time that lasts a few seconds or longer. The total duration of a GRB is about one minute, so these rest times are a non-negligible fraction of the total duration. Previous models could not explain where these rests are. This wobbling naturally explains that phenomenon. We observe the jet when it points at us. But when the jet wobbles to point away from us, we cannot see its emission. This is part of Einstein’s theory of relativity.”

Rare is getting rarer

These wobbly jets also offer new insights into the speed and nature of GRBs. While previous studies estimate that about 1% of collapsars produce GRBs, Gottlieb believes GRBs are actually much rarer.

If the jet were restricted to move along one axis, it would cover only a thin sliver of the sky – limiting the chance to observe it. But the jet’s wobbly nature means astrophysicists can observe GRBs in different orientations, increasing the odds of seeing them. According to Gottlieb’s calculations, GRBs are 10 times more observable than previously thought, meaning astrophysicists are missing 10 times fewer GRBs than previously thought.

“The idea is that we observe GRBs in the sky at a certain speed, and we want to learn more about the actual speed of GRBs in the universe,” Gottlieb explains. “The observed and actual rates are different because we can only see the GRBs that are pointing at us. That means we have to assume something about the angle these jets cover in the sky, to derive the true speed of GRBs. That is “say, what proportion of the GRBs are we missing. Wobbling increases the number of detectable GRBs, so the correction from the observed to the actual speed is smaller. If we miss fewer GRBs, there are fewer GRBs in the air overall.”

If this is true, Gottlieb argues, then most jets either fail to launch at all or never manage to escape the collapsar to produce a GRB. Instead, they remain buried inside.

Mixed energy

The new simulations also revealed that some of the magnetic energy in the jets is partially converted into thermal energy. This suggests that the jet has a hybrid composition of magnetic and thermal energies, producing the GRB. This is an important step forward in understanding the mechanisms that drive GRBs, and is the first time researchers have inferred the jet composition of GRBs at the time of emission.

“By studying jets, we can ‘see’ what happens deep inside the star when it collapses,” Gottlieb said. “Otherwise it is difficult to learn what happens in a collapsed star, because light cannot escape from the interior of the stars. But we can learn from the jet emissions — the history of the jet and the information it carries from the systems that launch them.”

The major advances of the new simulation lie partly in the computing power. Using the code “H-AMR” on supercomputers at 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 (CPUs). Highly efficient at manipulating computer graphics and image processing, GPUs accelerate the creation of images on a screen.

The study, “Black hole to photosphere: 3D GRMHD simulations of collapsars reveal wobbly and hybrid composite beams,” was supported by the Fermi Cycle 14 Guest Investigator program (prize numbers 80NSSC22K0031, 80NSSC21K1951 and 80NSSC21K1938), the National Science Foundation (prize numbers AST – 2107839, AST-1815304, AST-1911080, AST-2031997, AST-2107806 and 2020747), NASA (price number 80NSSC18K0565), and the Israel Science Foundation (price number 1657/18).

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