Astrophysicists have developed the first 3D simulation of the full evolution of a jet from its birth by a rotating black hole until its emission away from a collapsing star.
Simulations show that as the star collapses, its material falls onto the disk orbiting the black hole. This falling material tilts the disc, thus tilting the flux that oscillates as it struggles to return to its original trajectory.
The wobbly plane explains the longstanding mystery of why gamma-ray bursts flash and shows that these bursts are much rarer than previously thought.
Since these jets generate gamma-ray bursts (GRBs) – the most energetic and brightest events in the universe since the Big Bang – simulations have shed light on these strange, intense bursts of light. Their new findings include an explanation of the long-running question of why quiet moments are mysteriously punctuated — a flicker between powerful emissions and an eerily quiet stillness. New simulations also show that GRBs are much rarer than previously thought.
The new study was published June 29 in Astrophysical Journal Letters. It represents the first full 3D simulation of the plane’s full evolution – from its birth near a black hole to its emission after escaping from a collapsing star. The new model is also the highest fidelity simulation ever of a large jet aircraft.
“These jets are the most powerful events in the universe,” said Ore Gottlieb of Northwestern University, who led the study. “Previous studies have tried to understand how it works, but those studies were limited in computational power and many assumptions had to be included. We were able to model the entire evolution of the jet from the beginning – since its birth by a black hole – without assuming anything about the airframe. We followed the flow from the black hole all the way to the emission site and found processes that were overlooked in previous studies.”
Gottlieb is a Rothschild Fellow at the Northwestern Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The paper was co-authored with CIERA member Sasha Chekovskoye, associate professor of physics and astronomy at the Weinberg College of Arts and Sciences at Northwestern.
weird wobbly
The brightest phenomenon in the universe, GRBs appear when the core of a massive star collapses under its gravity to form a black hole. When gas falls into a rotating black hole, it activates—and shoots a jet into the collapsing star. The jet hits the star until it eventually escapes, accelerating at speeds approaching the speed of light. After liberation from the star, a bright GRB is born.
“The jet generates a GRB when it reaches about 30 times the size of a star — or a million times the size of a black hole,” Gottlieb said. “In other words, if the black hole was the size of a beach ball, the jet would need to span the entire area of France before it can produce a GRB.”
Due to the magnitude of this scale, previous simulations were unable to model the full evolution of the aircraft’s birth and subsequent flight. Using assumptions, all previous studies have found that a jet propagates along one axis and never deviates from that axis.
But Gottlieb’s simulation showed something completely different. When a star collapses into a black hole, material from that star falls onto the magnetized disk of gas that orbits the black hole. Falling matter causes the disc to tilt, which in turn tilts the flow. As the plane struggles to realign its original course, it wobbles within the collapse.
This oscillation provides a new explanation for why GRBs flash. During quiet moments, the jet doesn’t stop — its rays shoot away from Earth, so telescopes simply can’t observe it.
“Emissions from gaseous recycling gases are always erratic,” Gottlieb said. “We see spikes in the emission and then a quiet time that lasts for a few seconds or more. The entire GRB has a duration of about 1 minute, so these quiet times are a significant portion of the total duration. Previous models have not been able to explain where these quiet times came from. This oscillation naturally gives an explanation for this phenomenon. We notice the plane when it points at us. But when the plane is swinging away from us, we can’t see its emission. This is part of Einstein’s theory of relativity.”
Rare becomes rarer
These oscillating jets also provide new insights into the rate and nature of GRBs. Although previous studies estimated that about 1 percent of collapses produce GRBs, Gottlieb believes that GRBs are actually much rarer.
If the plane were forced to move along one axis, it would only cover a thin slice of the sky – limiting its observability. But the oscillating nature of the plane means astrophysicists can observe GRBs in different directions, increasing the likelihood of their detection. According to Gottlieb’s calculations, GRBs can be observed 10 times more than previously thought, which means astrophysicists are missing 10 times less than previously thought.
“The idea is that we’re observing GRBs in the sky at a certain rate, and we want to know the true rate of GRBs in the universe,” Gottlieb explained. “Observed and real rates are different because we can only see GRBs that are pointing at us. This means that we need to assume something about the angle these jets cover in the sky, in order to infer the true rate of GRBs. So, what is the missing part of the GRBs. The fluctuation increases the number of detectable GRBs, so the correction from the observed rate to the true rate is smaller. If we lose fewer GRBs, there will be fewer GRBs overall in the sky. “
If true, as Gottlieb supposes, most aircraft either fail to launch at all or never succeed in escaping the crash to produce a GRB. Instead, they remained 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 indicates that the aircraft has a hybrid combination of magnetic and thermal energies, which produces GRB. In a major step forward in understanding the mechanisms that drive GRBs, this was the first time researchers had extrapolated the jet makeup of GRBs at the time of emission.
“Studying jets enables us to ‘see’ what’s happening in the depths of the star as it collapses,” Gottlieb said. “Other than that, it is difficult to know what happens in a collapsing star because light cannot escape from the core of the star. But we can learn from the jet emissions — the history of the aircraft and the information it carries from the systems that launch it.”
The main advance of the new simulation lies in part in its computational 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 that uses graphics processing units (GPUs) instead of CPUs. GPUs, which are very efficient in computer graphics and image processing, speed up the creation of images on the screen.