Scientists have created the first 3D reconstruction of a jet's entire evolution, from its conception by a rotating black hole to its emission far from the collapsing star.
Simulation shows that as the star collapses, its material falls on the disk that swirls around the black hole; This falling material tilts the disk, and, in turn, tilts the jet, which wilts as it struggles to return to its original trajectory.
The wobbling jet clarifies the long-standing mystery of why Gamma-ray bursts blink and shows that these bursts are even rarer than previously assumed.
Simulations reveal why Gamma-ray Bursts are the most energetic and luminous events in the universe since the Big Bang. Their new findings answer a longstanding question why GRBs are mysteriously interrupted by quiet moments blinking between intense emissions and an eerie quietness.
The new analysis was published on June 29 in Astrophysical Journal Letters. It is the first full 3D simulation of a jet's whole evolution, from its conception near the black hole to its emission after exiting the collapsing star.
Ore Gottlieb of Northwestern University, who conducted the research, believes these jets are the most powerful objects in the universe. Previous investigations have attempted to explain how they work, but those were hampered by computational power and had to include many assumptions. We were able to reconstruct the whole evolution of the jet from the very beginning until its collision by a black hole, and we discovered 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). She coauthored the paper with CIERA member Sasha Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.
GRBs are formed when a massive star collapses beneath its own gravity to form a black hole. The jet punches the star until it emerges, speeding up at speeds comparable to light.
Gottlieb estimated that the jet would be capable of generating a GRB when it reaches a size of the star or a million times the size of the black hole. In other words, the jet would need to expand over the whole size of France before it may produce a GRB.
Due to the enormousness of this scale, previous simulations have been unable to reconstruct the entire evolution of the jets' birth and subsequent journey. Using assumptions, all previous studies demonstrated that the jet propagates along one axis and never deviates from that axis.
However, Gottlieb's simulation revealed something quite different. Material from that star falls onto the black hole's magnetic disk, which then tilts the jet, which then wiggles inside the collapsar.
This wobbling provides a new explanation for why GRBs blink. During the quiet moments, the jet does not stop emitting light away from Earthso telescopes.
Gottlieb noted that GRB emissions are always irregular. We see spikes in emission then a quiescent time that lasts for a few seconds or more. These quiescent times are a non-negligible fraction of the total duration of a GRB. We observe the jet when it is pointing toward us, but we cannot see its emission.
Rare becomes rarer.
These wobbly jets offer new insights into GRBs' rates and nature. Although previous studies believed that GRBs would only be produced in 1% of collapsars, Gottlieb believes that GRBs are actually quite rare.
If the jet were restricted to only a few spots on the sky, it would only cover a tiny section of the sky, decreasing the likelihood of seeing it. However, astrophysicists can observe GRBs at different orientations, increasing the likelihood of spotting them. According to Gottlieb's calculations, GRBs are ten times more observable than previously thought, meaning that astrophysicists are missing ten times as many GRBs as previously thought.
Gottlieb explained that we observe GRBs on the sky at a certain rate, and we want to learn more about their true rate in the universe. That means we must assume something about the direction that these jets are pointing at us in order to infer the true rate of GRBs. Generally speaking, fewer GRBs means less GRBs are visible in the sky.
If this is true, Gottlieb posits, then most jets either fail to be launched at all or never succeed in escaping from the collapsar to produce a GRB. Instead, they remain buried inside.
The new research has also revealed that some of the magnetic energy in the jets converts to thermal energy. This implies that the jet has a hybrid composition of magnetic and thermal energy, which produces the GRB. This is the first time researchers have studied the jet composition of GRBs at the time of emission.
Studying jets allows us to observe what happens deep inside the star as it collapses, according to Gottlieb. Otherwise, it is difficult to know what happens in a collapsed star because light cannot escape from the stellar interior. However, we can learn from the jet emission the jet's history and the information it contains in the systems that launch them.
The main advantage of the new simulation lies in its computational power. Researchers used the code H-AMR on supercomputers at the Oak Ridge Leadership Computing Facility in Oak Ridge, Tennessee, to create the new simulation, which instead uses central processing units (CPUs). GPUs are extremely powerful at manipulating computer graphics and image processing.