Astrophysicists from Northwestern University have developed an innovative simulation of an entire evolution of a jet formed by a collapsing star. It’s the first-ever full 3D simulation of the jet’s evolution — from its birth near the black hole to its emission after escaping from the collapsing star, or “collapsar.”
The jets generate gamma ray bursts (GRB), which are the most energetic and luminous events in the universe since the Big Bang. Researchers say the new simulation shows these gamma ray bursts are rarer than previously thought.
“These jets are the most powerful events in the universe,” says study author Ore Gottlieb, an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences, in a statement. “Previous studies have tried to understand how they work, but those 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 very beginning — from its birth by a black hole — without assuming anything about the jet’s structure. We followed the jet from the black hole all the way to the emission site and found processes that have been overlooked in previous studies.”
Gamma ray bursts emerge when a massive star’s core collapses under its own gravity to form a black hole. The jet launches into the collapsing star after energizing when gas falls into the rotating black hole. The jet then generates a bright gamma ray burst after breaking free from the star.
“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,” explains Gottlieb. “In other words, if the black hole is the size of a beach ball, the jet needs to expand over the entire size of France before it can produce a GRB.”
Gottlieb’s simulation showed something different than from previous studies. The jet’s disk tilts when falling material from the star falls onto it. The jet then wobbles inside the collapsing star as it struggles to realign with its original trajectory.
The wobbling causes gamma ray bursts to blink. Since the jet’s emissions beam away from Earth, telescopes cannot observe it.
“Emissions from GRBs is always irregular,” says Gottlieb. “We see spikes in emission and then a quiescent time that lasts for a few seconds or more. The entire duration of a GRB is about one minute, so these quiescent times are a non-negligible fraction of the total duration. Previous models were not able to explain where these quiescent times were coming from. This wobbling naturally gives an explanation to that phenomenon. We observe the jet when it’s pointing 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.”
Previous studies predicted that about 1% of collapsing stars produce gamma ray bursts, but Gottlieb believes it’s much rarer than that. Gottlieb believes gamma ray bursts are 10 times more observable than previously thought, meaning astrophysicists are missing 10 times fewer gamma ray bursts.
“The idea is that we observe GRBs on the sky in a certain rate, and we want to learn about the true rate of GRBs in the universe,” says Gottlieb. “The observed and true rates are different because we can only see the GRBs that are pointing at us. That means we need to assume something about the angle that these jets cover on the sky, in order to infer the true rate of GRBs. That is, what fraction of GRBs we are missing. Wobbling increases the number of detectable GRBs, so the correction from the observed to true rate is smaller. If we miss fewer GRBs, then there are fewer GRBs overall in the sky.”
Gottlieb reports “most of the jets either fail to be launched at all or never succeed in escaping from the collapsar to produce a GRB.” However, they remain buried inside.
The study was published in The Astrophysical Journal Letters.