What makes gamma-ray bursts blink on and off?
The birth of a gamma-ray burst (GRB) is a stellar event. These incredibly violent blasts are the most energetic explosions in the universe. In just one second, a GRB can release more energy than our Sun has emitted over the course of its entire lifetime to date. GRBs also have a deadly reputation; they may have even played a role in one of Earth’s largest mass extinctions.
But for events so intense they can be seen across the universe, GRBs are difficult to study. This difficulty is further compounded by the environments in which researchers think they are born, which typically contain dense star-forming regions nearby. But new research published June 29 in The Astrophysical Journal Letters, produces the highest-resolution 3D GRB simulations to date and is a major step forward in understanding these mysterious blasts and why they act the way they do.
How to make a gamma-ray burst
GRBs come in two flavors: long and short. Long GRBs, those lasting anywhere from a second to several minutes, are released from so-called collapsars, when a quickly rotating massive star goes supernova and collapses into a black hole, ejecting jets of material along the way. These jets are what power the GRB.
Ore Gottlieb, a Sierra Fellow at Northwestern University in Evanston, Illinois, has made a career out of studying these high-energy astrophysical phenomena. “I’ve always been curious about stellar explosions,” he tells Astronomy. But beyond the explosions, Gottlieb hopes to learn more about the stars themselves. In particular, he wants to “understand how and why different stars explode in different ways.”
He had previously studied the jets emitted by collapsars by looking at the interactions between the GRB jets and the surrounding stellar material as the star is in the process of collapsing. His work used hydrodynamic simulations to model the interactions between the two. But “one thing that was always missing is: How do you start or launch the jet in the first place?” he says.
Probing the heart of the star itself required integrating relativistic physics into the already complex simulations. It was a daunting prospect.
A lot of area
Gottlieb says one of the biggest challenges they faced was the sheer differences in the scale involved in tracking a jet from inside a collapsar through outer space.
“The black hole is a million times smaller than the area where the GRB is emitted,” Gottlieb says. But by creating a model that could accurately resolve the jet across that vast space, the researchers were able to track its evolution from birth through emission.
Their approach was deceptively simple: “We took a star, put a black hole in the middle — assuming the star core has collapsed into a black hole already — and let the simulation run,” he says. While it sounds simple on paper, the simulations required were intense.
But the results were worth the effort according to Gottlieb, as the team came away with three key findings.
Wobbles and other weirdness
Long GRBs can last anywhere from one to hundreds of seconds. During that time, the intensity of the signal can be extremely variable. “It jumps rapidly … on timescales of maybe 10 milliseconds,” says Gottlieb.
But GRBs also have strange periods of quiescence that, before now, lacked an explanation. For anywhere from one to 10 seconds, the signal can “blink off,” dropping to zero and staying there before resuming its extremely rapid variability and then eventually petering out more slowly.
The new models provided a simple — but surprising, according to Gottleib — explanation for these quiescent periods: The jet isn’t gone, it’s simply just not pointed in our direction. As gas from the collapsing star falls onto the black hole, it lands on a swirling accretion disk of material around it. But the intense turmoil during the collapse causes the accretion disk to tilt, its angle relative to the black hole oscillating back and forth. Gottlieb says that since the jet emitted by the black hole and causing the GRB “is always perpendicular to the disk,” the unsteady disk causes the jet itself to wobble in turn. “So for a given observer, what he would see is that sometimes the emission is pointing towards the observer, and sometimes away, because of the wobbly motion of the jet.”