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What makes gamma-ray bursts blink on and off?

The most energetic events in the universe have a strange habit of going silent mid-blast, stumping astrophysicists. But new simulations suggest a surprising answer.
Artist's impression of a gamma-ray burst
When a massive star collapses into a black hole, jets generated by the black hole during the collapse can power a gamma-ray burst.
NASA's Goddard Space Flight Center

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 CIERA 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 Gottlieb — 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.”

Artist's impression of a black hole with a wobbling disk and jet
If the accretion disk around a black hole begins to tilt and wobble, it can cause the jet it generates to also wobble, moving across the sky.

The tilting of the jet, Gottlieb says, has another interesting implication. Astronomers estimate how many long GRBs occur throughout the universe by calculating how many they see in a given area of sky and extrapolating to the areas they aren’t currently observing. “If we see one GRB per gigaparsec per year,” Gottlieb explains, where 1 gigaparsec is equal to about 3.3 billion light-years, “then overall there are about 100 [GRBs occurring], because we missed 99 of them.”

But previous estimates assume that GRB jets propagate along only one axis — that is, they’re pointing in only one direction of the sky and stay pointed that way. However, a wobbling jet covers about 10 times as much area. So instead of assuming there are 100 long GRBs out there for every one astronomers observe, the estimate shrinks to around 10. “That means these events are much [rarer] than we previously thought,” Gottlieb says.

Finally, the researchers also discovered details about another poorly understood aspect of long GRBs: their emission system. The two current prevailing theories are that GRB emission is the result of either magnetic or non-magnetic processes inside the jet. “What we found is that the GRB jet is neither magnetized nor un-magnetized,” Gottlieb says. “So it means all physical processes, both magnetic and non-magnetic, should all be considered.”

More to ponder

As always, there are caveats to the results. One of the major ones, Gottlieb says, is that their work couldn’t consider neutrinos, which carry huge amounts of energy out of supernovae. Including them would have required adding yet another layer of physics to their already highly complex simulations.

Another important caveat is that the team’s simulations begin once the black hole has already formed, but its birth could also affect the jets it produces. The team hopes to investigate these added complications with future work.

Though more work can always be done, the team has opened some new and exciting avenues into long GRBs. According to Gottlieb, it’s already proving useful in studies of other stellar phenomena. One of the most exciting is investigating gravitational waves created from the formation of stellar jets. Analytic models of GRB jets suggest that if they produce gravitational waves, the waves are at frequencies too low for most current gravitational-wave detectors to pick up. But Gottlieb says the material inside the star that’s agitated by the jet might generate detectable gravitational waves.

Ultimately, however, the team is proud of the novel work they have done in producing what is, to the best of their knowledge, a first-of-its-kind simulation of long GRBs. And by learning more about the jets that produce GRBs, scientists can effectively see what is happening deep within these stars as they collapse, peering into a place otherwise completely invisible and unknown.



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