Closest cosmic explosion challenges popular theory

Andrew Taylor waited nine years to catch a gamma-ray burst in action.

His patience finally paid off on August 29, 2019 when Taylor, a researcher at the German Electron Synchrotron, and his teammates on the H.E.S.S. Collaboration caught one of these bright events in action. And it wasn’t anything like he’d anticipated. For one, it didn’t behave anything like the models said they “should.”

But that, in its own way, was a blessing.

“It might sound odd to you to not see what you expect to see, but that's normally the most exciting science,” Taylor tells Inverse. “I don’t think I want to live in a textbook universe.”

The gamma-ray burst came from the direction of the constellation of Eridanus in the southern hemisphere and seemed to take place only 1 billion light years from Earth, making it the closest gamma-ray burst observed so far.

Its proximity allowed the researchers to study it in great detail — and along the way found challenges to their idea of how gamma-ray bursts are produced.

Their observations are detailed in a study published Thursday in the journal Science.

WHAT’S NEW — The gamma-ray burst, dubbed GRB 190829A, was detected using the High Energy Stereoscopic System (H.E.S.S.) in Namibia.

Gamma-ray bursts are some of the most powerful and brightest explosions in the universe. They result from one of two cosmic scenarios:

• Supernovae, the death of a very massive star.
• The collision between two neutron stars, the collapsed cores of massive stars.

“When the gamma ray hits the atmosphere, it interacts with the atoms in the atmosphere, and particles are produced in the atmosphere, and those secondary particles make more particles and more particles and we get this cascade in the atmosphere,” Taylor says. “Then our telescopes see the radiation producing that cascade.”

Edna Ruiz-Velasco, a researcher at the Max Planck Institute for Nuclear Physics in Heidelberg, and co-author of the study, says that the proximity of the burst allowed them to study gamma ray bursts in detail.

“The radiation, the more it travels, the more it gets absorbed by photons that are floating around the universe,” Ruiz-Velasco tells Inverse. “The closer the subject is to us, the less there’s absorption.”

The team was able to observe the afterglow of the explosion for three days.

The afterglow is often much weaker than the initial explosion as the particle jet slams into surrounding gas, and previous hypotheses had suggested that they were produced by two different mechanisms.

But the observations showed that the high-energy gamma-ray emissions were surprisingly similar to the afterglow emissions and both faded in sync, suggesting that they must be produced by the same process.

“So it means that we've got something wrong in our models,” Taylor says. “We're not just seeing what we expect and then ticking a box. We have to reflect a bit about what assumptions went into our models, and test the assumptions to see what would have to change.”

HERE’S THE BACKGROUND — Gamma-ray bursts were first discovered in the 1960s, when military satellites caught a glimpse of the high-energy explosions while on the lookout for secret nuclear weapon testing on Earth.

Krista Smith, an assistant professor at Southern Methodist University’s physics department, says that at first gamma-ray bursts came as a complete surprise and no one knew what they were.

“It wasn't until the 90s that we started to observe the afterglows, and then we understood that these things were coming from other galaxies,” Smith tells Inverse.

In 1991, scientists launched the Compton Gamma Ray Observatory with the Burst and Transient Source Experiment (BATSE) which detected roughly one new gamma-ray burst every day.

Astronomers estimate that around 500 gamma-ray bursts are taking place at the same time across the universe.

“These things are probably going off more or less uniformly out in the universe, but the universe is enormous,” Smith, who was not involved in the study, says. “So the odds of us seeing a gamma-ray burst very nearby are quite low. These things happen mostly in distant galaxies because there simply are so many distant galaxies.”

WHY IT MATTERS — Gamma-ray bursts inform scientists of the original event that sparked them, improving their understanding of the interior structure of the star that exploded, or the state of the end of the life of a star.

“They're sort of simple if you think about it — it just throws off a blob of plasma at very high velocities,” Taylor says. “It's quite a simple laboratory because the physics behind it is simple, and yet it’s something that we can't test on Earth.”

WHAT’S NEXT — Although the window for observing the recent gamma-ray burst has long been closed, the researchers behind the new study are hoping to detect another nearby gamma-ray burst in order to test out new models.

“The next 10 years should be very exciting,” Taylor says. “As our instruments get better, we’ll get better at catching them.”

Abstract — Gamma-ray bursts (GRBs), which are bright flashes of gamma rays from extragalactic sources followed by fading afterglow emission, are associated with stellar core collapse events. We report the detection of very-high-energy (VHE) gamma rays from the afterglow of GRB 190829A, between 4 and 56 hours after the trigger, using the High Energy Stereoscopic System (H.E.S.S.). The low luminosity and redshift of GRB 190829A reduce both internal and external absorption, allowing determination of its intrinsic energy spectrum. Between energies of 0.18 and 3.3 tera–electron volts, this spectrum is described by a power law with photon index of 2.07 ± 0.09, similar to the x-ray spectrum. The x-ray and VHE gamma-ray light curves also show similar decay profiles. These similar characteristics in the x-ray and gamma-ray bands challenge GRB afterglow emission scenarios.