When a massive, dying star finally collapses into a black hole, it emits explosive jets of radiation known as a gamma ray burst that is second only to the Big Bang in terms of the energy produced.
Usually over in seconds, a minute at most, gamma ray bursts are extremely rare events that astronomers have spent years trying to observe and measure.
An international team of researchers led by astronomers from the University of Maryland, College Park finally succeeded last year when a mix of “preparation and luck” seemed to, well, align the stars in their favor.
“Every time until now we tried to catch these bursts, these explosions of the sky, by the time we arrived there with our telescope and start observing, the explosion is faded away,” said Eleonora Troja, an assistant research scientist in the University of Maryland’s astronomy department and a visiting research scientist at NASA’s Goddard Space Flight Center. “This time we managed to arrive on the spot while the burst was still ongoing.
“This is the first time that we managed to perform the measurements during the bursting phase. It’s the first time anybody has done something like this.”
Their precise measurements enabled the team to develop a detailed description of the gamma ray burst, published in the journal Nature in July. Though the findings need to be replicated, the study significantly advances the scientific understanding of how they work.
Researchers want to know more about gamma ray bursts to better understand the universe and its origins.
The events were first discovered in the 1960s when evidence of them was picked up by the United States’ network of Vela satellites, which were created to detect gamma rays from nuclear weapons tested in space as well as the atmosphere.
Gamma ray bursts, which release about as much energy in a moment as the sun will give off in its billions of years of life, have never been observed in the Milky Way galaxy, but some scientists theorize that one was responsible for an extinction event on Earth 440 million years ago that wiped out 70 percent of the then marine creatures. A direct hit from the high-energy stream of radiation from a burst as far as 6,500 light years away would deplete the protective ozone layer, potentially plunging the Earth into a new ice age.
The one captured by the Maryland-led team on June 25, 2016, called GRB160625B, occurred 9 billion light years away.
GRB160625B was observed by an array of telescopes around the world, including NASA’s Fermi Gamma-ray Space Telescope and telescopes in Russia and the Canary Islands.
The research team had worked to connect the global array of telescopes electronically so they would be alerted by a satellite when such an event was starting.
This gamma ray burst had a “precursor,” or an event that signals the burst is about to begin, something only about 20 percent of them have, that was detected by the Fermi space telescope. That made it easier for the other telescopes to quickly focus on the right part of space when the larger, longer gamma ray burst occurred three minutes later.
The event itself was extremely bright, Troja said, allowing the scientists to study it in great detail.
“We understood immediately this was an important event,” she said. “We were all excited. The first day I worked the entire day trying to coordinate the observations. It took some time to put all the pieces together because we had so much data. It took a few weeks before realizing that we got the right measurements to study the magnetic field.”
By studying polarization data, the team concluded that the explosive jets of radiation erupting at nearly the speed of light from the newly formed black hole were driven at first by a simultaneously produced magnetic field. But the field breaks down and matter takes over the gamma ray burst.
“This is very hard to find because it’s very hard to measure the magnetic field in these explosions,” Troja said.
Previously, scientists had theorized that either a magnetic field or matter controlled the energy streams, not both.
Daniel Perley, a senior lecturer at the Astrophysics Research Institute at the Liverpool John Moores University in England, cautioned that more gamma ray bursts would need to be studied to confirm the team’s findings.
Their finding about matter and magnetic field in the jets is “one of the things that’s a little harder to say for sure,” he said.
“It could be interpreted in other ways,” Perley said. “I think we need more observations like this before we can say something like that securely.”
The scientists also found what they believe is the physical mechanism that drives gamma ray bursts. Their data suggested that synchrotron radiation is behind the initial, “prompt” phase of the explosion, first picked by the Fermi telescope. Synchrotron radiation occurs when electrons are accelerated in a curved or spiral pathway.
“The data provide a unique window into what is powering [gamma ray bursts],” said Ryan J. Foley, an assistant professor in the Astronomy and Astrophysics Department at the University of California, Santa Cruz. “Many teams have tried to make this measurement, but this team did a great job convincingly measuring the polarization.”
Alexander Kutyrev, an associate research scientist in the University of Maryland Department of Astronomy and a co-author of the research paper, said scientists need to upgrade their technology to be able to consistently capture gamma ray bursts in the future. They need telescopes that can move more quickly when the events begin and that can detect polarization better.
“A fast reaction is key to the detection of the event,” he said. “You need to have a telescope basically sitting there looking at the sky all the time, ready to be moved.”
The massive explosions light up corners of the universe usually shrouded in darkness, Troja said. She said that’s one reason why it’s important to continue studying them, to learn more about the universe and its origins.
“They’re so bright, you can see them as lighthouses of the universe,” she said. “When there are these huge explosions we can see far distances from us. We want to use them as beacons to study the universe.”