Astronomers are trying to understand what caused a series of gamma ray bursts, or GRBs — the most powerful explosions in the universe.
These energetic bursts are typically unleashed by the incendiary death of a star, or when it is shredded by a black hole.
The observation of repeating bursts has never occurred before, given that the events that produce them are so catastrophic, said Antonio Martin-Carrillo, an assistant professor in astrophysics at the University College Dublin in Ireland. But in July, several bursts were detected in a single day.
“(It’s) unlike any other seen in 50-years of GRB observations,” Martin-Carrillo said in a statement. He is the co-lead author of a study based on observations of the event published August 29 in The Astrophysical Journal Letters.
Energetic gamma-ray bursts typically last milliseconds to minutes, but in this case, the signal repeated three times over 24 hours, making them aggregately 100 to 1,000 times longer than most single GRBs, said co-lead study author Andrew Levan, an astronomer at Radboud University in the Netherlands.
The exact reason for the bursts remains unknown, but astronomers have working theories. Zeroing in on what caused them could change the way researchers understand the death of stars.
“Is this event really rare so that we’ve only seen one of them in 50 years, or is it the tip of the iceberg where we might find many more of them if we look carefully?” Levan said. “Because gamma-ray bursts come from the catastrophic deaths of stars, this might really tell us a lot more about some of the most extreme ways that stars end their lives.”
On July 2, NASA’s Fermi Gamma-ray Space Telescope detected the succession of four gamma-ray bursts from a similar region of the sky, drawing the attention of astronomers who wondered if they could all be linked to the same source, said Martin-Carrillo. Further analysis later determined that three of the four bursts were related.
Fermi was only able to provide a rough location for the origin of the bursts. But better positional data captured by an X-ray telescope called the Einstein Probe — a mission undertaken by the Chinese Academy of Sciences, the European Space Agency and the Max Planck Institute for Extraterrestrial Physics — enabled astronomers to determine where to begin pointing their ground-based telescopes. The Einstein Probe had detected X-ray activity the day before the bursts, which was discovered retroactively.
Following up on the Einstein Probe’s data, NASA’s Neil Gehrels Swift Observatory’s X-ray instrument also provided more accurate location information to help orient telescopes in the right direction.
The research team was able to determine that the GRBs occurred outside our galaxy by tracing the radiation they emitted using the European Southern Observatory’s Very Large Telescope and its HAWK-I camera in Chile. Further confirmation of its extragalactic origin came from images taken by the Hubble Space Telescope.
The team suspects the host galaxy is a few billion light-years away, but research to determine the exact galaxy and distance is ongoing.
“Knowing the distance will help us considerably understand the true nature of this source,” Martin-Carrillo said.
Continued observations with other telescopes, such as the James Webb Space Telescope, could refine the measurements and ultimately determine the root cause of the repeated bursts.
Currently, the authors have considered two possible causes for the repeating bursts. One is an unusual type of supernova explosion that lasted much longer than those that typically release gamma rays, Levan said.
“If this is a massive star,” he said, “it is a collapse unlike anything we have ever witnessed before.”
Another potential cause is a white dwarf — a dead star as heavy as our sun but condensed to the size of Earth — being ripped apart by a rare intermediate-mass black hole.
“It’s either one of a kind in half a century, or it’s from a small group of events,” said Dr. Eric Burns, associate professor of physics and astronomy at Louisiana State University.
Burns was not involved in the new study, but he is part of a research group that intends to make a strong assertion in a study next month about what caused the event.
The recent series of bursts are why so many astronomers have dedicated time to carrying out diverse telescope observations to follow-up on the event, said Brendan O’Connor, a McWilliams Fellow at Carnegie Mellon University. O’Connor did not participate in this study but is leading his own research on the accompanying X-ray data.
Burns and O’Connor are coordinating and sharing what they find with one another, while formally working on separate results.
The new study presents critical information that solidifies an extragalactic origin for the event, O’Connor said.
“They provide a great summary of the likely culprits for producing this extremely long duration event,” O’Connor wrote in an email. “However, the precise nature of this event is still an open question and it is not cleanly described by any of the usual suspects.”
Understanding cosmic lighthouses
All the research about the gamma-ray burst event comes at a crucial time for those who study these energetic phenomena.
Burns runs a consortium called the InterPlanetary Network made up of a dozen satellites that identify where gamma-ray bursts originate.
“I think part of the reason that we haven’t seen events like this before is that we haven’t had the right instruments,” Burns said. “What we need is (to build) a new gamma-ray burst monitor that has the deep sensitivity we need but placing it far from Earth.”
Specifically, the monitor would orbit at L2, or Lagrange point 2, about 1 million miles (1.5 million kilometers) from Earth, where the Webb telescope is located. This vantage point is ideal because it enables telescopes to have an unimpeded view of the universe, unlike Hubble, which moves in and out of Earth’s shadow every 90 minutes.
The Astro2020 Decadal Survey, released in November 2021, identifies scientific priorities for the decade ahead, focusing on unlocking the secrets of the universe. One of its recommendations was for higher sensitivity gamma-ray telescopes to observe how space varies over time, Burns said.
Burns and his colleagues designed a mission to do just that, and won a NASA competition three years ago, but it wasn’t funded due to budget issues. The team is still pursuing the mission, but it’s unclear if it will happen under the current administration, Burns said.
“NASA is in a precarious position at the moment where they’re just trying to keep their current plans running,” he said, “so I’m not sure what the timeline will be.”
The InterPlanetary Network relies on a dozen satellites, and at the moment, Burns doesn’t know if they will be shut off before Congress passes a budget. The astronomy community depends on the network to highlight intriguing signals that warrant further observation, Burns added.
“None of us can plan what proposals to put in for, what telescopes to use, because we don’t know what we’ll even be finding anymore,” he said.
Studying and understanding gamma-ray bursts provides insight into the universe, Levan said. Within seconds, gamma-ray bursts can create as much energy as the sun does over the course of its ultimately 10 billion-year lifetime, he added. Currently, the sun is an estimated 4.6 billion years old.
“They are really bright, and so act as a lighthouse to the distant Universe saying ‘look here,’” Levan wrote in an email. “We can use them to study really distant objects.”
Gamma-ray bursts are like having an extreme laboratory in space unlike anything that can be built on Earth, Martin-Carrillo said. They can be used to test electromagnetism, particle physics and Albert Einstein’s theory of relativity.
“Their huge energy output means that we can detect them across the full Universe,” he wrote in an email, “and thus, they can provide a unique window into the early universe and the first stars that were produced.”
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