Gamma ray bursts are some of the most powerful explosions in the universe, unleashing as much energy in mere seconds as the Sun will in its entire 10 billion year lifespan. Typically, they're produced by stars dying in a spectacular supernova — a rapid collapse that completely obliterates the stellar object.
But now, astronomers say they've detected a gamma ray burst that utterly defies explanation: it repeated multiple times over the course of a single day, as if the star somehow suffered back-to-back — and back — deaths.
"This event is unlike any other seen in 50-years of GRB observations," Antonio Martin-Carrillo, an astronomer at University College Dublin, Ireland, and coauthor of a new study published in The Astrophysical Journal Letters detailing the detection, said in a statement.
"GRBs are catastrophic events, so they are expected to go off just once because the source that produced them does not survive the dramatic explosion," he emphasized.
The signal's existence is so stunning that no known scenario can fully explain the GRB, according to a release from the European Southern Observatory. How can a star suffer an explosion that should completely obliterate it multiple times?
First, let's consider how stars die. When a massive one at least several times heavier than the Sun burns through all its fuel, it near-instantly collapses under its own gravity in a supernova, which blows the stars to smithereens and blasts its scattered remains into space.
But not all supernovas are the same. Some occur in binary star systems that contain a super-dense stellar remnant called a white dwarf and an ordinary star. If their orbits get too close, the white dwarf begins to siphon huge amounts of material from its companion. When enough of the stolen stuff accumulates on the white dwarf's surface, it triggers an epic thermonuclear explosion that destroys both stars.
Astronomers have spotted a star exploding multiple times before — in this same white dwarf scenario, in fact. Instead of instantly resulting in a supernova, however, the pairs' explosive death is preceded by a smaller blast caused by a cloud of some of the siphoned helium.
But that type of scenario wouldn't produce a double — let alone triple — GRB, since only one of those blasts is a supernova proper.
And so this latest explosion, designated GRB 250702B, appears to be altogether something different. Scientists first took notice when it was detected on July 2 by NASA's Fermi Gamma-ray Space Telescope. In all, Fermi detected three distinctive bursts. Later, the astronomers realized that another observatory, the x-ray telescope called the Einstein Probe, had picked up the activity almost an entire day earlier.
"This is 100-1000 times longer than most GRBs," said co-lead author and Radboud University astronomer Andrew Levan in the statement.
More than that, it also appeared to be periodic, Martin-Carrillo added, which has also never been seen before.
The original suspicion was that the signal was coming from somewhere in the Milky Way because it was close to the plane that our galaxy sits on. To follow up, the team used a wide-field infrared camera called the HAWK-I on the Very Large Telescope in Chile. That's when they found that the blast was likely coming outside our galaxy — perhaps billions of light years away, based on its size and brightness.
"What we found was considerably more exciting: the fact that this object is extragalactic means that it is considerably more powerful," Martin-Carrillo said.
The astronomers have a few tentative theories, but none are satisfying. The simplest is that this is some massive star imploding under its own weight "where some material kept powering the central engine," Martin-Carrillo said, but this would produce a GRB that lasts only a few seconds.
"If this is a massive star, it is a collapse unlike anything we have ever witnessed before," Levan said.
Another explanation is that this was produced by a tidal disruption event: the grisly carnage that unfolds as a star is devoured by a black hole.
"However, unlike more typical TDEs, to explain the properties of this explosion would require an unusual star being destroyed by an even more unusual black hole," Martin-Carrillo said.
That black hole, he speculated, might be what's known as an intermediate-mass black hole, which would fall somewhere between the mass of your typical black hole that forms from a star dying and the supermassive black holes that dictate entire galaxies. An intermediate-mass black hole has never been directly observed, leaving what astronomers call a "mass gap."
Either explanation would be a first, Martin-Carrillo said.
"We are still not sure what produced this or if we can ever really find out," he added, "but, with this research, we have made a huge step forward towards understanding this extremely unusual and exciting object."
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