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A core-collapse supernova creates a black hole and emits a burst of gamma rays. |Image credit: Robert Lea (Created with Canva)
Astronomers didn’t notice a cosmic explosion with an energy level equivalent to the output of a billion suns until they caught an “echo” of this so-called gamma-ray burst.
Considering all the advances we’ve made in astronomy, and the fact that we have space telescopes capable of discovering objects that existed 13.3 billion years ago, some of the most powerful explosions in the universe since 2007 seem unlikely. big Bang can escape our notice. But usually, they do. This includes gamma ray burst (GRB), emitted when a massive star disappears supernova and birth black hole.
Despite their enormous energy output, gamma-ray bursts must be oriented Earth To be seen. However, even if missed, these cosmic explosions can be observed through their “echoes,” as the impact bounces off the surrounding gas and dust, causing the afterglow to fade away. Thanks to the Australian SKA Pathfinder (ASKAP) radio telescope in Western Australia, the detection of radio signal ASKAP J005512-255834 represents the most conclusive detection of such explosive echoes to date.
Ashna Gulati, a member of the discovery team at the University of Sydney, told Space.com: “GRBs are powerful pencil beam explosions of energy that occur after a black hole is formed due to the collapse of a star or the merger of dense objects.” “As these jets interact with the surrounding medium, they slow down and become spherical. If the GRB jet is not pointed at us, the initial jet may not be visible. But later, as the jet passes through the surrounding medium, we can see the faded afterglow of the initially invisible explosion – called the ‘orphan afterglow’.”
These orphan afterglows have been predicted for decades but have so far proven elusive because there are no bright bursts of high-energy radiation to signal their existence.
“This is the most convincing ‘Orphan Afterglow’ candidate to date and the only second candidate identified,” Gulati explained. “It’s similar to an echo in that we don’t capture the initial explosion, but we do see the interaction of the explosion with the environment. The gamma burst was missed because the initial jet was pointed away from us and the jet was too narrow, so it probably missed the detector.”
cosmic detective work
The team identified ASKAP J005512-255834 as an orphan afterglow after comparing its luminosity, energy and velocity to known explosion transients, including gamma bursts, supernovae and tidal disruption events (TDEs) in which black holes tear apart and swallow stars.
What really stood out to the team was that ASKAP J005512-255834 brightened rapidly over a few weeks and continued to emit radio waves for more than 1,000 Earth days while it faded. This makes explosions unique, because radio transients like these often evolve very quickly, or burst out more than once. The explosion in front of me was neither, but resembled the echo of a very powerful explosion.
Scientists were able to trace the origin of the explosion to a small, bright galaxy It is about 1.7 light-years away from us. The galaxy has an irregular structure and is undergoing intense star formation. However, the explosion did not occur at the center of this galaxy, but in an off-center, dense, star-forming region: possibly a star cluster. This suggested to the team that the echoes they saw could not be the result of a star being torn apart by a supermassive black hole in tidal expansion.
“The origin of this gamma burst was in a chaotic galaxy, so the stellar collapse likely occurred in a region of high star formation. The transient event was not located at the center of the galaxy where the supermassive black hole was located,” Gulati said. “In addition, the potential star cluster at the location of the transient did not have the mass required to host a supermassive black hole.”
Nonetheless, this does not completely rule out the possibility that the echo is the result of TDE.
Illustration of a tidal disruption event in which a black hole tears apart and consumes a star. |Image credit: Carl Knox – OzGrav, ARC Center of Excellence for Gravitational Wave Discovery, Swinburne University of Technology
It’s possible that the initial explosion involved a star being torn apart by an elusive intermediate-mass black hole.
The mass of these black holes ranges between the supermassive black holes that dominate galaxies, which are millions or even billions of times the mass of the Sun, and stellar-mass black holes, which are up to hundreds of times the mass of stars born when massive stars die in supernova explosions.
Whatever the cause of this orphan afterglow, the discovery provides astronomers with a template that could help discover more echoes of high-energy explosions.
“We now have a well-studied object that allows us to check what we are looking for when similar objects appear again,” Gulati concluded.
The team’s research was published on Tuesday (March 17) The Astrophysical Journal.
