A cosmic explosion with an energy level equivalent to the output of a billion suns went unnoticed by astronomers – until they caught the “echo” of this so-called gamma-ray burst, that is.
Considering all our advances in astronomy and the fact that we have space telescopes with the ability to detect objects that existed up to 13.3 billion years ago, it may seem unlikely that some of the most powerful explosions in the cosmos since Big Bang may escape our attention. But often they do. And this includes gamma ray burst (GRBs), which are launched when massive stars exit supernova and childbirth black holes.
Despite their enormous energy output, GRBs must be oriented toward Earth to be observed. But even when missed, these cosmic explosions can be observed via their “echoes” as the impact bounces off surrounding gas and dust, resulting in a gradually fading afterglow. And the detection of the radio signal ASKAP J005512-255834 – thanks to the Australian SKA Pathfinder (ASKAP) radio telescope located in Western Australia – represents the most conclusive detection of such an explosive echo to date.
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“GRBs are powerful pencil-beam bursts of energy that follow the birth of a black hole due to stellar collapse or the merger of compact objects,” discovery team member Ashna Gulati of the University of Sydney told Space.com. “As these jets interact with the surrounding medium, they slow down and become spherical. If a GRB jet is not pointed at us, the first jet may go unseen. But later, as that jet plows through the surrounding medium, we can see the faint afterglow of the first invisible explosion—called an ‘orphan afterglow.’
These orphaned afterglows have been predicted for decades, but have so far proved elusive due to the fact that there is no bright, high-energy radiation to signal their existence.
“This is the most compelling ‘orphan afterglow’ candidate to date and only the second candidate identified.” Gulati explained. “It’s akin to an echo because we didn’t pick up the first explosion, but we saw the explosion’s interaction with its surroundings. The GRB was missed because the first jet was pointed away from us and because the jets are too narrow, so it could have just missed the detector.”
Cosmic detective work
The team identified ASKAP J005512-255834 as an orphan afterglow after comparing its brightness, energy and speed with known explosive transients, including GRBs, supernovae and tidal disruption events (TDEs) in which black holes tear up and devour stars.
What really stood out to the team was the fact that ASKAP J005512-255834 brightened rapidly over the course of a few weeks, continuing to emit radio waves for over 1,000 Earth days as it faded. This sets the burst apart because radio transients like this usually develop very quickly or flare more than once. The explosion at hand did neither, but instead resembled the echo of a single, very powerful explosion.
The researchers were able to trace the origin of this explosive event to a small, bright galaxy is around 1.7 light-years away. The galaxy has an irregular structure and is in the midst of intense star formation. The explosive event did not take place in the heart of this galaxy, but took place in an off-center, dense, star-forming region of it: probably a star cluster. That indicated to the team that the echo they saw could not be the result of a star being ripped apart by a supermassive black hole in a TDE.
“The origin of this GRB is in a chaotic galaxy, so most likely a stellar collapse occurred in a region of high star formation. The transient is not in the center of the galaxy where supermassive black holes reside,” said Gulati. “A potential star cluster in the transient location also does not have the mass it would take to host a supermassive black hole.”
Nevertheless, this does not completely rule out the possibility that the echo is the result of a TDE.
There is a possibility that the original explosive event involved a star being torn apart by an elusive intermediate-mass black hole.
These are black holes with masses in between the masses of galaxy-dominant supermassive black holes, with masses equivalent to millions, or even billions, of times that of the Sun, and stellar-mass black holes with masses up to hundreds of times the mass of our star that are born when massive stars die in supernova explosions.
Whatever the cause of this orphaned afterglow, this discovery offers astronomers a template that could help detect more of these echoes of high-energy explosions.
“We now have a very well-studied object that allows us to check what we’re looking for when something like this object reappears,” Gulati concluded.
The team’s research was published on Tuesday (March 17) The Astrophysical Journal.
