A cosmic explosion with an energy level equivalent to the output of a billion of suns went unnoticed by astronomers — until they caught the “echo” of this so-called gamma-ray burst, that is.
Considering all of our advances in astronomy and the fact that we have space telescopes with the ability to spot 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 the Big Bang can escape our notice. But often, they do. And this includes gamma-ray bursts (GRBs), which are launched when massive stars go supernova and birth black holes.
Despite their huge energy outputs, GRBs have to be orientated toward Earth to be observed. However, even when missed, these cosmic blasts 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 explosions of energy that follow the birth of a black hole due to stellar collapse or compact object mergers,” 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 towards us, the initial jet can go unseen. But later, as that jet ploughs through the surrounding medium, we can see the fading afterglow of the initial unseen explosion — called an ‘orphan afterglow.'”
These orphan afterglows have been predicted for decades, but have thus far proved elusive due to the fact that there is no bright, high-energy burst of radiation to signal their existence.
“This is the most convincing ‘orphan afterglow’ candidate to date and only the second candidate to be identified.” Gulati explained. “It is akin to an echo because we didn’t capture the initial blast but we did see the blast’s interaction with the environment. The GRB was missed because the initial 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 luminosity, energy and velocity with known explosive transients including GRBs, supernovas, and tidal disruption events (TDEs) in which black holes rip up and devour stars.
What really stood out to the team was the fact that ASKAP J005512-255834 brightened rapidly over a matter of weeks, continuing to emit radio waves for over 1,000 Earth-days as it faded. This sets the explosion apart because radio transients like this usually evolve very quickly or flare more than once. The explosion at hand did neither, instead resembling the echo of a single, very powerful explosion.
The scientists were able to track the origin of this explosive event to a small, bright galaxy located 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 within the heart of this galaxy, though, instead taking place in an off-center, dense, star-birthing region of it: likely 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 point of this GRB resides 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,” Gulati said. “Also, a potential star cluster at the transient location does not have the mass it would take to hold a supermassive black hole.”
Still, this doesn’t 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 ripped apart by an elusive intermediate mass black hole.
These are black holes with masses in between that of galaxy-dominating 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 that of our star born when massive stars die in supernova explosions.
Whatever the cause of this orphan afterglow, this discovery offers astronomers a template that could help discover more of these echoes of high-energy explosions.
“We now have a very well-studied object that allows us to check what we are looking for when something like this object shows up again,” Gulati concluded.
The team’s research was published on Tuesday (March 17) The Astrophysical Journal.
