Unseen was a cosmic explosion that had the power of a million Suns.
The most intense explosions in the universe leave almost no trace.
Although the original explosion cannot be seen, our observations capture its long-lasting echo as it ploughs through the surrounding environment.
We have found the most obvious example of one of the hidden explosions in a new study accepted for publication by The Astrophysical Journal: The radio afterglow from a powerful, unnoticed gamma ray burst.
Only an extremely rare, but difficult-to-detect class of intermediate-mass Black Holes can explain what we’ve seen.
We’re either watching the slow motion aftermath of an extreme and rare event that the universe can create.
We usually overlook the explosions
Gamma-ray blasts are powerful, short-lived jets of radiation. In seconds they can release the same amount of energy that the Sun would emit in its lifetime. When massive stars form black holes, they are the result.
We only see a small portion of the jets’ emissions that are directed at us. The initial flash is not visible when it’s directed away from you. All we see is a slowly dissipating afterglow.
A gamma ray burst animation showing narrow high-energy jets. NASA
These “orphan” afterglows of gamma ray bursts, which have been known for years but are still difficult to find, has proved extremely challenging.
Astronomers must search for thousands of square degree of sky without a flash of high energy to signal their arrival.
These cosmic explosions were easy to overlook and difficult to recognize when they did appear.
The ghost of the cosmic appears
We scanned large areas of the sky using ASKAP, a 36 antenna radio telescope located at Inyarrimanha Ilgari Bundara (Western Australia) to search for radio transients. These are astronomical objects which appear over a period from weeks to years and then change. In order to capture rare events, we were able to detect their radio emissions.
We noticed that a new radio source had appeared in the data of one of these large-field surveys (named ASKAPJ005512-255834).
The star brightened quickly, emitting 1032 Watts into the space. This is equivalent to radio power produced by billions of Suns. It then faded slowly with time.
Radiant radio glow detected by ASKAP in RACS.
The source is detected turning on in observations beginning 2022. It remains detectable after that for over 1,000 days. Emil Lenc
Its behavior immediately sets it apart. Radio transients usually either change rapidly or flare up repeatedly. The source was neither.
It acted more like the echoing echoes of an enormously powerful single explosion.
ASKAPJ005512-255834 is bright in radio waves, but leaves almost no sign at other wavelengths. No counterpart was visible in visible light, X-rays or other wavelengths.
Orphan afterglows are exactly what astronomers look for: a fading and widening light from a focused, tightly-focused cosmic jet. The jet was initially not pointed at Earth but became visible after slowing down.
The neighbourhood where you live is busy, but billions of light years away
The rare transient occurs in a bright, small galaxy located around 1.7 billion miles from Earth. This galaxy is a very irregular one and it’s actively forming new stars.
It makes for a perfect environment to witness extreme events like stellar collapse or stellar disruption.
On the right, we see the same radio source detected by the Giant Metrewave Radio Telescope (India) in the image below. The Giant Metrewave Radio Telescope, located in India, detected the same source on the left. Ashna Gupta
It is not in line with the nucleus of the galaxy. It appears instead to be within a compact region of star formation, perhaps a cluster of nuclear stars.
It raises questions as to what kind of environment can support such powerful cosmic events.
What else could it be?
ASKAP J005512-255834 was so strange that we needed to investigate to find out what could be.
The alternative explanations (which we ruled out), included stars, pulsars, and supernovae, were carefully scrutinized.
Only a black hole of intermediate mass can produce the radio behavior observed. This is a very rare type of black hole that lies between the stellar remnants in galaxies’ centres and supermassive ones.
This explanation is not completely ruled out, even though such events at radio wavelengths are extremely rare.
If confirmed, this would be the first instance of its type. This discovery is just as exciting as an orphaned gamma ray burst.
Radio waves reveal a hidden world
Is this a lucky discovery, or is it the first look at a previously hidden population? We simply did not have the means to find out until recently.
ASKAP is the strongest orphaned gamma ray burst AFTERGLOW yet discovered.
Our radio telescope was used to look for the long-lasting echo of an unknown explosion.
We hope that by using the same method, we can uncover more orphaned afterglows. They will finally have a home in our cosmic tale.
We may then be able build up a complete picture of the population of gamma ray bursts, even those who never flashed, but quietly linger in the radio skies.
