An unseen but extremely powerful cosmic burst has sent out sonic boom through the cosmos. This rare occurrence is an example of what astronomers call an orphan afterglow – the phenomenon astronomers have been chasing after for years, which now has been found although the explosion was totally unseen.
The colossal event was the result of the collapse of a massive star in a distant galaxy that is about 300 million light years away from Earth , and it is known as Gamma Ray Burst (GRB). It released an enormous amount of energy as the star collapsed to form either a neutron star, or a black hole.
GRBs are hard to come by, but how the astronomers managed to get hold of the sonic boom generated by one is astounding. Also, the jets generated by gamma rays are usually thin, so it’s tricky to detect them. But to add another layer of complexity, the jets weren’t pointed directly to us making them almost impossible to be noticed. But, here’s what they did.
Generally, the blast is accompanied by two jets of gamma rays dashing out from the collapsing star at nearly the speed of light in opposite directions. When these jets are directed towards earth, astronomers identify these powerful streams of energy as flash of gamma-rays.
But, as you know, this time the jets weren’t pointed at us. Instead, they ploughed into the gas surrounding the original star, and sent out an immense shockwave which corresponds to a sonic boom. Also, as gas around the star got heated up following the crash, it produced a glow that sent out radio energy throughout the universe.
“This is the first time anyone has been able to capture the sonic boom from an unseen GRB explosion,” said Bryan Gaensler, a co-author of the study in a statement. “In the past, people have either seen the explosion and then seen the boom, or on one or two occasions have seen the boom and then looked back and recovered the explosion after the fact. But here we have seen the boom, and yet the preceding explosion seems to be completely missing as viewed from Earth.”
Prior to discovery, Bryan Gaensler and his team made a side by side comparison of data from previous radio surveys of the sky to the data from the Very Large Array Sky Survey (VLASS). Looking into images from old maps of the sky, they found one radio source that VLASS today can no longer catch a glimpse on.
But digging into the radio source in other old data, they were able to pinpoint where the GBS originated from. It came from a relatively nearby galaxy, which back in the 1990s was the brightest explosions known.
The discovery is a significant step towards this ultimate quest to understanding the nature of Gamma Ray Bursts (GRB) and their jets. Also, taking into account that jets generated by GRBs are pointed in random directions relative to earth, the fractions we see depend on the magnitude of the jets.
“By comparing the number of ‘orphaned’ afterglows to those with GRBs preceding them, we can measure that fraction much more precisely than before,” explained Casey Law, the lead author of the paper.
The team says their findings could help them identify GRBs more accurately than ever with their state of the art astronomical radio surveys that can scan large portions of the sky. In the next 7 years, they hope to make three complete scans of 80 percent of the sky.
“This shows the exciting capabilities of the new generation of wide-field radio surveys. There are dramatic and dynamic explosions and flares happening out there, but we can only find them if we can constantly patrol the sky to see what’s changing,” said Gaensler.
Their findings have been being published in the Astrophysical Journal Letters.