Gamma-ray bursts are some of the most powerful events in the universe. We can observe them due to brief emissions of gamma rays collimated in tight beams, which disappear as quickly as they appear. A complete understanding of this phenomenon is still lacking, but we could be getting some more info from the other end of the light spectrum: radio waves.
Astronomers have turned to archival data on bright radio signals that have slowly faded away over time. In a study published in the Astrophysical Journal Letters, they looked at one such source, FIRST J141918.9+39403. It was one of the brightest objects in the sky in the early 1990s and now, 23 years later, it is only detectable by the largest radio telescopes.
“We thought, ‘That was weird,’” lead author Casey Law, from the University of California, Berkeley, said in a statement. “Its peak brightness in the ‘90s was quite high, so it was a big, big change: about a factor of 50 decrease in brightness. We basically went through every radio survey, every radio dataset we could find, every archive in the world to piece together the story of what happened to this thing.”
Combining 10 sets of radio observations they were able to show that the signal faded over the course of 23 years. The team looked at possible causes for the phenomenon and the most likely explanation is that this is the product of a gamma-ray burst. Researchers think that a star roughly 40 times the mass of the Sun went supernova in a dwarf galaxy 284 million light-years from Earth. As the star collapsed into a neutron star or a black hole, it emitted a powerful jet of material and an associated gamma-ray. What was spotted in radio waves was the ghostly aftershock of that cataclysmic event.
“We believe we are the first to find evidence for gamma-ray bursts that could not be detected with a gamma-ray telescope,” Law added. “These are known as ‘orphan’ gamma-ray bursts, and many more such orphan GRBs are expected in new radio surveys that are now underway.”
Researchers will look for more of the orphan gamma-ray burst both in archival data and in new surveys. Work like this shows that past observations continue to play a key role in discoveries, and investment in astronomical observatories last far beyond their lifetime.