'Ghost' objects seen in the sky could be the aftermath of an explosion of a massive star, according to the University of California, Berkeley astronomer Casey Law.
The objects blazed into existence in the 1990s, but then faded out over the next 25 years.
Based on the extreme brightness of the radio source and the type of galaxy in which the flare-up occurred, Law believes that it was the afterglow of the explosion of a major star, which would have emitted an undetected long-duration gamma-ray burst.
Gamma-ray bursts, whose origins are still contentious, are among the most intense flashes in the universe because much of their energy is collimated into a tight beam, like that from a lighthouse.
The fact that these explosions are followed by a decades-long radio afterglow provides a way for astronomers to find the rest of these explosive events, not just those heralded by a gamma-ray burst.
Law emphasised the value of mining archived observational data in search of astronomical events
Law suggests that finding many more gamma-ray bursts will help resolve a major question in astronomy today: What are these massive stellar explosions that generate gamma-ray bursts, and what's left behind?
He believes in a theory that the explosion - whether preceded by the merger of two very large stars or neutron stars, or marking the death of a single, massive star - produces a rapidly spinning and highly magnetized neutron star, known as a magnetar.
The surrounding matter emits intense radio waves that slowly fade away, during which time the magnetar spins down and occasionally emits fast radio bursts, another mysterious "transient" event in the universe.
The radio source was a bright spot in a radio survey of the sky conducted in the early 1990s by the Very Large Array in New Mexico. It was on a par with the brightest radio sources in the universe: quasars and active galactic nuclei fueled by stars and gas falling into the massive black holes in the cores of galaxies.
"We thought, 'That was weird'," Law said. "Its peak brightness in the 1990s 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."
Pulling out information from these rich and diverse data sets are helping us do good science
He and his colleagues discovered 10 other sets of radio observations of that area of the sky, in the constellation Boötes, that allowed them to document the object's appearance and disappearance.
They concluded that the radio emissions first reached Earth in 1992 or 1993, although their first detection was around the source's peak brightness in 1994. They then faded away over the next 23 years.
The mystery object is located inside a dwarf galaxy 284 million light years from Earth that is still forming stars: a special environment that has previously been associated with fast radio bursts and, independently, gamma-ray bursts and the formation of magnetars.
Law concluded that the radio emissions from the dwarf galaxy were the 25-year-long afterglow from the explosion of a massive star, more than 40 times the mass of the sun, which would have produced a long gamma-ray burst that went undetected. Most gamma-ray bursts last less than a minute.
Law emphasised the value of mining archived observational data in search of astronomical events that pop up and fade out over years to decades, what his team refers to as "anti-transients."
"Part of the story is about how much of the sky is changing, even on this long timescale, and how hard it is to test that," he said. "It is also partly about the value of new data science techniques. Pulling out information from these rich and diverse data sets are helping us do good science."
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