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The orphan burst flared and died out over a period of 25 years. Now it's time, its peak brightness in 1993 was more than 50 times what it is today. NRAO image.
Astronomers typically study objects that are visible after night, but like supernovas, Casey Law is scouring vast amounts of data in search of bright objects that disappear, never to be seen again.
That search turned on the first of the same things in the sky: in this case, an extremely bright source of radio broadcasts that blazed into existence in the 1990s and then faded out over next 25 years.
Based on the extreme brightness of the radio source and the type of galaxy in which the flare-up occurred, Law argues that it was the afterglow of the explosion of a massive star, which would have emitted an undetected long-lasting gamma-ray burst. . Gamma-ray bursts, whose origins are still contentious, are among the most intense flashes in the universe because much of their explosive energy is collimated into a tight beam, like that from a lighthouse.
"We believe we are the first to find evidence for gamma-ray bursts that could not be detected with a gamma-ray telescope," said Law, an assistant research astronomer in the Department of Astronomy at the University of California, Berkeley. "These are known as 'orphan' gamma-ray bursts, and many more such orphan GRBs are expected in new radio surveys that are now underway."
Gamma-ray bursts, such as last year gravitational waves from the merger of two neutron stars, are rarely seen because of the source of the gamma rays – a relativistic jet of material emerging from the explosive merger – must be pointing directly to Earth. Perhaps only one in 100 explosions can be seen from Earth by NASA's Fermi Gamma-ray Space Telescope, for example.
The fact that these explosions are followed by explosive events is not an easy one.
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 afterward?
Law favors the theory that the explosion – whether preceded by the merger of two stars or stars, or marking the death of a single, massive star – produces a rapidly spinning and highly magnetized star neutron, 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.
He commented on his findings and conclusions in an article recently accepted by the journal Astrophysical Journal Letters and accessible now on the arXiv server.
The value of archived data
The radio source (FIRST J141918.9 + 394036), nowhere to be found in the sky, but still detectable by large radio telescopes, 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 one of 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.
An artist's depiction of a gamma ray burst, emitted in oppositely directed beams after a massive stellar explosion. While the waves often miss Earth, these explosions create a telltale transient radio glow that can be detected. NRAO image.
"We thought, 'That was weird,'" Law said. "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 all went through every radio survey, every radio dataset we could find, every single archive in the world to piece together the story of what happened to this thing. "
He and his colleagues discovered 10 other sets of radio observations of that area of the sky, in the constellation Boötes, which allowed them to be the object of their appearance and disappearance.
They concluded that the radio emissions first reached Earth in 1992 or 1993, though their first detection was around the source in 1994. It then faded away over a period of 23 years. It was fainter in a 2010 survey and barely visible in 2015. It was invisible in a 2017 Very Large Array Sky Survey.
The mystery object is located inside a galaxy 284 million light years from Earth that is still forming stars, and independently, gamma-ray bursts and the formation of magnetars. This led to the fact that the radio broadcasts were the 25-year-long afterglow of the explosion of a massive star, perhaps more than 40 times the mass of the sun, which would have produced a long gamma-ray burst that went undetected. Most GRBs last less than a minute.
One theory is that the resulting magnetar, because of its high rotation rate and huge magnetic fields, emits periodic fast radio bursts – each just a millisecond long – as it winds down to a run-of-the-mill pulsar.
While the law is excited by the possibility of uncovering many more orphan gamma-ray bursts, he emphasizes the value of mining archived observational data in search of astronomical events anti-transient. "
"It's changing the way it's done, and it's going to be that," he said. "It is also about the value of new data science techniques. Pulling out information from these rich and diverse data sets is helping us do good science. "
Law's co-authors are Bryan Gaensler of the University of Toronto's Dunlap Institute, Brian Metzger and Lorenzo Sironi of Columbia University and Eran Ofek of the Weizmann Institute in Israel. Law's research is supported by the National Science Foundation (1611606).
Paper: Discovery of the Luminous, Decades-Long, Extragalactic Transient Radio FIRST J141918.9 + 394036
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