Observations by the NSF’s Jansky Very Large Array, pictured here, show that a suspected fast radio burst afterglow is actually radio emission from an active galactic nucleus. Credit: NRAO
Last Feb a team of astronomers reported detecting an afterglow from a mysterious event called a fast radio burst, which would pinpoint the precise position of the burst’s origin, a longstanding goal in studies of these mysterious events. These findings were quickly called into question by follow-up observations. New research by Harvard astronomers Peter Williams and Edo Berger shows that the radio emission believed to be an afterglow actually originated from a distant galaxy’s core and was unassociated with the fast radio burst.
“Part of the scientific process is investigating findings to see if they hold up. In this case, it looks like there’s a more mundane explanation for the original radio observations,” says Williams. As their name suggests, fast radio bursts (or FRBs) are brief yet powerful spurts of radio energy lasting only a few milliseconds. The first ones were only identified in 2007. Their source has remained a mystery. The new event, FRB 150418, is only the second one to be identified in real time. The afterglow was used to link the FRB to a host galaxy located 6 B light-years from Earth.
In late Feb and March, Williams and Berger investigated the supposed host galaxy in detail using the NSF’s Jansky Very Large Array radio telescopes. If the initial observations had been an afterglow, it should have completely faded away. Instead they found a persistent radio source whose strength varied randomly by a factor of 3, often reaching levels that matched the initial brightness of the claimed afterglow. The initial study also saw this source, but unluckily missed any rebrightenings. Berger said. “The radio emission from this source goes up and down, but it never goes away. That means it can’t be associated with the fast radio burst.”
The emission instead originates from an active galactic nucleus that is powered by a supermassive black hole. Dual jets blast outward from the black hole, and complex physical processes within those jets create a constant source of radio waves. The variations may be due to “scintillation,” where interstellar gases make an intrinsically steady radio beacon appear to flicker. The source itself might also be varying as the active galactic nucleus periodically gulps a little more matter and flares in brightness.
“Now we have firm evidence for the origins of both short and long gamma-ray bursts. With more data and more luck, I expect that we’ll eventually solve the mystery of fast radio bursts too,” he adds. https://www.cfa.harvard.edu/news/2016-10
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