Scientists decode mystery of afterglow from a mysterious event

Scientists have found that a radio emission believed to be an afterglow from a mysterious event called fast radio burst, actually originated from a supermassive black hole in the core of a distant galaxy. Last year, researchers reported detecting the afterglow from 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.

“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,” said Peter Williams, from Harvard University.

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.

“We don’t even know if they come from inside our galaxy or if they’re extragalactic,” said Edo Berger, from Harvard. Most FRBs have been identified in archival data, making immediate follow-up impossible. The new event, FRB 150418, is only the second one to be identified in real time.

Radio observations reported in the journal Nature purportedly showed a fading radio afterglow associated with the FRB. That afterglow was used to link the FRB to a host galaxy located about 6 billion light-years from Earth.

Williams and Berger studied the supposed host galaxy in detail using the US National Science Foundation’s Jansky Very Large Array network of radio telescopes.

If the initial observations had been an afterglow, it should have completely faded away. Instead researchers found a persistent radio source whose strength varied randomly by a factor of three, often reaching levels that matched the initial brightness of the claimed afterglow. The initial study also saw this source, but unluckily missed any rebrightenings.

“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,” said Berger. 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 we see from Earth may be due to a process called “scintillation,” where interstellar gases make an intrinsically steady radio beacon appear to flicker, just like Earth’s atmosphere makes light from stars twinkle.

The source itself might also be varying as the active galactic nucleus periodically gulps a little more matter and flares in brightness. The study appears in the Astrophysical Journal Letters.