A week after a major astronomical announcement, doubt has been thrown on the claim that the source of a Fast Radio Burst (FRB) has been found. The findings challenging last week's big news have yet to be peer reviewed, but if they stand up they will send the search to understand FRBs back to square one.
Since their discovery in 2007, FRBs have been a major source of mystery to astronomers. An immense flash of radio waves lasting just milliseconds, FRBs were initially found in archival records from the Parkes Radio Telescope, far too late for astronomers to learn anything about their source.
In 2014 an FRB was observed in real time, and last week Dr. Evan Keane of Swinburne University and 40 other authors reported an afterglow to FRB 150418 in Nature. Keane apparently confirmed FRBs come from beyond the Earth and narrowed down the range of possible options for what was causing this strange phenomenon.
Almost immediately, Dr. Peter Williams and Professor Edo Berger, both of the Harvard-Smithsonian Center for Astrophysics, challenged Keane's work. In a preprint available on arXiv.org they wrote, “We argue that the properties of the long-term radio emission from the proposed host point to a different interpretation: that the observed variable radio emission is instead due to AGN [active galaxy nucleus] activity and that the variable emission and galaxy are unrelated to FRB 150418.”
Galaxies with an AGN are very bright, including at radio frequencies, and this is attributed to radiation released as black holes swallow material from around them. In a message to IFLScience, Williams wrote, “Not only are they one of the few classes of sources bright enough to explain the emission level seen, but they are frequently variable emitters. We argue that the dimming seen by Keane et al. is just an unlucky result of getting a small number of samples of something that's burbling up and down all the time.”
The galaxy identified as being the source of FRB 150418. Williams and Berger say it may instead have an Active Galactic Nucleus, and coincidentally be in the same direction as the FRB. David Kaplan and Dawn Erb
Williams and Berger argued AGNs variable at the frequencies Keane studied are more common than his team allowed for. On Monday they went further, announcing on The Astronomer's Telegram that they observed a substantial brightening of radio emissions from this source over the weekend.
Initially, this "afterglow" seen 6 days later had been used to measure the distance to the FRB and also theorize its source, thought to be merging binary stars. But the fact that it is still brightening suggests that the source was actually an AGN, Williams and his team suggest, because an FRB is not expected to last this long. This would mean that the FRB may not be associated with the distance measurement; it could be much closer or much further, or even at the same location, but that's not known at the moment.
It would be a coincidence for an FRB to appear in exactly the same area of the sky as an AGN, but the preprint argues the chance “is non-negligible.” If Williams is right, the FRB's distance remains mysterious.
IFLScience attempted to contact some of the authors of the Nature paper, without success. Williams' work has not been subjected to the rigorous peer review of a leading journal.
Nevertheless, Williams told IFLScience, “We are going to check in on this galaxy every week with the Very Large Array for a few months. If it is an AGN as we theorize, the odds are very good that we will see it varying up and down over time. If we see that, I think it will be very hard to argue that the FRB is necessarily associated with this galaxy.”