Title: No Precise Localization for FRB 150418: Claimed Radio Transient is AGN Variability
Authors: P. K. G. Williams, E. Berger
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
Status: Resubmitted to Astrophysical Journal Letters
Fast Radio Bursts, or FRBs for short, have been getting a lot of limelight this year, and for good reason—it’s been an year with a lot of FRB firsts. One of astronomy’s mysteries, FRBs last for only a few milliseconds, yet can contain as much energy as the sun produces in 10,000 years. Yet we don’t know what produces these energetic radio burps, nor even if they were produced by something from our own galaxy or in galaxies far, far away. But this year, for the first time, we saw one go off multiple times—not just twice, but 11 times total (here’s an astrobite covering that discovery). We also found another FRB go off with a radio afterglow that faded over six days, allowing us to pinpoint its location for the first time—in a distant galaxy. This discovery was ushered in with considerable excitement. Knowing that at least this FRB was produced in a distant galaxy presents an exciting opportunity to study the stuff between the galaxies (what astronomers called the intergalactic medium) as well as, of course, bring us another step closer to understanding FRBs and the extreme physics believed to produce them.
Or did we? The authors of today’s paper argue that the case for an extragalactic home for FRB 150418 isn’t airtight. They give three reasons why this might be the case. First, the authors point out that while the discoverers of FRB 150418 argued that it’s highly unlikely that an unrelated fading radio glow randomly occurs in the same location as FRB 150418, they didn’t take into account the fact that the glow may have been caused by a flickering or “variable” radio source. The authors point to radio surveys that indicate that a flicker in the radio sky is 70 times more likely to have come from a variable source rather than a transient (one that appears and fades only once), especially close to the plane of our own Milky Way, where FRB 150418 was seen.
Another piece of evidence that the discoverers of FRB 150418 used to argue for an extragalactic origin is an unusual property common to all FRBs: its large dispersion measure. A radio burst is made up of photons of different energies. If these photons chance upon any free electrons, they interact with them, which slows them down. Radio photons at lower energies tend to be slowed more than the more energetic ones. Thus when you observe a radio burst, you’ll see the energetic photons first, then the ones a little less energetic than that, and so on. How long the lowest energy photons in the burst are delayed is dependent on the number of free electrons that the burst passes through, which is quantified by the burst’s dispersion measure, or DM for short. FRB 150418 has a dispersion measure that’s about four times higher than what the free electrons in the Milky Way can produce. It turns out that the intergalactic medium also contains free electrons, so we can explain FRB 150418’s high dispersion via a long, lonely journey between the galaxies. It also turns out that the distance it had to have traveled matches up well with the redshift of the galaxy in which the radio afterglow was seen—the second piece of evidence FRB 150418’s discoverers give for its extragalactic home. However, the second argument of today’s is that FRB 150418’s dispersion measure can be explained by galaxies at a large range of redshifts.
FRB150418’s discoverers point to a few gamma-ray bursts that have had similar radio afterglows, and suggest that they may have similar origins. The authors’ third point is that the afterglow can be modeled just as well as by a steady radio emission that appears to twinkle or “scintillate,” much like the stars do in the night sky, as the radio waves pass through the interstellar medium of the Milky Way. They also point out that the afterglow is in the same location as a known, faint active galactic nucleus (AGN). AGN are well-known to vary in brightness over short periods of time, and the brightness and duration of FRB 150418’s afterglow matches the variation seen in the AGN.
Thus there is as of yet no confirmed birthplace for FRB 150418. While the locations of FRBs are still in question, we’re poised to discover many more FRBs in the future. So stay tuned for future discoveries of FRBs—hopefully it won’t be long until we discover the birthplaces of these mysterious radio bursts!
Cover Image: The Parkes Radio Telescope, by which FRB 150418 was discovered. Photo by CSIRO, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=35474256.