The four new FRBs gathered by Thornton's team at Parkes have eliminated most lingering doubts. The burst with the smallest wavelength dispersion seems to have reached us after passing through some 5.5 billion light-years of space; the burst with the largest dispersion appears to be from nearly twice as far out, originating some 10.4 billion light-years away. One of the four bursts also bears the likely imprint of turbulence in the intergalactic medium, a subtle stretching out of its pulse shape probably caused by electron scattering. Making those same measurements for further FRBs would give astronomers the unprecedented ability to estimate the strength of intergalactic magnetic fields.
From sources unknown
Even though their extragalactic origins are largely confirmed, the FRBs' sources remain unknown. In follow-up multiwavelength observations, no trace of an FRB precursor or afterglow has been found, and because of the relatively low angular resolution of most current radio telescopes, astronomers have not been able to pinpoint any galaxy as a site for any FRB's source. Based on each burst's brevity, brightness and far-distant origin, whatever generated them gave off a truly enormous amount of energy in radio waves, all from an emission region no bigger than a few hundred kilometers. And yet, according to Thornton's colleague Matthew Bailes, an astronomy professor at Swinburne University of Technology in Melbourne, Australia, the signal from a mobile phone broadcasting from the surface of Earth's moon would be brighter than a typical FRB by a factor of 1,000. Such apparent faintness, Thornton says, means that "it would take Parkes operating full-time for a million years to collect enough FRBs to equal the kinetic energy found in a single flying mosquito."
The high occurrence rate of FRBs paired with their immense intrinsic luminosity has, however, largely ruled out a handful of hypothesized sources. The FRBs are too bright to be radio-wave burps of evaporating supermassive black holes at galactic centers, and they are far too frequent to be easily explained as the echoes from energetic mergers of neutron star pairs. Similarly, gamma-ray bursts occur only about once a day, not often enough to be obviously associated with FRBs. One new hypothesis, espoused by Heino Falcke of Radboud University Nijmegen in the Netherlands and Luciano Rezzolla of the Max Planck Institute for Gravitational Physics in Potsdam, posits that FRBs are the farewell messages of dying stars dubbed "blitzars," putative rapidly spinning "supramassive" neutron stars that would otherwise become black holes. As a blitzar loses energy and spins down over time, it suddenly crosses a threshold where it can no longer support its weight, and as it collapses an FRB is emitted. "When the black hole forms, the magnetic fields will be cut off from the star and snap like rubber bands," Falcke explained in a press release. "As we show, this can indeed produce the observed giant radio flashes. All other signals you normally would expect—gamma rays, x-rays—simply disappear behind the event horizon of the black hole."
Bailes finds the blitzar explanation difficult to swallow; reconciling it with the estimated FRB occurrence rate would require the majority of neutron stars to be in this unlikely supramassive state. Additionally, "the problem with many of these more exotic scenarios is they aren't easily falsifiable," he says. "We wouldn't be able to detect anything else from these things after they're locked away behind a black hole's event horizon."
Bailes's preferred FRB source is something called a magnetar, rare neutron stars with the most powerful magnetic fields ever measured. In the so-called Christmas event of 2004, Bailes notes, astronomers observed a magnetar flare up on the far side of the Milky Way in a "giant burst." In a millisecond the magnetar erupted with a bit more energy than the sun releases in 300,000 years; astronomers believe the burst was caused by a starquake, a sudden rearrangement of the magnetar's structure to release built-up stresses associated with its whirling magnetic field. "Even a tiny fraction of that energy converted to radio waves would give the kind of luminosity we need to satisfy an FRB's energetics, and the millisecond timescale is perfect for our duration," he says.