Solution to FRB conundrum also reveals clues about their origin
11 September 2015
Since first reported in 2007, the origin of the bright, millisecond-duration pulses known as Fast Radio Bursts (FRBs) has remained a mystery to astronomers. There have been more theories proposed to explain them than the 17 events so far detected. Where do they come from – our Solar System, our Galaxy, or beyond? The dispersion sweeps of FRBs (the delay of the pulse arrival time with wavelength caused by propagating through plasma in space) indicate that they have travelled through so much material on their way to Earth that they must be cosmological in origin. There just is not enough plasma in the interstellar medium of the Milky Way to explain their long dispersion sweeps.
Adding to the mystery, CAASTRO PhD student Emily Petroff (Swinburne University) and colleagues analysed the distribution of FRB detections across the sky and reported (in their 2014 paper) that their rate is about four times higher at high Galactic latitudes than close to the Galactic plane. This result is doubly puzzling because (a) the distribution of extragalactic pulses should not be related to their position with respect to the Galactic disk and (b) the rate of pulses of Galactic origin should be higher closer to the Galactic plane, the exact opposite of what is observed.
CAASTRO members Dr Jean-Pierre Macquart (Curtin University) and Prof Simon Johnston (CSIRO) now provide an explanation in their recent publication. Radio pulses received at the Earth have propagated through the turbulent interstellar medium of our own Galaxy irrespective of whether they were generated inside or outside of it. The density fluctuations in this medium can randomly amplify the amplitude of a pulse. This effect is equivalent to the Earth's atmosphere causing an apparent twinkling of stars.
The intensity fluctuations change both with time and with observing frequency; and the more material, the more quickly they change with frequency. At low Galactic latitudes, FRBs propagate through so much turbulent Galactic material that the intensity fluctuations change very rapidly with frequency – so much so that radio telescopes average over many tens to hundreds of intensity fluctuations over the observing band. When averaged across the bandwidth of the telescope, the intensity is very close to the mean intensity of the pulse. However, at high Galactic latitudes the FRB radiation is subject to only one or two intensity fluctuations across the telescope observing band, and the observed radiation can be either greatly diminished or enhanced.
The extent to which this matters for FRBs depends on their brightness distribution – the ratio of intrinsically bright and faint pulses. If the distribution is steep, many more faint events will be enhanced to become apparently bright than bright events will be diminished to become faint. The net result is that many faint events which would otherwise have been undetectable are rendered detectable. Thus, there is a direct connection between the brightness distribution of FRBs and the degree to which interstellar scintillation enhances their apparent event rate at high Galactic latitudes.
The researchers used the difference in event rates at high and low Galactic latitudes to infer the steepness of the FRB brightness distribution. Their results show that the distribution is significantly steeper than expected from homogenously distributed events, and this in turn rules out many of the proposed models in which FRBs originate from the nearby Universe. By far the most likely interpretation is that the population of FRBs occurs at cosmological distances and that their abundance has changed throughout cosmic history.