Absence of fast radio bursts at intermediate Galactic latitude
9 July 2015
The majority of the enigmatic bright pulses called fast radio bursts (FRBs) have been found at high Galactic latitudes. The bursts are thought to originate in distant Galaxies, and the sparser interstellar medium at high latitudes might make them easier to detect. But the total FRB rate over the entire sky is estimated at 10,000 FRBs per day, meaning that bursts should also be relatively common at lower Galactic latitudes. A CAASTRO team led by Swinburne PhD student Emily Petroff searched for FRBs in a newly completed survey around the Galactic plane: the High Time Resolution Universe (HTRU) survey.
Although four FRBs had been reported from the HTRU survey at high latitudes (Thornton et al. 2013), no FRBs were found in the intermediate latitude survey. The non-detection came as a surprise since the intermediate latitude survey spent double the time on sky of the Thornton study, and between 3 and 10 FRBs were predicted to be discovered in the data. This result provided the first evidence that FRBs are preferentially found at higher latitudes, out of the Galactic plane, a finding that has since been confirmed through analysis of other surveys.
It is still unknown as to what could be causing this anisotropy. A first hypothesis put forward by the authors was that FRBs traveling through the larger amount of ionised material in the Galactic plane might be obscured or scattered out as they travel though the interstellar medium. However, modelling of strong Galactic effects on simulated pulses only showed obscuration for 20% of the total survey pointings, not enough to explain the absence of detections. Even if strong scattering effects were in play the team’s result is inconsistent with an isotropic distribution of FRBs at the 99% confidence level given the current FRB rate.
Ultimately this work makes a Galactic progenitor for FRBs unlikely as a Galactic population would preferentially occur in the plane, not the halo. Only a handful of FRBs are currently published in the literature and more sources are needed to fully understand the true FRB rate and the sky distribution of sources.