Could Fast Radio Bursts be of cosmological origin?
High time resolution radio surveys over the last decade have discovered a population of millisecond-duration transient bursts called Fast Radio Bursts (FRBs) of unknown. Only 18 of these bursts have been detected to date, and their origin – whether extragalactic or at even cosmological distances – is still uncertain.
CAASTRO PhD student Manisha Caleb (ANU and Swinburne University of Technology) and colleagues have now scrutinised the FRB properties: energy distribution, spatial density as a function of redshift and properties of the Interstellar and Intergalactic Media. The researchers ran simulations to test whether a cosmological population is a feasible scenario and to compare their simulations to data from the High Time Resolution Universe survey that used the Effelsberg radio telescope in Germany and the 64-m Parkes radio telescope in Australia.
Their Monte Carlo simulations were based on two scenarios for the co-moving numbers of FRBs: a constant co-moving density model and a model in which the number of FRBs is proportional to the known cosmic star formation history (SFH). The most interesting property of the simulated events is their distribution of detections above some fluence (so-called logN-logF curves): if the sources have an even approximately typical luminosity (i.e. are standard candle-like), then the slope of this relation is a probe of their spatial distribution. For standard candles in the standard model of cosmology – LCDM – the slope varies smoothly from -3/2 for the nearby universe, gradually becoming flatter as further distances are probed. To illustrate, at a redshift of z ~0.7, which is typical of FRBs found to date, standard candles yield a relation with a slope of ~ -1. The observed slope of the logN-logF of the 9 FRBs analysed in this study is -0.9 +/- 0.3. The team’s simulations were able, in both scenarios for the number density of the sources with redshift, to match this slope well, yielding -0.8 +/- 0.3 for the cosmic SFH and -0.7 +/- 0.2 for the constant density case. They concluded that the properties of the observed FRBs are generally consistent with arising from sources at cosmological distances.
The researchers also simulated FRB rates at the upgraded Molonglo telescope, UTMOST, and at Parkes for the Multibeam and the planned Phased Array Feed (PAF) receivers. They applied conservative assumptions about the spectral index of FRBs and the sensitivity of the instruments. According to those simulations, UTMOST has the capability, at full design sensitivity, to dominate the FRB detection rate. Uncertainty in the final PAF design sensitivity make predictions difficult for Parkes but its wide sky coverage has the potential to increase the FRB discovery rate close to the fluence limit. The fully sensitive UTMOST will dominate the event detection rate at all fluences.
Manisha Caleb, Chris Flynn, Matthew Bailes, Ewan Barr et al. in the Monthly Notices of the Royal Astronomical Society (2016): “Are the distributions of Fast Radio Burst properties consistent with a cosmological population?”