Magneticum sheds new light on recently discovered Fast Radio Bursts (FRBs)
by Klaus Dolag
Fast radio bursts (FRBs) are a newly identified and as-yet-unexplained class of transient objects. The ten known FRBs currently in the literature are characterised by short (~ ms), bright (> 1 Jansky) bursts of radio emission; none have been seen to repeat, and all but two occurred at high Galactic latitude, |b| > 20°. The implied all-sky event rate is enormous, around 10000 per day.
The radio signals from FRBs experience a frequency-dependent dispersion delay as they propagate through ionised gas, just as is routinely seen for radio pulsars. However, for most observed FRBs, the very high dispersion measures (DMs), in the range , are more than an order of magnitude larger than the DM contribution expected from the interstellar medium (ISM) of the Milky Way in these directions.
We have used the Magneticum set of cosmological hydrodynamic simulations to investigate the contribution from cosmological large-scale structure and found that the foreground-subtracted DMs are consistent with a cosmological origin, corresponding to a source population observable to a maximum redshift z ~ 0.6-0.9. We considered models for the spatial distribution of FRBs in which they are randomly distributed in the Universe, track the star-formation rate of their host galaxies, track total stellar mass, or require a central supermassive black hole. Current data do not discriminate between these possibilities, but the predicted DM distributions for different models will differ considerably once we begin detecting FRBs at higher DMs and higher redshifts. We additionally consider the distribution of FRB fluences, and showed that the observations are consistent with FRBs being standard candles, each burst producing the same radiated isotropic energy. The data imply a constant isotropic burst energy of if FRBs are embedded in host galaxies, or if FRBs are randomly distributed. These energies are one to two orders of magnitudes larger than had previously been inferred. Within the constraints of the available small sample of data, our analysis favours FRB mechanisms for which the isotropic radiated energy has a narrow distribution in excess of 1040erg.
