On the FRB luminosity function - - II. Event rate density

The luminosity function of Fast Radio Bursts (FRBs), defined as the event rate per unit cosmic co-moving volume per unit luminosity, may help to reveal the possible origins of FRBs and design the optimal searching strategy. With the Bayesian modelling, we measure the FRB luminosity function using 46 known FRBs. Our Bayesian framework self-consistently models the selection effects, including the survey sensitivity, the telescope beam response, and the electron distributions from Milky Way/ the host galaxy/ local environment of FRBs. Different from the previous companion paper, we pay attention to the FRB event rate density and model the event counts of FRB surveys based on the Poisson statistics. Assuming a Schechter luminosity function form, we infer (at the 95 per cent confidence level) that the characteristic FRB event rate density at the upper cut-off luminosity L^*=2.9_-1.7^+11.9× 10⁴⁴ erg s^-1 is φ^*=339_-313⁺¹⁰⁷⁴ Gpc^-3 yr^-1, and the lower cut-off luminosity is L₀≤ 9.1× 10⁴¹ erg s^-1. The event rate density of FRBs is found to be 3.5_-2.4^+5.7 × 10⁴ Gpc^-3 yr^-1 above 10⁴² erg s^-1, 5.0_-2.3^+3.2 × 10₃ Gpc^-3 yr^-1 above 10⁴³ erg s^-1, and 3.7_-2.0^+3.5× 10² Gpc^-3 yr^-1 above 10⁴⁴ erg s^-1. As a result, we find that, for searches conducted at 1.4 GHz, the optimal diameter of single-dish radio telescopes to detect FRBs is 30-40 m. The possible astrophysical implications of the measured event rate density are also discussed in the current paper.