Understand
We describe a model of the short gamma-ray burst (SGRB) population under a `quasi-universal jet' scenario in which jets can differ in their on-axis peak prompt emission luminosity $L_c$, but share a universal angular luminosity profile $\ell(\theta_v)=L(\theta_v)/L_c$ as a function of the viewing angle $\theta_v$.
- The model is fitted, through a Bayesian hierarchical approach inspired by gravitational wave (GW) population analyses, to 3 observed SGRB samples simultaneously: the Fermi/GBM sample of SGRBs with spectral information in the catalogue (367 events); a flux-complete sample of 16 Swift/BAT SGRBs also detected by GBM, with a measured redshift; and a sample of SGRBs with a binary neutron star (BNS) merger counterpart, which only includes GRB~170817A at present.
- The results favour a narrow jet core with half-opening angle $\theta_c=2.1_{-1.4}^{+2.4}$ deg (90\% credible intervals from our fiducial `full sample' analysis) whose on-axis peak luminosity is distributed as $p(L_c) \propto L_c^{-A}$ with $A=3.2_{-0.4}^{+0.7}$ above a minimum luminosity $L_c^\star = 5_{-2}^{+11}\times 10^{51}$ erg s$^{-1}$.
- For $\theta_v>\theta_c$, the luminosity scales as a power law $\ell\propto \theta_v^{-\alpha_L}$ with $\alpha_L=4.7_{-1.4}^{+1.2}$, with no evidence for a break.