Constraining the Structure of GRB Jets Through the log(N)-log(S) Distribution

Abstract

A general formalism is developed for calculating the luminosity function and the expected number $N$ of observed GRBs above a peak photon flux $S$ for any GRB jet structure. This new formalism directly provides the true GRB rate without the need for a `correction factor'. We apply it to the uniform jet (UJ) and universal structured jet (USJ) models for the structure of GRB jets and perform fits to the observed log(N)-log(S) distribution from the GUSBAD catalog which contains 2204 BATSE bursts. A core angle $θ_c$ and an outer edge at $θ_{max}$ are introduced for the structured jet, and a finite range of half-opening angles $θ_{min}\leqθ_j\leqθ_{max}$ is assumed for the uniform jets. The efficiency $ε_γ$ for producing gamma-rays, and the energy per solid angle $ε$ in the jet are allowed to vary with $θ_j$ (the viewing angle $θ_{obs}$) in the UJ (USJ) model, $ε_γ\proptoθ^{-b}$ and $ε\proptoθ^{-a}$. We find that a single power-law luminosity function provides a good fit to the data. Such a luminosity function arises naturally in the USJ model, while in the UJ model it implies a power-law probability distribution for $θ_j$, $P(θ_j)\proptoθ_j^{-q}$. The value of $q$ cannot be directly determined from the fit to the observed log(N)-log(S) distribution, and an additional assumption on the value of $a$ or $b$ is required. Alternatively, an independent estimate of the true GRB rate would enable one to determine $a$, $b$ and $q$. The implied values of $θ_c$ (or $θ_{min}$) and $θ_{max}$ are close to the current observational limits. The true GRB rate for the USJ model is found to be $R_{GRB}(z=0)=0.86^{+0.14}_{-0.05} Gpc^{-3} yr^{-1}$.

9 pages, 5 figures, 1 table; accepted for publication in ApJ