The Expected Dipole in the Distribution of Cosmological Gamma-Ray Bursts

Abstract

If gamma-ray bursts originate at cosmological distances then their angular distribution should exhibit a dipole in the direction of the solar motion relative to the cosmic microwave background. This is due to the combined effects of abberation, an anisotropic shift of the burst event rate, and an angular variation in the distance out to which bursts can be detected. We derive the amplitude of the expected dipole for an open and flat cosmological model, and for various possible evolution rates of the burst population. Although our dimensionless velocity with respect to the CMB rest frame is of order $10^{-3}$, the dipole amplitude is of order $10^{-2}$, an order of magnitude larger. The results depend very weakly on the value of $Ω_0$, but are sensitive to the spectral index of the bursts' photon spectra, and to the rate of evolution of the burst population. There is no dependence on the value of the Hubble constant. A clear detection of the dipole will require a larger sample of bursts than currently available (of order $10^4$). Future statistical analyses of the hypothesis that bursts originate at cosmological distances should take this effect into account, rather than assuming a perfectly isotropic distribution, for obtaining the correct statistical significance of their results.