THE INTRINSIC LUMINOSITY OF GAMMA-RAY BURSTS AND THEIR HOST GALAXIES

THE Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory has shown that, although gamma-ray bursts are distributed isotropically on the sky, there is an apparent dearth of weak events compared to those expected from a homogeneous distribution of sources1,2. This suggests that the bursts originate either in an extended galactic halo3 (that is, locally) or at cosmological distances4,5. The intensity distribution of the bursts can be used to constrain their source properties and spatial distribution, and here we address this question by considering the BATSE data with those from the Pioneer Venus Orbiter, correcting for the different reponse of the two instruments. We show that the composite intensity distribution can after all be fitted by a simple, homogeneous model of gamma-ray burst sources with identical intrinsic luminosities where the faintest BATSE events are at a redshift of z approximately 0.80+/-0.05. We obtain an upper limit of about -18 on the absolute magnitude of the host galaxies by assigning model-derived distances to the brightest extragalactic objects found15 within the error boxes of eight well localized gamma-ray bursts. All but the faintest active galaxies are excluded as the source of the bursts. The bursts may instead be associated with normal galaxies, but only if the host-galaxy magnitudes are close to our upper limit.