Fast X-Ray Transients and Gamma-Ray Bursts: Constraints on Beaming

The detection of X-ray afterglows of γ-ray burst (GRB) sources has enabled the optical discovery that GRBs are at cosmological distances, which confirms that the enormous luminosities required must produce a relativistically expanding fireball. It is not yet clear whether this expansion is quasi-spherical or beamed, or if the beaming differs between the prompt GRB and afterglow, although the differences for both source emission and population models are profound. We show that relative beaming (prompt vs. afterglow) may be constrained by analysis of the "fast transient" X-ray sources cataloged by Pye & McHardy from the Ariel 5 survey as well as several other surveys (e.g., HEAO 1 experiments A-1/A-2). After removing contamination from likely stellar transients, and scaling from X-ray/γ-ray fluence ratios recently derived from Ginga and BeppoSAX, approximately half of the sources were possible prompt emission or afterglows from GRBs with γ-ray fluence of 1.2 × 10−5 ergs cm−2. Since the rate of these candidate afterglow events, 0.15 day−1, is consistent with the BATSE log N-log S distribution, no difference in prompt versus afterglow beaming is required. However, more sensitive X-ray monitors or hard X-ray imaging surveys, with better temporal and spatial resolution, could provide strong constraints on GRBs and beaming.