Constraints on Off-Axis X-Ray Emission from Beamed GRBs

We calculate the prompt x–ray emission as a function of viewing angle for beamed Gamma–Ray Burst (GRB) sources. Prompt x-rays are inevitable due to the less highly blueshifted photons emitted at angles ∼> 1/γ relative to the beam symmetry axis, where γ is the expansion Lorentz factor. The observed flux depends on the combinations γ∆θ and γθv, where ∆θ is the beaming angle and θv is the viewing angle. We use the observed source counts of γ–ray–selected GRBs to predict the minimum detection rate of prompt x–ray bursts as a function of limiting sensitivity. We compare our predictions with the results from the Ariel V catalog of fast x–ray transients, and find that Ariel’s sensitivity is not great enough to place significant constraints on γ and ∆θ. We estimate that a detector with fluence limit ∼ 10−7 erg cm−2 in the 2–10 keV channel will be necessary to distinguish between geometries. Because the x-ray emission is simultaneous with the GRB emission, our predicted constraints do not involve any model assumptions about the emission physics but simply follow from special-relativistic considerations. Subject headings: gamma rays: bursts