An Achromatic Break in the Afterglow of the Short GRB 140903A: Evidence for a Narrow Jet

We report the results of our observing campaign on GRB 140903A, a nearby (z = 0.351) short-duration (T ₉₀ ∼ 0.3 s) gamma-ray burst discovered by Swift. We monitored the X-ray afterglow with Chandra up to 15 days after the burst and detected a steeper decay of the X-ray flux after t _j ≈ 1 day. Continued monitoring at optical and radio wavelengths showed a similar decay in flux at nearly the same time, and we interpret it as evidence of a narrowly collimated jet. By using the standard fireball model to describe the afterglow evolution, we derive a jet opening angle θ _j ≈ 5° and a collimation-corrected total energy release E ≈ 2 × {10}⁵⁰ erg. We further discuss the nature of the GRB progenitor system. Three main lines disfavor a massive star progenitor: the properties of the prompt gamma-ray emission, the age and low star formation rate of the host galaxy, and the lack of a bright supernova. We conclude that this event likely originated from a compact binary merger.