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 <SUB>90</SUB> ∼ 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 <SUB> j </SUB> ≈ 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 <SUB> j </SUB> ≈ 5° and a collimation-corrected total energy release E ≈ 2 × {10}<SUP>50</SUP> 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.