We report the results of our observing campaign on GRB 140903A , a nearby ( z = 0.351 ) short duration ( T _ { 90 } \sim 0.3 s ) gamma-ray burst discovered by Swift .
We monitored the X-ray afterglow with Chandra up to 21 days after the burst , and detected a steeper decay of the X-ray flux after t _ { j } \approx 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 \theta _ { j } \approx 5 deg and a collimation-corrected total energy release E \approx 2 \times 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 was likely originated by a compact binary merger .