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When a gamma-ray burst (GRB) emitter stops emission abruptly, the observerreceives rapidly fading emission from high latitudes with respect to the lineof sight, known as the ``curvature effect''. Identifying such emission from GRBprompt-emission lightcurves would constrain the radius of prompt emission fromthe central engine and the composition of GRB jets. We perform a dedicatedsearch of high-latitude emission (HLE) through spectral and temporal analysesof a sample of single-pulse bursts detected by the Gamma-ray Burst Monitor onboard the {\it Fermi} satellite. We identify HLE from a subsample of bursts andconstrain the emission radius to be $R_{\rm GRB} \sim (10^{15}-10^{16})$ cmfrom the central engine. Some bursts have the HLE decay faster than predictedby a constant Lorentz factor jet, suggesting that the emission region isundergoing acceleration during prompt emission. This supports thePoynting-flux-dominated jet composition for these bursts. The conclusion isconsistent with previous results drawn from spectral-lag modeling of promptemission and HLE analysis of X-ray flares.

Testing High-latitude Curvature Effect of Gamma-Ray Bursts with {\it Fermi} Data: Evidence of Bulk Acceleration in Prompt Emission