Search for Relativistic Curvature Effects in Gamma‐Ray Burst Pulses

We analyze the time profiles of individual gamma-ray burst (GRB) pulses, thatare longer than 2 s, by modelling them with analytical functions that are basedempirical descriptions of GRB spectral evolution. These analytical profiles areindependent of the emission mechanism and can be used to model both the riseand decay profiles Using this method, we have studied a sample of 77 individualGRB pulses, allowing us to examine the fluence, pulse width, asymmetry, andrise and decay power-law distributions. We find that the rise phase is bestmodelled with a power law of average index $r = 1.31 \pm 0.11$ and that theaverage decay phase has an index o.f $d = 2.39 \pm 0.12$. We also find that theratio between the rise and decay times (the pulse asymmetry) exhibited by theGRB pulse shape has an average value of 0.47 which varies little from pulse topulse and is independent of pulse duration or intensity. We compare theseparameters with those predicted to occur if individual pulse shapes are createdpurely by relativistic curvature effects in the context of the fireball model,a process that makes specific predictions about the shape of GRB pulses. Thedecay index distribution obtained from our sample shows that the average GRBpulse fades faster than the value predicted by curvature effects, with only 39%of our sample being consistent with the curvature model. We discuss severalrefinements of the relativistic curvature scenario that could naturally accountfor these observed deviations