Search for Relativistic Curvature Effects in Gamma-Ray Burst Pulses

We analyze the time profiles of individual gamma-ray burst (GRB) pulses, that are longer than 2 s, by modelling them with analytical functions that are based on physical first principles and well-established empirical descriptions of GRB spectral evolution. These analytical profiles are independent of the emission mechanism and can be used to model both the rise and decay profiles allowing for the study of the entire pulse light-curve. Using this method, we have studied a sample of 77 individual GRB pulses, allowing us to examine the fluence, pulse width, asymmetry, and rise and decay power-law distributions. We find that the rise phase is best modelled with a power law of average index r = 1.31 ± 0.11 and that the average decay phase has an index of d = 2.39± 0.12. We also find that the ratio between the rise and decay times (the pulse asymmetry) exhibited by the GRB pulse shape has an average value of 0.47 which varies little from pulse to pulse and is independent of pulse duration or intensity. Although this asymmetry is largely uncorrelated to other pulse properties, a statistically significant trend is observed between the pulse asymmetry and the decay power law index, possibly hinting at the underlying physics. We compare these parameters with those predicted to occur if individual pulse shapes are created purely by relativistic curvature effects in the context of the fireball model, a process that Department of Physics and Astronomy, Rice University, Houston, TX 77005 Center for Space Science and Astrophysics, Stanford University, Stanford, CA 94305 Present address: Stockholm Observatory, AlbaNova, SE-106 91 Stockholm, Sweden