Deviation of light curves of gamma-ray burst pulses from standard forms due to the curvature effect of spherical fireballs or uniform jets

We check in this paper if the phenomenon of a reverse S-feature deviation, of profiles of the pulses of spherical fireballs or uniform jets with a sufficiently large opening angle, from the standard decay form revealed previously is common for the pulses comprising a fast rise and an exponential decay (the so-called FRED pulses). Our analysis shows that, in addition to the standard decay form shown in Qin et al. (2004), there exists a marginal decay curve. The reverse S-feature phenomenon is found common in the sample of Kocevski et al. (2003), when compared with both the standard decay form and the marginal decay curve. According to the analysis we propose to take the marginal decay curve as a criteria to check if an observed pulse could be taken as a candidate suffered from the curvature effect. We introduce two quantities A1 and A2 to describe the deviations within and beyond the FWHM position of the decay phase, respectively. The values of A1 and A2 of pulses of the sample are calculated, which suggest that for most of these pulses their corresponding local pulses might last a sufficiently large interval relative to their rising time