Both the light curve and spectral evolution of the radiation from a relativistic fireball with extremely short duration are studied, in order to examine the curvature effect for different forms of the radiation spectrum. Assuming a δ function emission we get formulas that get rid of the impacts from the intrinsic emission duration, applicable to any forms of spectrum. It shows that the same form of spectrum could be observed at different times, with the peak energy of the spectrum shifting from higher energy bands to lower bands following Epeak ∝ t −1. When the emission is early enough the t 2 fν(t) form as a function of time will possess exactly the same form that the intrinsic spectrum as a function of frequency has. Assuming fν ∝ ν −β t −α one finds α = 2 + β which holds for any intrinsic spectral forms. This relation will be broken down and α> 2 + β or α ≫ 2 + β will hold at much later time when the angle between the moving direction of the emission area and the line of sight is large. An intrinsic spectrum in the form of the Band function is employed to display the light curve and spectral evolution. Caused by the shifting of the Band function spectrum, a temporal steep decay phase and a spectral softening appear simultaneously. The softening phenomenon will appear at different frequencies. It occurs earlier for higher frequencies and later for lower frequencies. The terminating softening time ts,max depends on the observation frequency, following ts,max ∝ ν −1. This model predicts that the softening duration would be linearly correlated with ts,max; the observed βmin and βmax are determined by the low and high energy indexes of the Band function; both βmin and βmax are independent of the observation frequency
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