Abstract:
The softening process observed in the steep decay phase of early
x-ray afterglows of Swift bursts has remained a puzzle since its discovery. The
softening process can also be observed in the later phase of the bursts and its
cause has also been unknown. Recently, it was suggested that, influenced by
the curvature effect, emission from high latitudes would shift the Band function
spectrum from a higher energy band to a lower band, and this would give rise to
the observed softening process accompanied by a steep decay of the flux density.
The curvature effect scenario predicts that the terminating time of the softening
process would be correlated with the duration of the process. In this paper, on the
basis of the data from the UNLV GRB (University of Nevada, Las Vegas, Gamma-
Ray Burst) group web-site, we found an obvious correlation between the two
quantities. In addition, we found that the softening process can be divided into
two classes: the early type softening (ts,max ≤ ‘4000’ s) and the late type softening
(ts,max > ‘4000’ s). The two types of softening show different behaviors in the
duration versus terminating time plot. In the relation between the variation rates
of the flux density and spectral index during the softening process, a discrepancy
between the two types of softening is also observed. According to their timescales
and the discrepancy between them, we propose that the two types are of different origins: the early type is of internal shock origin and the late type is of external
shock origin. The early softening is related to the steep decay just following the
prompt emission, whereas for the late decay one typically conceives the transition
from flat decay to late afterglow decay. We suspect that there might be a great
difference in Lorentz factor between the two classes, which is responsible for the
observed discrepancy.
