  By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19
cases that monotonously decay as a single power law (SPL) with an index of 1 ~
1.7 from tens (or hundreds) to ~ 10^5 seconds post the GRB trigger, apparently
different from the canonical lightcurves characterized by a shallow-to-normal
decay transition. No statistical difference is found in their prompt
gamma-rays, and the X-ray properties of the two samples are also similar,
although the SPL sample tend to have a slightly lower NH value of the host
galaxies and larger energy release compared with the canonical sample. The SPL
XRT lightcurves in the burst frame gradually merge into a conflux.The normal
decay segment for the canonical sample has the same feature. Similar to the
normal decay segment, the SPL lightcurves satisfy the closure relations of
external shock models. If the X-rays are the afterglow of the GRB fireball, our
results indicate that the shallow decay would be due to energy injection and
the total energy budget after injection for both samples of GRBs is comparable.
More intriguing, we find that a prior X-ray emission model proposed by Yamazaki
is more straightforward to interpret the data. We show that the zero times (T0)
of the X-rays atisfy a log-normal distribution, and the negligible T0's of the
SPL sample are consistent with being the tail of T0 distributions at low end.
Referenced to T0, the canonical XRT lightcurves well trace the SPL lightcurves.
The T0's of the canonical lightcurves in our analysis are usually much larger
than the offsets of the known precursors from the main GRBs, indicating the
X-rays would be an emission component from external shocks prior to GRB
trigger. The lack of detection of a jet-like break in most XRT lightcurves
implies that the opening angle of its jet would be usually large.
