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By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19cases 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, apparentlydifferent from the canonical lightcurves characterized by a shallow-to-normaldecay transition. No statistical difference is found in their promptgamma-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 hostgalaxies and larger energy release compared with the canonical sample. The SPLXRT lightcurves in the burst frame gradually merge into a conflux.The normaldecay segment for the canonical sample has the same feature. Similar to thenormal decay segment, the SPL lightcurves satisfy the closure relations ofexternal shock models. If the X-rays are the afterglow of the GRB fireball, ourresults indicate that the shallow decay would be due to energy injection andthe 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 Yamazakiis 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 theSPL 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 largerthan the offsets of the known precursors from the main GRBs, indicating theX-rays would be an emission component from external shocks prior to GRBtrigger. The lack of detection of a jet-like break in most XRT lightcurvesimplies that the opening angle of its jet would be usually large.

astrophysical journal/the astrophysical journal

A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law  Decaying Lightcurves vs. Canonical Lightcurves and Implications for a Unified  Origin of X-rays