Shallow Decay of Early X-Ray Afterglows from Inhomogeneous Gamma-Ray Burst Jets

Almost all the X-ray afterglows of gamma-ray bursts (GRBs) observed by theSwift satellite have a shallow decay phase in the first thousands of seconds.We show that in an inhomogeneous jet (multiple-subjet or patchy-shell) modelthe superposition of the afterglows of off-axis subjets (patchy shells) canhave the shallow decay phase. The necessary condition for obtaining the shallowdecay phase is that gamma-ray bright subjets (patchy shells) should havegamma-ray efficiency higher than previously estimated, and should be surroundedby gamma-ray dim subjets (patchy shells) with low gamma-ray efficiency. Ourmodel predicts that events with dim prompt emission have the conventionalafterglow light curve without the shallow decay phase like GRB 050416A.Comment: 4 pages, 2 figures, accepted for publication in ApJ