We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts ( GRBs ) well-observed by the Swift X-ray Telescope ( XRT ) .
The early X-ray afterglows show a canonical behavior , where the light curve broadly consists of three distinct power law segments : ( i ) an initial very steep decay ( \propto t ^ { - \alpha } with 3 \lesssim \alpha _ { 1 } \lesssim 5 ) , followed by ( ii ) a very shallow decay ( 0.5 \lesssim \alpha _ { 2 } \lesssim 1.0 ) , and finally ( iii ) a somewhat steeper decay ( 1 \lesssim \alpha _ { 3 } \lesssim 1.5 ) .
These power law segments are separated by two corresponding break times , t _ { break, 1 } \lesssim 500 s and 10 ^ { 3 } { s } \lesssim t _ { break, 2 } \lesssim 10 ^ { 4 } s. On top of this canonical behavior , many events have superimposed X-ray flares , which are most likely caused by internal shocks due to long lasting sporadic activity of the central engine , up to several hours after the GRB .
We find that the initial steep decay is consistent with it being the tail of the prompt emission , from photons that are radiated at large angles relative to our line of sight .
The first break in the light curve ( t _ { break, 1 } ) takes place when the forward shock emission becomes dominant , with the intermediate shallow flux decay ( \alpha _ { 2 } ) likely caused by the continuous energy injection into the external shock .
When this energy injection stops , a second break is then observed in the light curve ( t _ { break, 2 } ) .
This energy injection increases the energy of the afterglow shock by at least a factor of f \gtrsim 4 , and augments the already severe requirements for the efficiency of the prompt gamma-ray emission .