We examine a sample of 48 Swift /UVOT long Gamma-ray Burst light curves and find a correlation between the logarithmic luminosity at 200s and average decay rate determined from 200s onwards , with a Spearman rank coefficient of -0.58 at a significance of 99.998 % ( 4.2 \sigma ) .
We discuss the causes of the log L _ { 200 s } - \alpha _ { > 200 s } correlation , finding it to be an intrinsic property of long GRBs , and not resulting from the selection criteria .
We find two ways to produce the correlation .
One possibility is that there is some property of the central engine , outflow or external medium that affects the rate of energy release so that the bright afterglows release their energy more quickly and decay faster than the fainter afterglows .
Alternatively , the correlation may be produced by variation of the observers viewing angle , with observers at large viewing angles observing fainter and slower decaying light curves .