  We investigate the clustering of afterglow light curves observed at X-ray and
optical wavelengths. We have constructed a sample of 61 bursts with known dis
tance and X-ray afterglow. This sample includes bursts observed by BeppoSAX,
XMM-Newton, Chandra, and SWIFT. We correct the light curves for cosmological ef
fects and compare the observed X-ray fluxes one day after the burst. We check
for correlations between the observed flux and the burst spectral and temporal
properties. We confirm the previous result of Boer and Gendre (2000) that X-ray
afterglow light curves cluster in luminosity, even when we consider the l ast
SWIFT data. We observe this clustering only for the afterglow light curves; the
inclusion of prompt-related data broaden the distribution. A similar clu
stering is observed for the optical light curves; GRB sources can be divided in
three classes, namely optical and X-ray bright afterglows, optical and X-ray
dim ones, and optically bright -X-ray dim ones. We argue that this clustering
is related to the fireball total energy, the external medium density, the
fraction of fireball energy going in relativistic electrons and magnetic
fields. These parameters can be either fixed to a standard va lue, or
correlated. We finally propose a method for the estimation of the GRB source
redshift based on the observed X-ray flux one day after the burst and optical
properties. Using this method, we compute a redshift of 1.4 +/- 0.2 for GRB
980519 and of 1.9 \+/- 0.3 for GRB 040827. We tested this method on three
recently detected SWIFT GRBs with known redshift, and found it in good
agreement with the reported distance from optical spectroscopy .
