Clustering of the optical-afterglow luminosities of long gamma-ray bursts

We studied the optical afterglows of the 24 pre-SWIFT gamma-ray bursts (GRBs) with both known spectroscopic redshift and published estimates of the optical extinction in the source frame. We found an unexpected clustering of the optical-afterglow luminosities measured 12 h ( source frame time) after the trigger. For 21 out of 24 bursts, the distribution of the optical luminosities is narrower than the distribution of the X-ray luminosities, and even narrower than the distribution of the ratio between the monochromatic optical luminosities and the total isotropic, emitted prompt energy. Three bursts stand out from the distribution of the other sources, being underluminous by a factor similar to 15. We compare this result with another somewhat analogous result concerning the luminosity of the X-ray afterglows studied earlier. We constructed the optical to X-ray spectral energy distribution for all our GRBs. For all but a minority of them, the optical and the X-ray emissions are consistent with being produced by the same radiation process. We discuss our results in the framework of the standard external-shock synchrotron model. Finally, we consider the behavior of the first GRBs of known redshifts detected by SWIFT. We find that these SWIFT GRBs entirely confirm our findings.