A Statistical Comparison of the Optical/UV and X-ray Afterglows of Gamma-Ray Bursts using the Swift Ultra-violet Optical and X-ray Telescopes

摘要

We present the systematic analysis of the UVOT and XRT light curves for asample of 26 Swift Gamma-Ray Bursts (GRBs). By comparing the optical/UV andX-ray light curves, we found that they are remarkably different during thefirst 500s after the BAT trigger, while they become more similar during themiddle phase of the afterglow, i.e. between 2000s and 20000s. If we takeliterally the average properties of the sample, we find that the mean temporalindices observed in the optical/UV and X-rays after 500s are consistent with aforward-shock scenario, under the assumptions that electrons are in the slowcooling regime, the external medium is of constant density and the synchrotroncooling frequency is situated between the optical/UV and X-ray observing bands.While this scenario describes well the averaged observed properties, someindividual GRB afterglows require different or additional assumptions, such asthe presence of late energy injection. We show that a chromatic break (a breakin the X-ray light curve that is not seen in the optical) is present in theafterglows of 3 GRBs and demonstrate evidence for chromatic breaks in a further4 GRBs. The average properties of these breaks cannot be explained in terms ofthe passage of the synchrotron cooling frequency through the observed bands,nor a simple change in the external density. It is difficult to reconcilechromatic breaks in terms of a single component outflow and instead, morecomplex jet structure or additional emission components are required.