An unexpectedly rapid decline in the X-ray afterglow emission of long γ-ray bursts

Swift response The Swift satellite, launched in November last year, is designed to study γ-ray bursts (GRBs) as soon as they happen. GRBs are the most powerful explosions known in the Universe, and Swift's ability to study the early phases of the X-ray afterglow was expected to yield exciting results. Swift has now bagged its first two long GRBs: in both, the X-ray afterglow emission declined rapidly in the first few hundred seconds, then flattened out. The steep decline was unexpected, and neither it nor the spectral properties of the afterglow can be explained by current models. ‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see 1 , 2 . As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands 1 , 2 . Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.