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


‘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 see1,2. 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 bands1,2. 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.

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Figure 1: The steep early X-ray light curves of five GRBs observed by XRT16.
Figure 2: The X-ray light curves of GRB050126 and GRB050219a as seen by BAT and XRT.
Figure 3: Evolution of the two GRB X-ray light curves from the prompt phase to the afterglow phase.


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We acknowledge support from ASI, NASA and PPARC.

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Correspondence to G. Tagliaferri.

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Tagliaferri, G., Goad, M., Chincarini, G. et al. An unexpectedly rapid decline in the X-ray afterglow emission of long γ-ray bursts. Nature 436, 985–988 (2005).

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