1. INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
2. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
3. Università degli studi di Milano-Bicocca, Piazza delle Scienze 3, I-20126 Milano, Italy
4. Department of Astronomy & Astrophysics, Pennsylvania State University, Pennsylvania 16802, USA
5. ASI Science Data Center, Via Galileo Galilei, I-00044 Frascati, Italy
6. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
7. Universities Space Research Association, 10227 Wincopin Circle, Suite 212, Columbia, Maryland 21044, USA
8. National Research Council, 2101 Constitution Avenue, NW, Washington DC 20418, USA
9. Department of Astronomy, University of Texas, RLM 15.308, Austin, Texas 78712-1083, USA
10. Department of Physics, University of Nevada, Box 454002, Las Vegas, Nevada 891, USA
11. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
12. INAF – Istituto di Astrofisica Spaziale e Cosmica. Via Ugo La Malfa 153, I-90146 Palermo, Italy
13. INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monteporzio, Italy

Correspondence to: G. Tagliaferri¹ Correspondence and requests for materials should be addressed to G.T. (Email: tagliaferri@merate.mi.astro.it).

Abstract

'Long' -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 -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.

1. INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
2. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
3. Università degli studi di Milano-Bicocca, Piazza delle Scienze 3, I-20126 Milano, Italy
4. Department of Astronomy & Astrophysics, Pennsylvania State University, Pennsylvania 16802, USA
5. ASI Science Data Center, Via Galileo Galilei, I-00044 Frascati, Italy
6. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
7. Universities Space Research Association, 10227 Wincopin Circle, Suite 212, Columbia, Maryland 21044, USA
8. National Research Council, 2101 Constitution Avenue, NW, Washington DC 20418, USA
9. Department of Astronomy, University of Texas, RLM 15.308, Austin, Texas 78712-1083, USA
10. Department of Physics, University of Nevada, Box 454002, Las Vegas, Nevada 891, USA
11. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA
12. INAF – Istituto di Astrofisica Spaziale e Cosmica. Via Ugo La Malfa 153, I-90146 Palermo, Italy
13. INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, I-00040 Monteporzio, Italy

Correspondence to: G. Tagliaferri¹ Correspondence and requests for materials should be addressed to G.T. (Email: tagliaferri@merate.mi.astro.it).

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