The variable X-ray light curve of GRB 050713A: the case of refreshed shocks

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy e-mail: guetta@mporzio.astro.it
² JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, 440 UCB, Boulder CO, USA
³ NASA, Goddard Space Flight Center, Greenbelt, MD, USA
⁴ XMM-Newton SOC, ESAC, ESA, Apartado 50727, 28080 Madrid, Spain
⁵ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy
⁶ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁷ International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
⁸ Astrophysics Research Institute, Liverpool John Moores University, Twelver Quays, Birkenhead CH41 1LD, UK
⁹ IAC: Instituto de Astrofísica de Canarias (IAC), C/. Via Láctea, 38200 La Laguna, Tenerife, Spain
¹⁰ Instituto de Astrofisica de Andalucia (IAA-CSIC), Apartadode Correos, Granada, Spain
¹¹ A. Gomboc Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

Received: 17 February 2006
Accepted: 21 July 2006

Abstract

We present a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main Gamma Ray Burst (GRB) event. The X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken power-law that is often seen in Swift GRBs. Our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. Our sparsely sampled light curve of the optical afterglow can be fitted with a single power-law without large flares. The optical decay index appears flatter than the X-ray one, especially at later times.