The X-ray light curve of gamma-ray bursts: clues to the central engine

¹ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
e-mail: grazia.bernardini@brera.inaf.it
² ICRANet, p. le della Repubblica 10, 65100 Pescara, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Yunnan Observatory & Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, 650011 Yunnan Province, PR China
⁵ International Center for Astrophysics, Korean Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon, Republic of Korea
⁶ University of Padova, Astronomy Dept., v. dell’Osservatorio 3, 35122 Padova, Italy
⁷ University of Milano Bicocca, Physics Dept., P.zza della Scienza 3, Milano 20126, Italy

Received: 16 August 2011
Accepted: 29 November 2011

Abstract

Aims. We present the analysis of a large sample of gamma-ray burst (GRB) X-ray light curves in the rest frame to characterise their intrinsic properties in the context of different theoretical scenarios.

Methods. We determine the morphology, timescales, and energetics of 64 long GRBs observed by Swift/XRT without flaring activity. We furthermore provide a one-to-one comparison to the properties of GRBs with X-ray flares.

Results. We find that the steep decay morphology and its connection with X-ray flares favour a scenario in which a central engine origin. We show that this scenario can also account for the shallow decay phase, provided that the GRB progenitor star has a self-similar structure with a constant envelope-to-core mass ratio  ~0.02−0.03. However, difficulties arise for very long duration (t_p ≳ 10⁴ s) shallow phases. Alternatively, a spinning-down magnetar whose emitted power refreshes the forward shock can quantitatively account for the shallow decay properties. In particular we demonstrate that this model can account for the plateau luminosity vs. end time anticorrelation.