GRB 070311: a direct link between the prompt emission and the afterglow^*

¹ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy e-mail: cristiano.guidorzi@brera.inaf.it
² INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Dunsink Observatory – DIAS, 31 Fitzwilliam Street, Dublin 2, Ireland
⁴ School of Physical Sciences and NCPST, Dublin City University, Dublin 9, Ireland
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
⁶ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 København Ø, Denmark
⁸ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, via P. Gobetti 101, 40129 Bologna, Italy
⁹ XMM-Newton Science Operations Centre, European Space Agency, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3.004, 18080 Granada, Spain
¹¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monte Porzio Catone, Italy
¹² ASI Science Data Center, via G. Galilei, 00044 Frascati (Roma), Italy (INAF personnel resident at ASDC.)
¹³ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
¹⁴ Università dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
¹⁵ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA, USA
¹⁶ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK

Received: 11 July 2007
Accepted: 7 August 2007

Abstract

Context.The prompt emission of gamma-ray bursts is mostly thought to be produced in internal shocks of relativistic shells emitted by the progenitor at different times, whereas the late multi-band afterglow is interpreted as the synchrotron emission of electrons swept up by the fireball expanding through the surrounding interstellar medium. The short timescale variability observed in flares superimposed on the X-ray/optical afterglow of several bursts, recently made possible by Swift, has been interpreted as evidence for prolonged activity of the inner engine through internal shocks. Yet, it is not clear whether this applies to all the observed bursts and, in particular, whether the bursts exhibiting single γ-ray pulses with no short timescale variability at late times could also be entirely interpreted as external shocks.

Aims.We present prompt γ-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt γ-ray pulse as well as for the early and late multi-band afterglow of GRB 070311.

Methods.We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays.

Results.The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the γ-ray burst and possibly accompanied by a faint γ-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3  10⁴ and 2  10⁵ s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt γ-ray pulse and an underlying power law.

Conclusions.This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock.