Letter to the Editor

GRB 090426: Discovery of a jet break in a short burst afterglow^⋆

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: ana@tls-tautenburg.de
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
³ Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748, Garching, Germany
⁴ Institute of Astronomy, University of Cambridge, Madingley Road CB3 0HA, Cambridge, UK
⁵ School of Physics, University College Dublin, Dublin 4, Republic of Ireland
⁶ Clemson University, Department of Physics and Astronomy, Clemson, SC 29634-0978, USA
⁷ CRESST and the Observational Cosmology Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA
⁸ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 4 February 2011
Accepted: 16 April 2011

Abstract

Context. The link between the duration of GRBs and the nature of their progenitors remains disputed. Short bursts (with durations of less than  ~2 s) are less frequently observed, technically more difficult to localize, and exhibit significantly fainter afterglows.

Aims. It is of critical importance to establish whether the burst duration can reliably distinguish the different GRB population models of collapsars and compact stellar mergers. The Swift GRB 090426 provides an unique opportunity to address this question. Its duration (T₉₀ = 1.28 s) places GRB 090426 firmly in the short burst population, while the high redshift (z = 2.609), host galaxy properties, and prompt emission spectral characteristics are more similar to those of long-duration GRBs.

Methods. On the basis of data obtained with the Tautenburg 2 m telescope (Germany) and the 7-channel imager GROND (La Silla, Chile), we compiled the most finely sampled light curve available for a short burst optical/NIR afterglow. The light curve was then analysed in a standard fashion. GROND and XRT data were used to determine the broad-band spectral energy distribution of the afterglow across more than three orders of magnitude.

Results. Our data show that a light curve break exists at 0.4 days, which is followed by a steep decay. This light curve decay is achromatic in the optical/NIR bands, and interpreted as a post-jet break phase. The X-ray data do not disagree with this interpretation.

Conclusions. The half-opening angle of the suspected jet as well as the luminosity of the optical afterglow provide additional evidence that GRB 090426 is probably linked to the death of a massive star rather than to the merger of two compact objects.