Letter to the Editor

Extensive multiband study of the X-ray rich GRB 050408^*,**

A likely off-axis event with an intense energy injection

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos 3004, 18080 Granada, Spain e-mail: deugarte@iaa.es
² Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchokskaya, Karachaevo-Cherkesia, 369167 Russia
³ Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
⁴ Nikolaev State University, Nikolska 24, 54030, Nikolaev, Ukraine
⁵ Dpto. de Física (EPS), Universidad de Jaén, 23071 Jaén, Spain
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ Astrophysics Research Inst., Liverpool John Moores Univ., Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
⁸ The UCL Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁹ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin d'Hères, France
¹⁰ Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan, PR China
¹¹ Institute for Physics and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
¹² Tokyo Institute of Technology, Ookayama, Meguro, Tokyo 152-8550, Japan
¹³ Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo 184, Japan
¹⁴ Kiso Observatory, Institute of Astronomy, University of Tokyo, Mitake-mura, Kiso-gun, Nagano 397-0101, Japan
¹⁵ Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan
¹⁶ Saitama University, Sakura-ku, Saitama 338-8570, Japan
¹⁷ TÜBITAK National Observatory, Akdeniz Üniversitesi, 07058, Antalya, Turkey
¹⁸ Akdeniz University, Physics Department, 07058 Antalya, Turkey
¹⁹ Middle East Technical University, Physics Department, Inonu Bulvari, Ankara, 06531, Turkey
²⁰ Departments of Astronomy, Kazan State University, Kremlevskaya Str., 18, Kazan, 420008, Russia
²¹ Academy of Sciences of Tatarstan, Bauman Str., 20, Kazan, 420111, Russia
²² Space Research Institute (IKI), 84/32 Profsoyuznaya, Moscow, 117997, Russia
²³ Raman Research Institute, Bangalore 560 080, India
²⁴ National Centre for Radio Astrophysics, Ganeshkhind, Pune, 411007, India

Received: 30 October 2006
Accepted: 14 December 2006

Abstract

Aims.Understand the shape and implications of the multiband ligth curve of GRB 050408, an X-ray rich (XRR) burst.

Methods.We present a multiband optical light curve, covering the time from the onset of the γ-ray event to several months after, when we only detect the host galaxy. Together with X-ray, millimetre and radio observations we compile what, to our knowledge, is the most complete multiband coverage of an XRR burst afterglow to date.

Results.The optical and X-ray light curve is characterised by an early flattening and an intense bump peaking around 6 days after the burst onset. We explain the former by an off-axis viewed jet, in agreement with the predictions made for XRR by some models, and the latter with an energy injection equivalent in intensity to the initial shock. The analysis of the spectral flux distribution reveals an extinction compatible with a low chemical enrichment surrounding the burst. Together with the detection of an underlying starburst host galaxy we can strengthen the link between XRR and classical long-duration bursts.