The nature of the X-ray flash of August 24 2005^*,**

Photometric evidence for an on-axis z burst with continuous energy injection and an associated supernova?

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
² Stockholm Observatory, Department of Astronomy, AlbaNova, 106 91 Stockholm, Sweden
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 3.004, 18.080 Granada, Spain
⁴ Columbia Astrophysics Laboratory, 550 West 120th Street, Columbia University, New York, NY 10027-6601, USA
⁵ Centre for Astrophysics Research, University of Hertfordshire, Collage Lane, Hatfield, Herts, AL10 9AB, UK
⁶ Institute of Theoretical Astrophysics, PO Box 1029, 0315 Oslo, Norway
⁷ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁸ Space Research Institute (IKI), 84/32 Profsoyuznaya Str, Moscow 117997, Russia
⁹ Ulugh Beg Astronomical Institute, Tashkent 700052, Uzbekistan
¹⁰ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
¹² Crimean Laboratory of Sternberg Astronomical Institute MSU, Nauchny, Crimea, 98409, Ukraine
¹³ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Received: 2 November 2006
Accepted: 18 January 2007

Abstract

Aims.Our aim is to investigate the nature of the X-Ray Flash (XRF) of August 24, 2005.

Methods.We present comprehensive photometric R-band observations of the fading optical afterglow of XRF 050824, from 11 min to 104 days after the burst. In addition we present observations taken during the first day in the  bands and two epochs of spectroscopy. We also analyse available X-ray data.

Results.The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law, albeit with a rather shallow slope of (). Our late R-band images reveal the host galaxy. The rest-frame B-band luminosity is ~. The star-formation rate as determined from the [O II] emission line is ~ yr^-1. When accounting for the host contribution, the slope is  ± 0.01 and a break in the lightcurve is suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still shows excess scatter in the lightcurve in the form of wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario, with an on-axis viewing angle and a wide jet opening angle (). If the energy injections are episodic this could potentially help explain the bumps and wiggles.
Spectroscopy of the afterglow gives a redshift of  ± 0.005 from both absorption and emission lines. The spectral energy distribution (SED) of the afterglow has a power-law () shape with slope  ± 0.04. This can be compared to the X-ray spectral index which is  ± 0.1. The curvature of the SED constrains the dust reddening towards the burst to  mag.