Multi-wavelength analysis of the field of the dark burst GRB 031220

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monte Porzio Catone, Italy e-mail: melandri@mporzio.astro.it
² Dipartimento di Fisica, Università di Cagliari, S. P. Monserrato, 09042 Cagliari, Italy
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, via Fosso del Cavaliere 100, 00133, Roma, Italy
⁴ Istituto de Astrofisica de Andalucia (IAA-CSIC), Camino Bajo de Huetor 24, 18.008 Granada, Spain
⁵ Subaru Telescope – National Astronomical Observatory of Japan, Hilo, HI 96720, USA
⁶ Department of Physics – Tokyo Institute of Technology, Meguro, Tokyo 152-0033, Japan
⁷ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802-6305, USA

Received: 16 June 2005
Accepted: 13 October 2005

Abstract

We have collected and analyzed data taken in different spectral bands (from X-ray to optical and infrared) of the field of GRB 031220 and we present results of such multiband observations. Comparison between images taken at different epochs in the same filters did not reveal any strong variable source in the field of this burst. X-ray analysis shows that only two of the seven Chandra sources have a significant flux decrease and seem to be the most likely afterglow candidates. Both sources do not show the typical values of the colour but they appear to be redder. However, only one source has an X-ray decay index (1.3 ± 0.1) that is typical for observed afterglows. We assume that this source is the best afterglow candidate and we estimate a redshift of 1.90 ± 0.30. Photometric analysis and redshift estimation for this object suggest that this GRB can be classified as a Dark Burst and that the obscuration is the result of dust extinction in the burst circum medium or inside the host galaxy.