The dark nature of GRB 051022 and its host galaxy ^*

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 3.004, 18.080 Granada, Spain e-mail: ajct@iaa.es
² Institute de Radioastronomie Milimetrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d'Héres, France 

³ Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany 

⁴ Nikolaev State University, Nikolskaya 24, 54030 Nikolaev, Ukraine
⁵ Special Astrophysical Observatory (SAO-RAS), Nizhnij Arkhyz, Karachai-Cirkassian Rep. 369167, Russia 

⁶ Instituto de Astrofísica de Canarias (IAC), Via Láctea s/n, La Laguna, Tenerife, Spain
⁷ Consejo Superior de Investigaciones Científicas (CSIC), Spain

⁸ Mullard Space Science Labratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

⁹ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263 129, India

¹⁰ Centre for Research and Education in Science and Technology (CREST), Indian Institute of Astrophysics Shidlaghatta Road, Hosakote 562 114, India

¹¹ Raman Research Institute (RRI), Bangalore 560 080, India

¹² Indian Institute of Astrophysics, Koramangala, Bangalore 560 034, India

¹³ Isaac Newton Group of Telescopes, PO Box 321, 38700 Santa Cruz de la Palma (Tenerife), Spain

¹⁴ Departamento de Física, EPS, Universidad de Jaén, Campus Las Lagunillas s/n, A3, 23071 Jaén, Spain
¹⁵ Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India

Received: 14 November 2006
Accepted: 20 August 2007

Abstract

Aims.We present multiwavelength (X-ray/optical/near-infrared/millimetre) observations of GRB 051022 between 2.5 h and ~1.15 yr after the event. It is the most intense gamma-ray burst (~10^-4 erg cm^-2) detected by HETE-2, with the exception of the nearby GRB 030329.

Methods.Optical and near infrared observations did not detect the afterglow despite a strong afterglow at X-ray wavelengths. Millimetre observations at Plateau de Bure (PdB) detected a source and a flare, confirming the association of this event with a moderately bright () galaxy.

Results.Spectroscopic observations of this galaxy show strong [O II], Hβ and [O III] emission lines at a redshift of 0.809. The spectral energy distribution (SED) of the galaxy implies A _V (rest frame) = 1.0 and a starburst occuring ~25 Myr ago, during which the star-forming-rate reached ~50 /yr. In conjunction with the spatial extent (~1″) it suggests a very luminous () blue compact galaxy, for which we also find . The X-ray spectrum shows evidence of considerable absorption by neutral gas with cm^-2 (rest frame). Absorption by dust in the host galaxy at certainly cannot account for the non-detection of the optical afterglow, unless the dust-to-gas ratio is quite different than that seen in our Galaxy (i.e. large dust grains).

Conclusions.It is likely that the afterglow of the dark GRB 051022 was extinguished along the line of sight by an obscured, dense star forming region in a molecular cloud within the parent host galaxy. This galaxy is different from most GRB hosts being brighter than L^* by a factor of 3. We have also derived a /yr and predict that this host galaxy will be detected at sub-mm wavelengths.