X-ray flashes or soft gamma-ray bursts?

The case of the likely distant XRF 040912

¹ Laboratoire d'Astrophysique de Toulouse, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France e-mail: gstratta@ast.obs-mip.fr
² Laboratoire d'Astrophysique de Marseille, Traverse du Siphon-Les trois Lucs, 13012 Marseille, France
³ Space Sciences Laboratory, University of California at Berkeley, 445 Campbell Hall, Berkeley, CA 94720, USA
⁴ INAF/IASF Roma, via fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos 3004, 18080 Granada, Spain
⁹ Instituto de Astrofísica de Canarias (IAC), C. via Láctea s/n, La Laguna, 38200 Tenerife, Spain
¹⁰ Observatoire Haute Provence (CNRS/OAMP), Saint Michel l'Observatoire, France
¹¹ Instituto Nacional de Pesquisas Espaciais, Avenida Dos Astronautas 1758, São José dos Campos 12227-010, Brazil
¹² Center for Space Research, MIT, 70 Vassar Street, Cambridge, MA 02139, USA
¹³ Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
¹⁴ Centre d'Étude Spatiale des Rayonnements, Observatoire Midi-Pyrénées, 9 avenue de Colonel Roche, 31028 Toulouse, France
¹⁵ Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM, 87545, USA
¹⁶ University of California at Berkeley, Space Sciences Laboratory, Berkeley, CA, 94720-7450, USA
¹⁷ Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
¹⁸ RIKEN (Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
¹⁹ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, 400 005, India
²⁰ Tsukuba Space Center, National Space Development Agency of Japan, Tsukuba, Ibaraki 305-8505, Japan
²¹ Department of Physics, Aoyama Gakuin University, Chitosedai 6-16-1 Setagaya-ku, Tokyo 157-8572, Japan
²² NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
²³ National Astronomical Observatory, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
²⁴ Faculty of engineering, Miyazaki University, Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan
²⁵ Department of Astronomy and Astrophysics, University of California at Santa Cruz, 477 Clark Kerr Hall, Santa Cruz, CA95064, USA

Received: 14 June 2006
Accepted: 28 September 2006

Abstract

Context.The origin of X-ray Flashes (XRFs) is still a mystery and several models have been proposed. To disentangle among these models, an important observational tool is the measure of the XRF distance scale, so far available only for a few of them.

Aims.In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties.

Methods.We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs).

Results.We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of  keV and a burst fluence ratio . A non-fading optical source with  mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to mag (3σ limiting magnitude) at 13.6 h after the burst. The host galaxy spectrum detected from 3800 Å to 10 000 Å, shows a single emission line at 9552 Å. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Lyα cut-off down to 3800 Å are consistent with the hypothesis of the [OII] line at redshift . The intrinsic spectral properties rank this XRF among the soft GRBs in the diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF (i-XRF) and are possibly associated with a different progenitor population. This scenario may call for a new definition of XRFs.