GRB 051028: an intrinsically faint gamma-ray burst at high redshift?

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 3.004, 18080 Granada, Spain e-mail: ajct@iaa.es
² Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
³ Max-Planck Institut für Astronomie, Koennigstuhl 17, 69117 Heidelberg, Germany
⁴ Indian Institute of Astrophysics, 560034 Bangalore, India
⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Instituto de Astrofísica de Canarias, via Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁷ Laboratoire d'Astrophysique de Marseille, 13376 Marseille, France
⁸ Nikolaev State University, Nikolskaya 24, 54030 Nikolaev, Ukraine
⁹ American Association of Variable Star Observers, Cambridge, MA, USA

Received: 22 December 2005
Accepted: 14 September 2006

Abstract

Aims.We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-2 in order to derive its afterglow emission parameters and to determine the reason for its optical faintness when compared to other events.

Methods.Observations were taken in the optical (2.0 m Himalayan Chandra Telescope, 1.34 m Tautenburg, 4.2 m William Herschel Telescope) and in X-rays (Swift/XRT) between 2.7 h and ~10 days after the onset of the event.

Results.The data can be interpreted by collimated emission in a jet with a typical value of which is moving in a homogeneous interstellar medium and with a cooling frequency still above the X-rays at 0.5 days after the burst onset. GRB 051028 can be classified as a “gray” or “potentially dark” GRB. On the basis of the combined optical and Swift/XRT data, we conclude that the reason for the optical dimness is not extra absorption in the host galaxy, but rather the GRB taking place at high-redshift. We also notice the very striking similarity with the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of 3.967, although GRB 051028 is ~3 mag fainter. We suggest that the bumps could be explained by multiple energy injection episodes and that the burst is intrinsically faint when compared to the average afterglows detected since 1997. The non-detection of the host galaxy down to is also consistent with the burst arising at high redshift, compatible with the published pseudo-z of .