Spectroscopy and multiband photometry of the afterglow of intermediate duration γ-ray burst GRB 040924 and its host galaxy

¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: kwrsema@science.uva.nl
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
³ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3004, 18080 Granada, Spain
⁵ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ Danish National Space Center, Juliane Maries Vej 30, Copenhagen 0, 2100, Denmark
⁸ ESA-European Space Astronomy Centre, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
⁹ Instituto de Astrofísica de Canarias, C/. Via Lactea s/n, 38205 La Laguna, Tenerife, Spain
¹⁰ University of Canterbury, Private Bag 4800 Christchurch 8140, New Zealand
¹¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile

Received: 9 June 2007
Accepted: 14 December 2007

Abstract

Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB 040924. This GRB had a rather short duration of T₉₀ ~2.4 s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor.

Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data.

Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ~0.03 days. We determine the redshift z = 0.858 ± 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the [Ne III] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne III] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter.

Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star.