The obscured hyper-energetic GRB 120624B hosted by a luminous compact galaxy at z = 2.20^⋆,⋆⋆

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: adeugartepostigo@gmail.com
² Dark Cosmology Centre, Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
³ INAF − Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, LC, Italy
⁴ Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, ETS Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
⁵ Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
⁶ Department of Astronomy and Astrophysics, Department of Physics, and Center for Particle and Gravitational Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁷ European Southern Observatory, Vitacura Casilla 19001, 19 Santiago de Chile, Chile
⁸ Harvard-Smithsonian Center for Astrophysics, Submillimeter Array, 645 North A’ohoku Place, Hilo, HI 96720, USA
⁹ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

Received: 12 June 2013
Accepted: 4 September 2013

Abstract

Context. Gamma-ray bursts (GRBs) are the most luminous explosions that we can witness in the Universe. Studying the most extreme cases of these phenomena allows us to constrain the limits for the progenitor models.

Aims. In this Letter, we study the prompt emission, afterglow, and host galaxy of GRB 120624B, one of the brightest GRBs detected by Fermi, to derive the energetics of the event and characterise the host galaxy in which it was produced.

Methods. Following the high-energy detection we conducted a multi-wavelength follow-up campaign, including near-infrared imaging from HAWKI/VLT, optical from OSIRIS/GTC, X-ray observations from the Chandra X-ray Observatory and at submillimetre/millimetre wavelengths from SMA. Optical/NIR spectroscopy was performed with X-shooter/VLT.

Results. We detect the X-ray and NIR afterglow of the burst and determine a redshift of z = 2.1974 ± 0.0002 through identification of emission lines of [O ii], [O iii] and H-α from the host galaxy of the GRB. This implies an energy release of E_iso,γ = (3.0 ± 0.2) × 10⁵⁴ erg, amongst the most luminous ever detected. The observations of the afterglow indicate high obscuration with A_V > 1.5. The host galaxy is compact, with R_1/2 < 1.6 kpc, but luminous, at L ~ 1.5 L^∗ and has a star formation rate of 91 ± 6 M_⊙/yr as derived from Hα.

Conclusions. As for other highly obscured GRBs, GRB 120624B is hosted by a luminous galaxy, which we also prove to be compact, with very intense star formation. It is one of the most luminous host galaxies associated with a GRB, showing that the host galaxies of long GRBs are not always blue dwarf galaxies, as previously thought.