The luminous host galaxy, faint supernova and rapid afterglow rebrightening of GRB 100418A^⋆

¹ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada  
e-mail: deugarte@iaa.es
² Dark Cosmology Centre, Niels Bohr Institute, Juliane Maries Vej 30, Copenhagen Ø 2100, Denmark
³ The George Washington University, 725 21st street, NW Washington DC 20052, USA
⁴ GWU/Astronomy, Physics and Statistics Institute of Sciences (APSIS), USA
⁵ Astroparticules et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
⁶ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763 0355, Santiago, Chile
⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁸ Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, 12800 Prague, Czech Republic
⁹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
¹⁰ Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País-Vasco UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
¹¹ Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
¹² Kazan Federal University, Kremlevskaya Str., 18, 420008 Kazan, Russia
¹³ Academy of Sciences of Tatarstan, Bauman Str., 20, 420111 Kazan, Russia
¹⁴ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’Héres, France
¹⁵ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
¹⁶ National Research University Higher School of Economics, Moscow, Russia
¹⁷ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
¹⁸ Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹⁹ Astronomical Institute of the Czech Academy of Sciences, Frikova 298, 251 65 Ondrejov, Czech Republic
²⁰ TUBITAK National Observatory, Akdeniz Universitesi Yerleskesi, 07058 Antalya, Turkey
²¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²² Universitá degli studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
²³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
²⁴ Harvard-Smithsonian Center for Astrophysics, Submillimeter Array, 645 North A’ohoku Place, Hilo, HI, 96720, USA
²⁵ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
²⁶ Monash Centre for Astrophysics, Monash University, PO Box 27, Clayton, Victoria 3800, Australia
²⁷ School of Physics and Astronomy, Monash University, PO Box 27, Clayton, Victoria 3800, Australia
²⁸ INAF-Istituto Astrofisica e Planetologia Spaziali, Via Fosso Cavaliere 100, 00133 Rome, Italy
²⁹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
³⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³² Australian Astronomical Observatory, PO Box 915, North Ryde, NSW, 1670 Australia

Received: 13 June 2018
Accepted: 24 July 2018

Abstract

Context. Long gamma-ray bursts (GRBs) give us the chance to study both their extreme physics and the star-forming galaxies in which they form.

Aims. GRB 100418A, at a redshift of z = 0.6239, had a bright optical and radio afterglow, and a luminous star-forming host galaxy. This allowed us to study the radiation of the explosion as well as the interstellar medium of the host both in absorption and emission.

Methods. We collected photometric data from radio to X-ray wavelengths to study the evolution of the afterglow and the contribution of a possible supernova (SN) and three X-shooter spectra obtained during the first 60 h.

Results. The light curve shows a very fast optical rebrightening, with an amplitude of ∼3 magnitudes, starting 2.4 h after the GRB onset. This cannot be explained by a standard external shock model and requires other contributions, such as late central-engine activity. Two weeks after the burst we detect an excess in the light curve consistent with a SN with peak absolute magnitude M_V = −18.5 mag, among the faintest GRB-SNe detected to date. The host galaxy shows two components in emission, with velocities differing by 130 km s⁻¹, but otherwise having similar properties. While some absorption and emission components coincide, the absorbing gas spans much higher velocities, indicating the presence of gas beyond the star-forming regions. The host has a star formation rate of SFR = 12.2 M_⊙ yr⁻¹, a metallicity of 12 + log(O/H) = 8.55, and a mass of 1.6 × 10⁹ M_⊙.

Conclusions. GRB 100418A is a member of a class of afterglow light curves which show a steep rebrightening in the optical during the first day, which cannot be explained by traditional models. Its very faint associated SN shows that GRB-SNe can have a larger dispersion in luminosities than previously seen. Furthermore, we have obtained a complete view of the host of GRB 100418A owing to its spectrum, which contains a remarkable number of both emission and absorption lines.