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Published September 2018 | Submitted + Published
Journal Article Open

The optical/NIR afterglow of GRB 111209A: Complex yet not unprecedented

Abstract

Context. Afterglows of gamma-ray bursts (GRBs) are simple in the most basic model, but can show many complex features. The ultra-long duration GRB 111209A, one of the longest GRBs ever detected, also has the best-monitored afterglow in this rare class of GRBs. Aims. We want to address the question whether GRB 111209A was a special event beyond its extreme duration alone, and whether it is a classical GRB or another kind of high-energy transient. The afterglow may yield significant clues. Methods. We present afterglow photometry obtained in seven bands with the GROND imager as well as in further seven bands with the Ultraviolet/Optical Telescope (UVOT) on-board the Neil Gehrels Swift Observatory. The light curve is analysed by multi-band modelling and joint fitting with power-laws and broken power-laws, and we use the contemporaneous GROND data to study the evolution of the spectral energy distribution. We compare the optical afterglow to a large ensemble we have analysed in earlier works, and especially to that of another ultra-long event, GRB 130925A. We furthermore undertake a photometric study of the host galaxy. Results. We find a strong, chromatic rebrightening event at ≈0.8 days after the GRB, during which the spectral slope becomes redder. After this, the light curve decays achromatically, with evidence for a break at about 9 days after the trigger. The afterglow luminosity is found to not be exceptional. We find that a double-jet model is able to explain the chromatic rebrightening. The afterglow features have been detected in other events and are not unique. Conclusions. The duration aside, the GRB prompt emission and afterglow parameters of GRB 111209A are in agreement with the known distributions for these parameters. While the central engine of this event may differ from that of classical GRBs, there are multiple lines of evidence pointing to GRB 111209A resulting from the core-collapse of a massive star with a stripped envelope.

Additional Information

© 2018 ESO. Article published by EDP Sciences. Received 1 June 2017; Accepted 3 May 2018; Published online 01 October 2018. Partially based on observations obtained under programme 088.A-0051(C), PI: J. P. U. Fynbo. DAK wishes to dedicate this work to his father, R.I.P. 20. 08. 2015. You are sorely missed by so many. We thank the referee for the very valuable report which helped improve the clarity of the paper. DAK acknowledges Massimiliano De Pasquale, Daniele Malesani, Antonio de Ugarte Postigo, Christina C. Thöne, Thomas Kampf, Cristiano Guidorzi, and Raffaella Margutti for interesting discussions and helpful comments. DAK acknowledges financial support by the DFG Cluster of Excellence "Origin and Structure of the Universe," from MPE, from TLS, from the Spanish research project AYA 2014-58381-P, and from Juan de la Cierva Incorporación fellowship IJCI-2015-26153. We are indebted to Joe Lyman and Vicki Toy for supplying the bolometric light curves of GRB 120422A/SN 2012bz and GRB 130702A/SN 2013dx, respectively. SK, DAK, ARossi, and ANG acknowledge support by DFG grants Kl 766/16-1 and Kl 766/16-3, SSchmidl also acknowledges the latter. ARossi acknowledges support from the Jenaer Graduiertenakademie and by the project PRIN-INAF 2012 "The role of dust in galaxy evolution". TK acknowledges support by the DFG Cluster of Excellence Origin and Structure of the Universe, and by the European Commission under the Marie Curie Intra-European Fellowship Programme. RF acknowledges support from European Regional Development Fund-Project "Engineering applications of microworld physics" (No. CZ.02.1.010.00.016_0190000766). DARK is funded by the DNRF. FOE acknowledges funding of his Ph.D. through the DAAD, and support from FONDECYT through postdoctoral grant 3140326. SSchulze acknowledges support from CONICYT-Chile FONDECYT 3140534, Basal-CATA PFB-06/2007, and Project IC120009 "Millennium Institute of Astrophysics (MAS)" of Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo. SK, SSchmidl, and ANG acknowledge support by the Thüringer Ministerium für Bildung, Wissenschaft und Kultur under FKZ 12010-514. MN and PS acknowledge support by DFG grant SA 2001/2-1. ANG, DAK, ARossi and AU are grateful for travel funding support through MPE. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

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August 19, 2023
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