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

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

Highly luminous supernovae associated with gamma-ray bursts I. GRB 111209A/SN 2011kl in the context of stripped-envelope and superluminous supernovae

Context. GRB 111209A, one of the longest gamma-ray bursts (GRBs) ever observed, is linked to SN 2011kl, which is the most luminous GRB supernova (SN) detected so far. Several lines of evidence indicate that this GRB-SN is powered by a magnetar central engine. Aims. We place SN 2011kl into the context of large samples of SNe, addressing in more detail the question of whether this GRB-SN could be radioactively powered, and whether it represents an extreme version of a GRB-SN or an underluminous superluminous SN (SLSN). Methods. We modelled SN 2011kl using SN 1998bw as a template and derived a bolometric light curve including near-infrared data. We compared the properties of SN 2011kl to literature results on stripped-envelope and SLSNe. Results. A comparison in the k, s context, i.e. comparing SN 2011kl to SN 1998bw templates in terms of luminosity and light-curve stretch, clearly shows SN 2011kl is the most luminous GRB-SN to date and is spectrally very dissimilar to other events because it is significantly bluer/hotter. Although SN 2011kl does not reach the classical luminosity threshold of SLSNe and evolves faster than any of these objects, it resembles SLSNe more than the classical GRB-associated broad-lined Type Ic SNe in several aspects. Conclusions. GRB 111209A was a very energetic event, both at early (prompt emission) and at very late (SN) times. We show in a companion publication that with the exception of the extreme duration, the GRB and afterglow parameters are in agreement with the known distributions for these parameters. SN 2011kl, on the other hand, is exceptional both in luminosity and spectral characteristics, indicating that GRB 111209A was likely not powered by a standard-model collapsar central engine, further supporting our earlier conclusions. Instead, it reveals the possibility of a direct link between GRBs and SLSNe