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 UVOT telescope 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.

Modeling The Most Luminous Supernova Associated with a Gamma-Ray Burst, SN 2011kl

“…A broken power law showed no such excess (e.g., Greiner et al 2015;Kann et al 2016), so this excess is modeldependent. Moreover, a single power law was used by Cano et al (2016), and no such excess was observed, making it inconclusive.…”

“…The G15 LC did not include the NIR contribution and is a pseudobolometric LC. By adding the NIR contribution, Kann et al (2016) obtained a genuine bolometric LC (the K16 LC)…”

“…By using WMAP ΛCDM cosmology (Spergel et al 2003) and adopting H 0 = 71 km s −1 Mpc −1 , Ω M = 0.27, and Ω Λ = 0.73, Kann et al (2016) derived a luminosity distance (D L ) of 4076.5 Mpc for GRB 111209A/SN 2011kl. With a peak luminosity ∼ 3.63 +0.17 −0.16 × 10 43 erg s −1 (Kann et al 2016) 1 , SN 2011kl is the most luminous GRB-SN yet detected and is significantly more luminous than all other GRB-SNe, whose average peak luminosity is 1 × 10 43 erg s −1 with a standard deviation of 0.4 × 10 43 erg s −1 (Cano et al 2017).…”

“…With a peak luminosity ∼ 3.63 +0.17 −0.16 × 10 43 erg s −1 (Kann et al 2016) 1 , SN 2011kl is the most luminous GRB-SN yet detected and is significantly more luminous than all other GRB-SNe, whose average peak luminosity is 1 × 10 43 erg s −1 with a standard deviation of 0.4 × 10 43 erg s −1 (Cano et al 2017). The time of peak brightness of SN 2011kl is ∼ 14 days, slightly larger than the average (13.0 days with a standard deviation of 2.7 days; Cano et al 2017) of other GRB-SNe.…”

“…By subtracting the contribution from the ultraviolet (UV)/optical/near-infrared (NIR) afterglow (Kann et al 2017) of GRB 111209A, Kann et al (2016) obtained a new bolometric LC (hereafter the K16 LC) and modeled it with the radioactive heating model, finding 2.27 ± 0.64 M ⊙ of 56 Ni and 6.79…”