Colour variations in the GRB 120327A afterglow

Aims. We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate possible causes of the observed early-time colour variations. Methods. We collected data from various instruments and telescopes in X-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times. Results. The ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 10 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. At early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction.© 2017 ESO.
A.M., S.Co., S.Ca., B.S., P.D.A., and G.T. acknowledge support from the ASI grant I/004/11/3. E.Z. acknowledges the support by the International Cooperation Program CAPES-ICRANet financed by CAPES Brazilian Federal Agency for Support and Evaluation of Graduate Education within the Ministry of Education of Brazil. D.M. acknowledges support from the Instrument center for Danish Astrophysics (IDA). C.G.M. acknowledges support from the Royal Society, the Wolfson Foundation, and the Science and Technology Facilities Council. L.H. acknowledges support from SFI (07-RFP-PHYF295, 11/RFP.1/AST/3188) and the EU-FP7/GLORIA (grant No. 283783). M.I. and J.W.K. acknowledge the support from the National Research Foundation of Korea grant Nos. 2017R1A3A3001362 and 2016R1D1A1B03934815. D.K. acknowledges the financial support from the Slovenian Research Agency (P1-0188). This work has been supported by ASI grant I/004/11/0 and by PRIN-MIUR 2009 grants.