Colour variations in the GRB 120327A afterglow^⋆

¹ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 36, 23807 Merate (LC), Italy
e-mail: andrea.melandri@brera.inaf.it
² ICRANet-Rio, Centro Brasileiro de Pesquisas Físicas, Rua Dr. Xavier Sigaud 150, Rio de Janeiro, RJ 22290-180, Brazil
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Department of Physics, The George Washington University, Washington, DC 20052, USA
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ CEOU/Astronomy Program, Dept. of Physics & Astronomy, Seoul National University, Seoul, 151-742, South Korea
⁷ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁸ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Asakuchi, Okayama 719-0232, Japan
⁹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁰ Astrophysics Research Institute, Liverpool JMU, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹¹ Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
¹² Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
¹³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
¹⁴ Skynet Robotic Telescope Network, Dep. of Physics and Astronomy, Univ. of North Carolina, Chapel Hill, NC 27599-3255, USA
¹⁵ Department of Physics, North Carolina A&T State University, Greensboro, NC 27411, USA
¹⁶ Laboratoire Univers et Particules de Montpellier, Université Montpellier, Place Eugène Bataillon, 34095 Montpellier, France
¹⁷ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone ( Roma), Italy
¹⁸ ASI Science Data Centre, via del Politecnico snc, 00133 Roma, Italy
¹⁹ Centre for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
²⁰ Dipartimento di Fisica, Universitá di Ferrara, via Saragat 1, 44100 Ferrara, Italy
²¹ Ishigakijima Astronomical Observatory, NAO of Japan, 1024-1, Arakawa, Ishigaki, Okinawa 907-0024, Japan
²² University College Dublin, School of Physics UCD, Belfield, Dublin 4, Ireland
²³ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
²⁴ Astronomical Institute, Czech Academy of Sciences, (ASU CAS), Ondřejov, Czech Republic
²⁵ INAF–IASF Milano, via E. Bassini 15, 20133 Milano, Italy
²⁶ GEPI,Observatoire de Paris, CNRS, Univ. Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
²⁷ Korea Astronomy and Space Science Institute, Daejeon 34055, Korea

Received: 11 August 2017
Accepted: 21 September 2017

Abstract

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 × 10⁴ 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.