The 999th Swift gamma-ray burst: Some like it thermal

A multiwavelength study of GRB 151027A

¹ Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
e-mail: francesco.nappo@brera.inaf.it
² INAF–Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
³ SISSA, via Bonomea 265, 34136 Trieste, Italy
⁴ INAF–Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁵ Università degli Studi di Milano–Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁶ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, 34095 Montpellier, France
⁷ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
⁸ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁹ ASScience Data Center, via del Politecnico snc, 00133 Roma, Italy
¹⁰ Racah Institute of Physics, The Hebrew University of Jerusalem, 91907 Jerusalem, Israel
¹¹ INAF–IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy
¹² INAF–IASF Milano, via E. Bassini 15, 20133 Milano, Italy
¹³ GEPI–Observatoire de Paris, CNRS UMR 8111, Univ. Paris-Diderot, 5 place Jules Jannsen, 92190 Meudon, France

Received: 27 April 2016
Accepted: 1 July 2016

Abstract

We present a multiwavelength study of GRB 151027A. This is the 999th gamma-ray burst detected by the Swift satellite and it has a densely sampled emission in the X-ray and optical band and has been observed and detected in the radio up to 140 days after the prompt. The multiwavelength light curve from 500 s to 140 days can be modelled through a standard forward shock afterglow, but it requires an additional emission component to reproduce the early X-ray and optical emission. We present optical observations performed with the Telescopio Nazionale Galileo (TNG) and the Large Binocular Telescope (LBT) 19.6, 33.9, and 92.3 days after the trigger which show a bump with respect to a standard afterglow flux decay and are interpreted as possibly due to the underlying supernova and host galaxy (at a level of ~0.4 μJy in the optical R band, R_AB ~ 25). Radio observations, performed with the Sardinia Radio Telescope (SRT) and Medicina in single-dish mode and with the European Very Long Baseline Interferometer (VLBI) Network and the Very Long Baseline Array (VLBA), between day 4 and 140 suggest that the burst exploded in an environment characterized by a density profile scaling with the distance from the source (wind profile). A remarkable feature of the prompt emission is the presence of a bright flare 100 s after the trigger, lasting ~70 s in the soft X-ray band, which was simultaneously detected from the optical band up to the MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, the broadband (0.3–1000 keV) time resolved spectral analysis of the flare reveals the coexistence of a non-thermal (power law) and thermal blackbody components. The blackbody component contributes up to 35% of the luminosity in the 0.3–1000 keV band. The γ-ray emission observed in Swift-BAT and Fermi-GBM anticipates and lasts less than the soft X-ray emission as observed by Swift-XRT, arguing against a Comptonization origin. The blackbody component could either be produced by an outflow becoming transparent or by the collision of a fast shell with a slow, heavy, and optically thick fireball ejected during the quiescent time interval between the initial and later flares of the burst.