The 999th Swift gamma-ray burst: Some like it thermal: A multiwavelength study of GRB 151027A

We present a multiwavelength study of GRB 151027A. This is the 999th GRBdetected by the Swift satellite and it has a densely sampled emission in theX-ray and optical band and has been observed and detected in the radio up to140 days after the prompt. The multiwavelength light curve from 500 s to 140days can be modelled through a standard forward shock afterglow but requires anadditional component to reproduce the early X-ray and optical emission. Wepresent TNG and LBT optical observations performed 19.6, 33.9 and 92.3 daysafter the trigger which show a bump with respect to a standard afterglow fluxdecay and are possibly interpreted as due to the underlying SN and host galaxy(of 0.4 uJy in the R band). Radio observations, performed with SRT, Medicina,EVN and VLBA between day 4 and 140, suggest that the burst exploded in anenvironment characterised by a density profile scaling with the distance fromthe source (wind profile). A remarkable feature of the prompt emission is thepresence of a bright flare 100 s after the trigger, lasting 70 seconds in thesoft X-ray band, which was simultaneously detected from the optical band up tothe MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, thebroadband (0.3-1000 keV) time resolved spectral analysis of the flare revealsthe coexistence of a non-thermal (power law) and thermal blackbody components.The BB component contributes up to 35% of the luminosity in the 0.3-1000 keVband. The gamma-ray emission observed in Swift-BAT and Fermi-GBM anticipatesand lasts less than the soft X-ray emission as observed by Swift-XRT, arguingagainst a Comptonization origin. The BB component could either be produced byan outflow becoming transparent or by the collision of a fast shell with aslow, heavy and optically thick fireball ejected during the quiescent timeinterval between the initial and later flares of the burst.