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

摘要

We present a multiwavelength study of GRB 151027A. This is the 999th GRBuddetected by the Swift satellite and it has a densely sampled emission in theudX-ray and optical band and has been observed and detected in the radio up toud140 days after the prompt. The multiwavelength light curve from 500 s to 140uddays can be modelled through a standard forward shock afterglow but requires anudadditional component to reproduce the early X-ray and optical emission. Weudpresent TNG and LBT optical observations performed 19.6, 33.9 and 92.3 daysudafter the trigger which show a bump with respect to a standard afterglow fluxuddecay and are possibly interpreted as due to the underlying SN and host galaxyud(of 0.4 uJy in the R band). Radio observations, performed with SRT, Medicina,udEVN and VLBA between day 4 and 140, suggest that the burst exploded in anudenvironment characterised by a density profile scaling with the distance fromudthe source (wind profile). A remarkable feature of the prompt emission is theudpresence of a bright flare 100 s after the trigger, lasting 70 seconds in theudsoft X-ray band, which was simultaneously detected from the optical band up toudthe MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, theudbroadband (0.3-1000 keV) time resolved spectral analysis of the flare revealsudthe coexistence of a non-thermal (power law) and thermal blackbody components.udThe BB component contributes up to 35% of the luminosity in the 0.3-1000 keVudband. The gamma-ray emission observed in Swift-BAT and Fermi-GBM anticipatesudand lasts less than the soft X-ray emission as observed by Swift-XRT, arguingudagainst a Comptonization origin. The BB component could either be produced byudan outflow becoming transparent or by the collision of a fast shell with audslow, heavy and optically thick fireball ejected during the quiescent timeudinterval between the initial and later flares of the burst.