The 999thSwiftgamma-ray burst: Some like it thermal

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 seconds 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 seconds 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.

On the α–intensity correlation in gamma-ray bursts: subphotospheric heating with varying entropy

“…Indeed, they found for many time bins that the Band function is not constrained, even though the statistical significance S ≥ 20 (Vianello 2018) was required. We also note that other models have been used in the past, in particular, including multiple spectral components (e.g, Ryde 2005;Ryde & Pe'er 2009;Guiriec et al 2011;Axelsson et al 2012;Burgess et al 2014;Iyyani et al 2015;Yu et al 2015a;Nappo et al 2017) and spectral breaks (e.g. Barat et al 2000;Oganesyan et al 2017b,a;Ravasio et al 2018) for which the physical interpretation is different.…”

East Asia VLBI Network observations of the TeV Gamma-Ray Burst 190114C

“…Multi-epoch European VLBI Network (EVN) and VLBA observations of GRB 151027A played a pivotal role in favouring the wind density profile model, typical of the medium surrounding a massive star in the final stages of its evolution, rather than a homogeneous profile typical of the interstellar medium [32]. Given the exceptional brightness and energy spectrum of GRB 190114C, this seemed another ideal case to take advantage of high angular resolution and sensitivity provided by the VLBI technique.…”

Investigating Subphotospheric Dissipation in Gamma-Ray Bursts Using Joint Fermi–Swift Observations

“…GRB 151027A is the only GRB in our sample with an extra component reported in the literature. Nappo et al (2017) find a blackbody component at soft X-rays from joint fits to GBM, BAT, and XRT data. This was confirmed by Valan et al (2018), performing an analysis of XRT data using a power law plus a blackbody.…”

“…We note that the DREAM model provides adequate fits in the early times of XRT data, when the flux from the additional blackbody is low. When the blackbody becomes significant, we instead obtain a poor fit (see Nappo et al 2017;Valan et al 2018 for discussions on the blackbody flux). Nappo et al (2017) suggests that the blackbody originates from the re-acceleration of the fireball.…”

“…They performed fits using a doubly smoothly broken power-law, and interpret their results in terms of synchrotron radiation. Additionally, Nappo et al (2017) performed a time resolved joint analysis of data from several instruments, including GBM and XRT, of GRB 151027A, where they detect the presence of a thermal component at low energies. These different results are interesting also from the perspective of photospheric models, as these often have a curvature at energies around a few keV.…”