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Thermal components in the early X-ray afterglows of GRBs: likely cocoon emission and constraints on the progenitors
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. the Oskar Klein Centre, AlbaNova, SE-106-91 Stockholm, Sweden.
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. the Oskar Klein Centre, AlbaNova, SE-106-91 Stockholm, Sweden.ORCID iD: 0000-0003-0065-2933
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. the Oskar Klein Centre, AlbaNova, SE-106-91 Stockholm, Sweden.ORCID iD: 0000-0003-4000-8341
2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 474, no 2, p. 2401-2418Article in journal (Refereed) Published
Abstract [en]

The early X-ray afterglows of gamma-ray bursts (GRBs) are usually well described by absorbed power laws. However, in some cases, additional thermal components have been identified. The origin of this emission is debated, with proposed explanations including supernova shock breakout, emission from a cocoon surrounding the jet, as well as emission from the jet itself. A larger sample of detections is needed in order to place constraints on these different models. Here, we present a time-resolved spectral analysis of 74 GRBs observed by Swift X-ray Telescope in a search for thermal components. We report six detections in our sample, and also confirm an additional three cases that were previously reported in the literature. The majority of these bursts have a narrow range of blackbody radii around similar to 2 x 10(12) cm, despite having a large range of luminosities (L-peak similar to 10(47)-10(51) erg s(-1)). This points to an origin connected to the progenitor stars, and we suggest that emission from a cocoon breaking out from a thick wind may explain the observations. For two of the bursts in the sample, an explanation in terms of late prompt emission from the jet is instead more likely. We also find that these thermal components are preferentially detected when the X-ray luminosity is low, which suggests that they may be hidden by bright afterglows in the majority of GRBs.

Place, publisher, year, edition, pages
Oxford University Press, 2018. Vol. 474, no 2, p. 2401-2418
Keyword [en]
gamma-ray burst: general, X-rays: bursts
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-223805DOI: 10.1093/mnras/stx2920ISI: 000424343600068Scopus ID: 2-s2.0-85042616959OAI: oai:DiVA.org:kth-223805DiVA, id: diva2:1188238
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20180307

Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2018-05-21Bibliographically approved

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Valan, VlastaLarsson, JosefinAhlgren, Björn

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