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A Large Catalogue of Multi-wavelength GRB Afterglows I: Color Evolution And Its Physical Implication
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). KTH Royal Institute of Technology, Sweden; ICRANet, Italy; Chinese Academy of Sciences, Peopleʼs Republic of China.
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2018 (English)In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 234, no 2, article id 26Article in journal (Refereed) Published
Abstract [en]

The spectrum of gamma-ray burst (GRB) afterglows can be studied with color indices. Here we present a large comprehensive catalogue of 70 GRBs with multi-wavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. We categorize them into two samples based on how well the color indices are evaluated. The Golden sample includes 25 bursts mostly observed by GROND, and the Silver sample includes 45 bursts observed by other telescopes. For the Golden sample, we find that 95\% of the color indices do not vary over time. However, the color indices do vary during short periods in most bursts. The observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43\%) and in a constant density medium (30\%), (ii) early dust extinction (12\%), (iii) transition from reverse shock to forward shock emission (5\%), or (iv) an emergent supernova emission (10\%). We also study the evolutionary properties of the mean color indices for different emission episodes. We find that 86\% of the color indices in the 70 bursts show constancy between consecutive ones. The color index variations occur mainly during the late GRB-SN bump, the flare and early reversed-shock emission components. We further perform a statistical analysis of various observational properties and model parameters (spectral index β CI o, electron spectral indices p CI, etc.) using color indices. Overall, we conclude that ∼  90\% of colors are constant in time and can be accounted for by the simplest external forward shock model, while the varying color indices call for more detailed modeling.

Place, publisher, year, edition, pages
2018. Vol. 234, no 2, article id 26
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-150390DOI: 10.3847/1538-4365/aaa02aISI: 000424013500003OAI: oai:DiVA.org:su-150390DiVA, id: diva2:1167310
Available from: 2017-12-18 Created: 2017-12-18 Last updated: 2018-03-06Bibliographically approved
In thesis
1. Interpretation of gamma-ray burst X-ray and optical afterglow emission: From the central engine to the circumburst interaction
Open this publication in new window or tab >>Interpretation of gamma-ray burst X-ray and optical afterglow emission: From the central engine to the circumburst interaction
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gamma-ray bursts are the largest electromagnetic explosions known to happen in the Universe and are associated with the collapse of stellar progenitors into blackholes. After an energetic prompt emission phase, lasting typically less than a minute and emitted in the gamma-rays, a long-lived afterglow phase starts. During this phase strong emission is observed at longer wavelengths, e.g., in the X-ray and optical bands. This phase can last several weeks and carries important information about the energetics and structure of the burst as well as about  the circumburst medium (CBM) and its density profile. The standard afterglow model includes a single emission component which comes from synchrotron emission in a blast wave moving into the CBM. Additional factors that could give observable features include prolonged energy injection from the central engine, effects of the jet geometry, and viewing angle effects, which thus constitute an extended standard model.

In this thesis, I study the afterglow emission in a global approach by analysing large samples of bursts in search for general trends and characteristics. In paper I, I compare the light curves in the X-rays and in the optical bands in a sample of 87 bursts. I find that 62% are consistent with the standard afterglow model. Among these, only 9 cases have a pure single power law flux decay in all bands, and are therefore fully described by the model within the observed time window. Including the additional factors described above, I find that 91% are consistent with the extended standard model. An interesting finding is that in nearly half of all cases the plateau phase (energy injection phase) changes directly into the jet decay phase.  In paper II, I study the afterglow by analysing the temporal evolution of color indices (CI), defined as the magnitude difference between two filters. They can be used to study the energy spectrum with a good temporal resolution, even when high-resolution spectra are not available. I find that a majority of the CI do not vary with time, which means that the spectral slope does not change, even between different emission episodes. For the other cases, the variation is found to occur during limited periods. We suggest that they are due to the cooling frequency passing over the observed filter bands and, in other cases, due to the emergence of the underlying supernova emission. In paper III, I study the energetics of the GRBs that can be inferred from the afterglow observations. Using this information I analyse the limits it sets on what the central engine can be, if it is a magnetar or a spinning black hole. Assuming that the magnetar energy is emitted isotropically, I find that most bursts are consistent with a BH central engine and only around 20% are consistent with a magnetar central engine. As a consistency check, we derive the rotational energy and the spin period of the blackhole sample and the initial spin period and surface polar cap magnetic field for the magnetar sample and find them to be consistent to the expected values. We find that 4 of 5 of the short burst belong to the magnetar sample which supports the hypothesis that short GRB come from neutron star mergers.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2018
Keywords
gamma-ray bursts, central engine, afterglow
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-150386 (URN)978-91-7797-118-4 (ISBN)978-91-7797-119-1 (ISBN)
Public defence
2018-02-09, sal FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
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Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted. Paper 3: Submitted.

Available from: 2018-01-17 Created: 2017-12-18 Last updated: 2018-01-11Bibliographically approved

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