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A CORRELATED STUDY OF OPTICAL AND X-RAY AFTERGLOWS OF GRBs
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.
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Number of Authors: 132015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 1, article id 13Article in journal (Refereed) Published
Abstract [en]

We study an extensive sample of 87 gamma-ray bursts (GRBs) for which there are well-sampled and simultaneous optical and X-ray light curves. We extract the cleanest possible signal of the afterglow component. and compare the temporal behaviors of the X-ray light. curve, observed by Swift XRT, and optical data, observed by UVOT and ground-based telescopes for each individual burst. Overall we find that 62% of the GRBs. are consistent with the standard afterglow model. When more advanced modeling is invoked, up to 91% of the bursts in our sample may be consistent with the external-shock model. A large fraction of these bursts are consistent with occurring in a constant interstellar density medium (61%) while only 39% of them occur in a wind-like medium. Only nine cases have afterglow light curves that exactly match the standard fireball model prediction, having a single power-law decay in both energy bands that are observed during their entire duration. In particular, for the bursts with chromatic behavior, additional model assumptions must be made over limited segments of the light curves in order for these bursts to fully agree with the external-shock model. Interestingly, for 54% of the X-ray and 40% of the optical band observations, the end of the shallow decay (t(similar to-0.5)) period coincides with the jet-break (t(similar to-p)) time, causing an abrupt change in decay slope. The fraction of the burst that is consistent with the external-shock model is independent of the observational epochs in the rest frame of GRBs. Moreover, no cases can be explained by the cooling frequency crossing the X-ray or optical band.

Place, publisher, year, edition, pages
2015. Vol. 805, no 1, article id 13
Keywords [en]
methods: statistical, reference systems, X-rays: ISM
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-118539DOI: 10.1088/0004-637X/805/1/13ISI: 000354991300013OAI: oai:DiVA.org:su-118539DiVA, id: diva2:826513
Available from: 2015-06-25 Created: 2015-06-22 Last updated: 2017-12-21Bibliographically 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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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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