PhD Thesis: Interpretation of gamma-ray burst X-ray and optical afterglow emission
Friday 09 February 2018
to 14:00 at
Liang Li (Stockholm University, Department of Physics)
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.
Keywords: gamma-ray bursts, central engine, afterglow.