Physics from the Gamma-Ray Spectra of Blazars

Abstract

Blazars are numerous and often bright sources of cosmic γ-rays with several hundred
such objects currently detected in this regime. However, there are several outstanding
issues surrounding them. For example, it is uncertain exactly where in the kiloparsec-
scale blazar jet γ-rays are produced and what the physical mechanisms responsible for
the emission are. This thesis is an investigation in the physics and astronomy that can
be learnt from studying the γ-ray energy spectra of blazars. Two studies are presented
analysing bright blazars with the Fermi-Large Area Telescope (LAT). From the smooth
spectral curvature seen in many of the objects and the lack of sharp spectral breaks it is
concluded that the emission region is likely beyond the radius of the broad line region
of 0.1 pc. It is seen that nearby confusing objects can lead to apparent breaks being ob-
served in the spectra. However, a light curve analysis is performed of the bright blazar
3C 454.3 and it is seen that spectral breaks do sometimes occur when the object is in high
flux states. From the shift in the peak of the γ-ray emission it is concluded that the high
flux states are caused by an increase in the Lorentz factor of the emission region or by
changes in the population of electrons in the emission region, but the origin of the breaks
remains unexplained. Finally, a study is presented examining the effect that hypothetical
axions or axion-like particles (ALPs) would have on the spectra if these particles should
exist. It is found that under the right conditions a sudden flux boost could appear in
the spectrum at a few TeV. The chances of the future Cherenkov Telescope Array (CTA)
detecting the effects of photon-ALP mixing in AGN spectra is considered through simu-
lated observations. These effects potentially present themselves in the spectra two ways:
firstly the aforementioned flux boost and secondly due to the fact that the existence of
ALPs could mitigate the pair absorption that γ-rays undergo when they traverse inter-
galactic space. It is concluded that CTA will have good prospects of either detecting
these effects or else, if no effect is seen, setting limits on the mass and coupling constant
that ALPs could have.