The Broadband Behaviour of Bright TeV Gamma-Ray Emitting Blazars
This thesis presents a characterisation of the broadband behaviour of TeV gamma-ray emitting blazars that belong to the family of active galactic nuclei. Their high bolometric luminosity as well as their bright non-thermal emission up to at least TeV energies make them among the most energetic persistent sources in the universe. Blazars are also identified by large relativistic plasma jets that are aligned at a small angle to the observer’s line of sight. These jets have a size of order ∼kpc-Mpc and radiate non-thermal emission. The emission is strongly variable over the entire spectrum on all time scales and one can observe flux changes by an order of magnitude on sub-hour time scale in the most extreme cases. The origins of these phenomena are still poorly understood and the underlying mechanisms for the gamma-ray emission in blazars are still debated. These violent and extreme properties make blazars candidate sources of ultra-high-energy cosmic rays (E > 10^18 eV), whose origins are still unknown. Blazars are also potential factories of PeV neutrinos, which diffuse flux has been detected by the IceCube detector. The goal of this thesis is to provide a detailed characterisation of the broadband emission properties of blazars with the ultimate aim to understand their emission mechanisms. This work focuses on two TeV emitting blazars, belonging to the category of BL Lac type objects: 1ES 2344+514 and Mrk 421. These two objects exhibit on a temporary basis a synchrotron energy flux peaking at energies around or above 1 keV. Only a handful objects detected in the TeV band are known to exhibit such peculiar and extreme properties; they are dubbed as extreme high-frequency BL Lacs (EHBL). 1ES 2344+514 and Mrk 421 are also among the brightest and the closest TeV gamma-ray emitting blazars. Thanks to their brightness, their broadband evolution can be characterised on relatively short time scales by current instruments. Thanks to their proximity, their gamma-ray emission is not strongly absorbed by the extragalactic background light. Therefore, 1ES 2344+514 and Mrk 421 are ideal targets to better understand the most extreme phenomena observed in the jets of blazars. In order to achieve an optimal energy and temporal coverage from radio to TeV, the present work makes use of data from a large number of instruments: MAGIC, ACT, Fermi-LAT, Swift-BAT, Swift-XRT, Swift-UVOT, instruments from the Whole Earth Blazar Telescope network, NOT, KAIT, OVRO, Metsähovi and Medicina. This thesis also makes use of observations from the NuSTAR instrument, which is currently the most sensitive telescope in the hard X-ray regime (> 10 keV) and the only instrument capable of precisely measuring the flux on a sub-hour time scale at these energies. The NuSTAR data are crucial to constrain models given that within leptonic scenarios, the hard X-ray emission is expected to originate from electrons with similar energies than those responsible for the TeV emission measured by MAGIC and FACT. Based on this extensive multiwavelength coverage, several atypical results are reported and the complex behaviour of 1ES 2344+514 and Mrk 421 are characterised with a high precision. The emission is interpreted using leptonic scenarios set up for this work. The blazar 1ES 2344+514 was observed during a period of strong flux activity in summer 2016. The measured very-high-energy (VHE; E > 100 GeV) flux reached a similar level as the historical maximum. A high activity was also measured in the X-rays. The spectral properties during this peculiar event are investigated in detail. One finds that the X-ray spectrum is hard, indicating a low-energy component of the spectral energy distribution (SED) peaking around or above 10 18 Hz (∼4 keV). This value is about two orders of magnitude higher than during archival observations of 1ES 2344+514. 1ES 2344+514 showed an EHBL behaviour on a temporary basis in August 2016. The SED is interpreted with a leptonic and a proton-synchrotron scenario for the gamma-ray emission. Thanks to the wealth of multiwavelength data that has been collected, these models can be better constrained compared to previous works. One finds that both theoretical models are able to describe the SED but require a system out of equipartition by several orders of magnitude, challenging current acceleration process thought to occur in relativistic plasma jets. Mrk 421 was observed from radio to TeV over a ∼7-month period between November 2016 and June 2017. Based on the dense temporal and energy coverage, the correlation patterns of the source are characterised in order to better constrain the emission mechanisms. The VHE versus X-ray flux correlation is studied by binning the data into several sub-energy bands. The correlation is characterised up to the hard X-ray band. The fluxes are strongly correlated, in agreement with leptonic models, which predict that the X-ray and VHE fluxes are emitted by a single electron population. Nonetheless, the present work unveils that the correlation characteristics strongly vary across the sub-energy bands: the correlation ranges from a linear to a cubic trend. Over the multiwavelength campaign, several nights appear as outliers and follow different correlation patterns. They show an enhanced gamma-ray activity without a strong X-ray emission. Using simultaneous MAGIC/NuSTAR/Swift-XRT observations, this peculiar behaviour can be explained by an emitting region in the jet that is changing its size over time. The correlation between the X-ray and UV/optical emission is also studied. An anti-correlation at the significance level above 3σ is found. A hint of this pattern was measured only once until now and only at a marginal significance. The second occurrence of an anti-correlation reported in this work is an important result as it strongly supports the hypothesis that this behaviour has a real physical origin. It may be indicative of changes in the cooling and acceleration efficiencies of electrons in the jet. Finally, a short and bright VHE flare of Mrk 421 is investigated in detail. Interestingly, this VHE flare is identified by an absence of clear X-ray flux increase. This particular day appears as an evident outlier in the VHE versus X-ray correlation study. Using time-resolved SEDs and a leptonic modelling, this work proposes that such a behaviour may be caused by the appearance of a second electron population (which adds up to the electron population responsible for the quiescent state) with a narrow energy distribution located in a compact blob.