Multiwavelength study of the 2020 and 2021 flares of S5 1803+784 and BL Lacertae
| dc.contributor.author | Omojola, Joseph | |
| dc.date.accessioned | 2023-11-23T08:56:59Z | |
| dc.date.available | 2023-11-23T08:56:59Z | |
| dc.date.issued | 2022 | |
| dc.description | A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, University of the Witwatersrand, 2022 | |
| dc.description.abstract | This thesis presents a detailed spectral and temporal analysis of two blazars (S5 1803+784 and BL Lacertae) during their recent flaring states from 2020 to 2021. They are both low synchrotron peak (LSP) BL Lac objects and reached their highest daily fluxes yet observed during these flares. The time-resolved spectral energy distribution (SED) and the spectral characteristics are used to study the high-energy emission mechanism and the nature and location of the emitting region in the framework of the single-zone leptonic jet model. The phenomenological spectral fits provide some constraints on the characteristics of the jet and emitting region structure, gamma-ray emission process, and the likely driver of particle acceleration. The two sources have unique emission characteristics that suggest the existence of external soft photons from outside the jet (EC) undergoing inverse Compton scattering in addition to synchrotron self-Compton scattering of the synchrotron photons intrinsic to the jet. In the case of S5 1803+784, the temporal analysis indicates six distinct flares, four of which have symmetric profiles, while two are asymmetric. The broadband SED of S5 1803+784 during the period labeled as Flare A, which peaked on April 12, 2020, was fit to a single-zone model using both synchrotron+SSC only and SSC+EC components. The X-ray emission in the quiescent state comes from inverse Compton scattering, while during the flare, the X-ray forms part of the upper tail of the synchrotron emission due to the acceleration of the emitting particles in the photon radiation field. The bestfit parameters show that the dusty torus (DT) may be the primary source of the external photons responsible for the inverse Compton emission. The synchrotron+SSC only model only fits the data if some parameters have unphysical values, favouring the SSC+EC model. The singlezone leptonic jet model could fit the SED during the flare with an emitting region size of 𝑅 = 3.42 × 1016 cm and a distance from the central black hole 𝑅𝐻 = 5. 00 × 1017cm. The size of the emitting region becomes more compact during the flare; one possible scenario is a collapse of the emitting region size, which could be triggered by sudden rearrangement of the magnetic field lines within the emitting region. The hard power-law spectrum of the low energy spectral slope is consistent with magnetic dissipation as the likely driver of particle acceleration during the flare. The highest daily flux ever observed from BL Lacerate (64.86± 3.16) × 10−7 cm−2 s −1 in the 0.1-300GeV energy range was observed with a flare duration of less than six days. The shortest flare has a duration of 2.1 days; the short variability time scale during the flares was also observed in both the X-ray and the optical wavebands. The BL Lacertae flare labeled as Flare A1 has an asymmetric flare profile with flare-time gamma-ray flux variability of 84%, the highest observed up to that time from BL Lacertae, with a less than a day variability timescale. The size of the emitting region of 𝑅 < 2.70 × 1015 cm, estimated using both the VLBI measured Doppler factor and the variability time scale (𝑡𝑣𝑎𝑟) from the gamma-ray flux cannot fit the SED with either the single-zone synchrotron+SSC-only or the SSC+EC model; the inability of the single-zone model to satisfy this constraint points to the possibility of multiple emitting blobs and that the gamma-ray flux variability timescale (𝑡𝑣𝑎𝑟) is likely from such blobs. The single-zone SED model produces the best fit with an emitting region of size 𝑅 = 1.38 × 1017 cm and the distance of the emitting region from the black hole 𝑅𝐻 = 1. 37 × 1017cm, which were used to constrain and model the SED in two-zone emission SSC+EC models. The two-zone model can reproduce the emitting region's physical features and spectral characteristics during the flare. In both blazars' SED models, the emitting region is particle-dominated. The gamma-ray emitting region electron low-energy spectral slopes return a hard power-law consistent with magnetic turbulence driven by reconnecting field lines. The two blazars share some similarities in that they are both BL Lac objects and LSP in their quiescent states, and the same mechanism may be responsible for enhancing particle acceleration in the emitting region during the flares; they require different approaches to explain the emitting region structure and emission processes and the observed fluxes during the flares. While S5 1803+784 SED S5 shows synchrotron-dominated X-ray emission during the flare for the first time, BL Lacertae requires a two-zone model to satisfy the variability time scale constraint. | |
| dc.description.librarian | PC(2023) | |
| dc.faculty | Faculty of Science | |
| dc.identifier.uri | https://hdl.handle.net/10539/37149 | |
| dc.language.iso | en | |
| dc.phd.title | PhD | |
| dc.school | Physics | |
| dc.subject | Temporal analysis | |
| dc.subject | Spectral analysis | |
| dc.subject | BL Lacertae | |
| dc.title | Multiwavelength study of the 2020 and 2021 flares of S5 1803+784 and BL Lacertae | |
| dc.type | Thesis |