MAGIC Observations of the Blazar TON 116 in a Multi-wavelength Context

This thesis focuses on the long-standing discussion on the blazar sequence intriguing the astrophysical community since the end of the '90s. Blazars constitute a subclass of Active Galactic Nuclei (AGNs) with a powerful radio jet pointing towards the Earth. They are characterized by a double-peaked Spectral Energy Distribution (SED), whose low-energy bump is thought to be due to synchrotron emission of electrons streaming in the jet, while the high-energy hump is likely due to inverse-Compton (IC) up-scattering of synchrotron photons by the same parent electrons. This emission mechanism is known as Synchrotron Self Compton (SSC). Blazars are further subdivided in Flat Spectrum Radio Quasars (FSRQs) and BL Lac objects, and their overall spectral trend follows a sequence which allows to discriminate the source type. The nature of the blazar sequence is still debated as actual feature of the blazar class or due to observing bias. Here we investigate TON 116, a High-frequency-peaked BL Lac (HBL) object supposed to be a candidate outlier of the dubbed blazar sequence. As a matter of fact, if the largest proposed values for its uncertain redshift are assumed, it happens to constitute one of the overluminous HBL that are hardly accommodated in the picture of the blazar sequence. This motivated us to start an observing campaign in the Very High Energy (VHE, E > 100 GeV) domain, which has never been explored before for this source, with the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC), one of the currently operating atmospheric Cherenkov detectors. A detection at VHE has a crucial role in assessing the nature of the targeted source, as it allows to put a solid constraint on the redshift, which is uncertain for TON 116 (z > 0.483). Unfortunately, MAGIC has not detected the source, but the observations unveiled its VHE emission for the first time, effectively constraining the blue tail of the IC bump, and confirming the Fermi-LAT insight about an IC peak at ≲ 100 GeV and a highly suppressed spectrum well before 1 TeV. Moreover, we studied the source in a multi-wavelength context, exploiting observations carried out by further instruments in different spectral bands, namely Fermi-LAT (HE gamma-ray), Swift-XRT (X-ray), and Sierra Nevada Observatory (OSN, optical). All the analyzed datasets allow us to obtain an overall picture of the spectral behaviour of TON 116, which is further processed by very recently developed fitting tools (python-based agnpy-sherpa packages and convolutional-neural-network-based MMDC platform). The broadband SED of the source is fitted with three SSC one-zone models, considering a power-law (PL) and a broken power-law (BPL) for the electron distribution, and a normalization to the total density. Including the upper limits improves the SSC PL model at VHE, but predicts an extreme HBL nature for TON 116. For this reason, the SSC BPL model turns out to be the best, as it also accounts for the upper limits without including them in the fitting procedure. This model is very similar to the one indicated by MMDC platform, involving a simple PL and a normalization to the electron luminosity. The best-fit parameters are then compared with the ones reported in the literature, validating our results about the most plausible broadband emission of TON 116 and confirming its HBL nature.