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Characterization of very high energy gamma-radiation from active galactic nuclei observed by the MAGIC telescopes

Terzić, Tomislav

School

University of Zagreb Faculty of Science Department of Physics
Radmile Matejcic 2 51000 Rijeka Croatia

External urlhttp://bib.irb.hr/prikazi-rad?&rad=780389

Abstract:

Active galactic nuclei (AGN) are bright compact regions in the centres of galaxies, that emit radiation across the entire electromagnetic spectrum. They are believed to be powered by supermassive black holes (SMBH), which actively accrete matter. Some of that matter is ejected in the form of collimated jets of ultrarelativistic particles. Jets are sources of electromagnetic radiation of all wavelengths. A significant portion of en- ergy radiated by AGN is emitted in the form of γ-rays. In addition, γ-rays are produced through physical processes different from the ones responsible for lower energy radiation. Therefore, in order to understand how jets are formed and how particles are accelerated to ultrarelativistic energies, it is important to understand the mechanisms and locations of γ-ray production within AGN. To reach closer to this goal here we study the very high energy (VHE) γ-radiation from three sources belonging to different classes of AGN: M87 (radio galaxy), PKS 1222+21 (flat spectrum radio quasar) and H1722+119 (BL Lac object). For these sources we ob- tained VHE γ-ray spectra and light curves using the MAGIC ground-based Cherenkov telescopes. We used long term monitoring of M87 to evaluate the level of low emission state. We propose a structured jet “spine-layer” scenario to explain the emission from M87. It places the VHE γ-ray emission region in the vicinity of the SMBH, and suggests that, if viewed head on, the emission from M87 would resemble that from a typical BL Lac. PKS 1222+21 was first detected by the MAGIC telescopes. Very short variability timescale and absence of a cut-off in the VHE spectrum sets constraints on VHE γ-ray emission region, and on emission models. We performed a multiwavelength study us- ing contemporaneous data, concluding that the γ-ray emission region is most probably located outside of the broad line region, and that it is possible to explain the emission using a simple one-zone emission scenario. H1722+119 is a source with unknown red- shift, first detected by the MAGIC telescopes. We performed a multiwavelength study using contemporaneous data, and estimated the redshift of the source using γ-rays to z = 0.4. As expected, different types of emission models are required to explain emission from different types of sources. However, we also find that the VHE γ-radiation originates in different locations in the sources we studied.

Keywords:

active galactic nuclei, blazars, gamma-rays, imaging atmospheric Cheren- kov telescopes, M87, PKS 1222+21, 4C +21.35, H1722+119

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