The very high energy gamma-ray spectra of AGN

Abstract

A total of six extragalactic objects have been detected so far at very high energies (VHE). They are BL Lac objects, a sub-group of active galactic nuclei characterized by intense nonthermal radiation. The VHE spectra of two of these, 1ES 1959 + 650 and 1ES 2344 + 514, were measured in this work. Similar to the other four BL Lacs detected, their VHE spectrum and flux level is highly variable and shows a broadband spectrum characterized by two emission peaks: one in X-ray, the other at GeV to TeV energies. For one of these, 1ES 1959 + 650, simultaneous observations were carried out at other wavelengths and for the first time, a VHE flare without increased X-ray flux level was recorded. For the other object, 1ES 2344 + 514, no simultaneous X-ray observations were taken, making further modeling impossible. VHE gamma-ray astronomy can establish important upper limits on the density of the extragalactic background light (EBL). If one can somehow guess what the source spectrum is, then one can infer the EBL density from the measured attenuation in the spectra. As the VHE spectra of BL Lac object are very similar, the zeroth order assumption was made (and justified) in this work that they are actually the same; differences in attenuation arising solely due to the different distances to the objects. The upper limits derived here are not very constraining, but they do question one particular set of EBL measurements that are very high in the near infrared waveband. Galaxy formation models are typically not able to reproduce this high density. The analysis of VHE spectra is still being developed; in part because of the new array of four telescopes, VERITAS, being built at the moment. Monte-Carlo simulations are used in this work and changes in the simulation software had not seen a comparison to the previous version until this work. Differences were identified that impact the energy reconstruction. A method was developed to calibrate the absolute energy scale by automatically identifying cosmic-ray muons recorded by the telescope.