Speaker
Description
Blazars represent the most numerous class of High Energy (HE; E about 50 MeV to few 100 GeV) and Very High Energy (VHE; E about 100 GeV to 10 TeV) gamma-ray emitters.
The knowledge of their redshift is fundamental for understanding their emission and conducting population studies. Furthermore, the redshift is essential for indirect studies of the extragalactic background light as well as for searches regarding Lorentz invariance violation and axion-like particles. BL Lacs are a particular type of blazars characterized by their nearly featureless, continuum-dominated optical spectra, which pose a challenge to the measurement of their corresponding redshift. As a result, this important quantity is available for only about half of the currently known gamma-ray BL Lacs.
The upcoming Cherenkov Telescope Array Observatory (CTAO), a ground-based gamma-ray observatory with one site in La Palma in the Canary Islands and one planned for construction in Paranal, Chile, will detect several hundred VHE gamma-ray blazars in the near future. In preparation for CTAO operations, we initiated a program several years ago to measure redshifts for gamma-ray blazars likely to be detected by the CTAO. Following initial Monte Carlo simulations to select a suitable sample, we organized optical observing campaigns involving mainly spectroscopic observations to efficiently constrain their redshifts. This extensive, long-term program has resulted in six refereed publications, 54 new redshift and 11 redshift lower limit measurements.
In this talk we will describe our campaigns, highlight the main results achieved, and outline planned future developments.