Optical spectroscopy of blazars for the Cherenkov Telescope Array

Abstract

Context. Blazars are the most numerous class of high-energy (HE; E ∼ 50 MeV−100 GeV) and very high-energy (VHE; E ∼ 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacertae objects (BL Lacs), mainly due to the difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars. Aims. This paper is the first in a series of papers that aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory. Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements, but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium- to high-resolution spectroscopy of 19 blazar optical counterparts with the Keck II, SALT, and ESO NTT telescopes. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux. Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482, one tentative redshift, three redshift lower limits including one at z ≥ 0.449 and another at z ≥ 0.868. Four BL Lacs show featureless spectra.

Gamma-ray Cosmology and Tests of Fundamental Physics

“…Despite these difficulties, dedicated spectroscopic campaigns, preceding, coordinated with, or following up the gamma-ray discoveries (see Ref. [31] and references therein), have enabled to constrain the distances of over 80% of the extragalactic gamma-ray sources detected above 100 GeV. These redshift measurements and gamma-ray detections act as the starting blocks of the journey of gamma-rays over cosmological distances, whose multiple paths are discussed in the following sections.…”

Blazar-Boosted Dark Matter at Super-Kamiokande

“…For future prospects, it could be interesting to do a more refined analysis (improving, e.g., the size of the searching cone and/or the time correlation) through the combination of neutrino detectors data with the observations of blazars from telescopes, such as Fermi-LAT [53] and/or the planned Cherenkov Telescope Array project [54]. This, in principle, could enable us to select events from the right blazar at the optimal time, e.g.…”

Gamma-Ray Cosmology and Tests of Fundamental Physics

“…Of particular interest to the present discussion are source redshifts, whose spectroscopic measurements can be challenging when the emission lines from the host galaxy are overwhelmed by non-thermal emission from the outflows. Despite these difficulties, dedicated spectroscopic campaigns, preceding, coordinated with, or following up the gamma-ray discoveries (see [31] and references therein), have enabled to constrain the distances of over 80% of the extragalactic gamma-ray sources detected above 100 GeV. These redshift measurements and gamma-ray detections act as the starting blocks of the journey of gamma-rays over cosmological distances, whose multiple paths are discussed in the following sections.…”