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1. Optical spectroscopy of blazars for the Cherenkov Telescope Array – II

   https://doi.org/10.1093/mnras/stac3167  

   Kasai, E. ; Goldoni, P. ; Pita, S. ; Williams, D. A. ; Max-Moerbeck, W. ; Hervet, O. ; Cotter, G. ; Backes, M. ; Boisson, C. ; Becerra González, J. ; et al ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very high-energy (VHE) gamma rays to date with the imaging atmospheric Cherenkov telescopes. Apart from four nearby radio galaxies and two AGNs of unknown type, all known VHE AGNs are blazars. Knowledge of the cosmological redshift of gamma-ray blazars is key to enabling the study of their intrinsic emission properties, as the interaction between gamma rays and the extragalactic background light (EBL) results in a spectral softening. Therefore, the redshift determination exercise is crucial to indirectly placing tight constraints on the EBL density, and to studying blazar population evolution across cosmic time. Due to the powerful relativistic jets in blazars, most of their host galaxies’ spectral features are outshined, and dedicated high signal-to-noise (S/N) spectroscopic observations are required. Deep medium- to high-resolution spectroscopy of 33 gamma-ray blazar optical counterparts was performed with the European Southern Observatory, New Technology Telescope, Keck II telescope, Shane 3-metre telescope, and the Southern African Large Telescope. From the sample, spectra from 25 objects display spectral features or are featureless and have high S/N. The other eight objects have low-quality featureless spectra. We systematically searched for absorption and emission features and estimated, when possible, the fractional host galaxy flux in the measured total flux. Our measurements yielded 14 firm spectroscopic redshifts, ranging from 0.0838 to 0.8125, one tentative redshift, and two lower limits: one at $z > 0.382$ and the other at z > 0.629.

    

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2. Long-term Spectra of the Blazars Mrk 421 and Mrk 501 at TeV Energies Seen by HAWC

   https://doi.org/10.3847/1538-4357/ac58f6  

   Albert, A. ; Alfaro, R. ; Alvarez, C. ; Camacho, J. R. ; Arteaga-Velázquez, J. C. ; Arunbabu, K. P. ; Rojas, D. Avila ; Solares, H. A. ; Baghmanyan, V. ; Belmont-Moreno, E. ; et al ( April 2022 , The Astrophysical Journal)

   Abstract The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high-energy sky in the 300 GeV to >100 TeV energy range. HAWC has detected two blazars above 11 σ , Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501). The observations are comprised of data taken in the period between 2015 June and 2018 July, resulting in ∼1038 days of exposure. In this work, we report the time-averaged spectral analyses for both sources, above 0.5 TeV. Taking into account the flux attenuation due to the extragalactic background light, the intrinsic spectrum of Mrk 421 is described by a power law with an exponential energy cutoff with index α = 2.26 ± 0.12 stat − 0.2 + 0.17 sys and energy cutoff E c = 5.1 ± 1.6 stat − 2.5 + 1.4 sys TeV, while the intrinsic spectrum of Mrk 501 is better described by a simple power law with index α = 2.61 ± 0.11 stat − 0.07 + 0.01 sys . The maximum energies at which the Mrk 421 and Mrk 501 signals are detected are 9 and 12 TeV, respectively. This makes these some of the highest energy detections to date for spectra averaged over years-long timescales. Since the observation of gamma radiation from blazars provides information about the physical processes that take place in their relativistic jets, it is important to study the broadband spectral energy distributions (SEDs) of these objects. For this purpose, contemporaneous data in the gamma-ray band to the X-ray range, and literature data in the radio to UV range, were used to build time-averaged SEDs that were modeled within a synchrotron-self Compton leptonic scenario. 

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3. Optical spectroscopy of blazars for the Cherenkov Telescope Array

   https://doi.org/10.1051/0004-6361/202040090  

   Goldoni, P. ; Pita, S. ; Boisson, C. ; Max-Moerbeck, W. ; Kasai, E. ; Williams, D. A. ; D’Ammando, F. ; Navarro-Aranguiz, V. ; Backes, M. ; Barres de Almeida, U. ; et al ( June 2021 , Astronomy & Astrophysics)

   null (Ed.)

   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. 

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4. Normalization of the extragalactic background light from high-energy γ -ray observations

   https://doi.org/10.1051/0004-6361/201834240  

   Biasuzzi, B. ; Hervet, O. ; Williams, D. A. ; Biteau, J. ( July 2019 , Astronomy & Astrophysics)

   Extragalactic background light (EBL) plays an important role in cosmology since it traces the history of galaxy formation and evolution. Such diffuse radiation from near-UV to far-infrared wavelengths can interact with γ -rays from distant sources such as active galactic nuclei (AGNs), and is responsible for the high-energy absorption observed in their spectra. However, probing the EBL from γ -ray spectra of AGNs is not trivial due to internal processes that can mimic its effect. Such processes are usually taken into account in terms of curvature of the intrinsic spectrum. Hence, an improper choice of parametrization for the latter can seriously affect EBL reconstruction. In this paper, we propose a statistical approach that avoids a priori assumptions on the intrinsic spectral curvature and that, for each source, selects the best-fit model on a solid statistical basis. By combining the Fermi -LAT observations of 490 blazars, we determine the γ -ray-inferred level of EBL for various state-of-the-art EBL models. We discuss the EBL level obtained from the spectra of both BL Lacs and flat spectrum radio quasars (FSRQ) in order to investigate the impact of internal absorption in different classes of objects. We further scrutinize constraints on the EBL evolution from γ -ray observations by reconstructing the EBL level in four redshift ranges, up to z  ∼ 2.5. The approach implemented in this paper, carefully addressing the question of the modeling of the intrinsic emission at the source, can serve as a solid stepping stone for studies of hundreds of high-quality spectra acquired by next-generation γ -ray instruments. 

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5. Multiwavelength Observations of the Blazar VER J0521+211 during an Elevated TeV Gamma-Ray State

   https://doi.org/10.3847/1538-4357/ac6dd9  

   Adams, C. B. ; Batista, P. ; Benbow, W. ; Brill, A. ; Brose, R. ; Buckley, J. H. ; Capasso, M. ; Christiansen, J. L. ; Errando, M. ; Feng, Q. ; et al ( June 2022 , The Astrophysical Journal)

   Abstract We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly binned light curve is (8.8 ± 0.4) × 10 −7 photons m −2 s −1 , or ∼37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z ≤ 0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED and the radio morphology of the jet from the MOJAVE program are consistent with the source being an intermediate-frequency-peaked BL Lac object. 

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