Absorption features in sub-TeV gamma-ray spectra of BL Lac objects

Abstract

The production site of gamma rays in blazars is closely related to their interaction with the photon fields surrounding the active galactic nucleus. In this paper, we discuss an indirect method that may help to unveil the presence of ambient structures in BL Lac objects through the analysis of their gamma-ray spectrum. Gamma rays, passing through structures at different distances from the black hole, interact via $\gamma \gamma$ pair production with the corresponding photon fields and produce absorption features in their spectral energy distribution. An interaction with a putative broad-line region may reduce the gamma-ray flux only if its production site were very close to the central engine. However, if jet photons interact with a bath of optical-UV seed photons produced by a narrow-line region extended over the parsec scale, the consequent $\gamma \gamma$ process may cause absorption features detectable at a few hundreds GeV. The detection of such absorption features is facilitated in sources with spectra reaching TeV energies, and specifically HBLs and EHBLs (extreme blazars) may represent exceptional probes to investigate this topic. We discuss recent observations of an extreme blazar named 2WHSP J073326.7+515354 (or PGC 2402248), which shows evidence of such an absorption feature in its gamma-ray spectrum and narrow emission lines in the optical spectrum, suggesting the presence of narrow-line regions in its large-scale environment. Finally, we discuss how sub-TeV absorption features in the spectra of BL Lac objects may affect their broadband modeling, and eventually represent a powerful diagnostic tool to constrain the gamma-ray production site and the jet environment.

arXiv commentAccepted for publication in ApJ

References in corpus (34)

• Unified Schemes for Radio-Loud Active Galactic Nuclei
• The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission
• The Host Galaxies and Classification of Active Galactic Nuclei
• The Co-Evolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe
• The Spectral Energy Distribution of Fermi bright blazars
• The extragalactic optical-infrared background radiations, their time evolution and the cosmic photon-photon opacity
• The Cosmic Infrared Background: Measurements and Implications
• Extragalactic Background Light Inferred from AEGIS Galaxy SED-type Fractions
• Structured jets in TeV BL Lac objects and radiogalaxies
• Constraints on the Physical Parameters of TeV Blazars
• Reverberation Mapping of High-Luminosity Quasars: First Results
• The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements
• A simplified view of blazars: clearing the fog around long-standing selection effects
• The blazar sequence: a new perspective
• Swift observations of the very intense flaring activity of Mrk 421 during 2006: I. Phenomenological picture of electron acceleration and predictions for the MeV/GeV emission
• Stochastic Acceleration and the Evolution of Spectral Distributions in SSC Sources: A Self Consistent Modeling of Blazars' Flares
• Extreme synchrotron BL Lac objects
• Log-parabolic spectra and particle acceleration in blazars. III: SSC emission in the TeV band from Mkn 501
• GeV breaks in blazars as a result of gamma-ray absorption within the broad-line region
• The extragalactic background light revisited and the cosmic photon-photon opacity
• The Blazar Main Sequence
• The spectrum of the Broad Line Region and the high-energy emission of powerful blazars
• An optical spectroscopic survey of the 3CR sample of radio galaxies with z<0.3. I. Presentation of the data
• On the origin of gamma rays in Fermi blazars: beyond the broad line region
• The NuSTAR view on Hard-TeV BL Lacs
• The hard TeV spectrum of 1ES 0229+200: new clues from Swift
• Extended silicate dust emission in PG QSOs
• A new hard X-ray selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties
• The mystery of spectral breaks: Lyman continuum absorption by photon-photon pair production in the Fermi GeV spectra of bright blazars
• Testing emission models on the extreme blazar 2WHSP J073326.7+515354 detected at very high energies with the MAGIC telescopes
• Radio morphology-accretion mode link in FRII low-excitation radio galaxies
• Ionized Gas Outflows in Low Excitation Radio Galaxies Are Radiation Driven
• Meeting the Challenge from Bright and Fast Gamma-Ray Flares of 3C 279
• Optical spectral characterization of the the TeV extreme blazar 2WHSP J073326.7+515354

Cited by in corpus (1)

• Blazars at Very High Energies: Emission Modelling