Properties and Possible Physical Origins of $\gamma$-ray Emission in Extreme Synchrotron Blazars

Abstract

Extreme synchrotron blazars, characterized by a first peak in their broadband spectral energy distributions (SEDs) at frequencies exceeding $10^{17}$ Hz, often exhibit a second peak beyond 1~TeV. These sources serve as ideal laboratories for studying particle acceleration and radiation mechanisms in relativistic jets. In this work, we systematically analyze the $\sim$16-year Fermi-LAT observational data for 25 extreme high-synchrotron-peaked BL Lacs (EHBLs). The results indicate that the majority of these sources display stable or low flux levels in the GeV band, with only 6 sources showing significant variability at a confidence level exceeding 5$\sigma$. The time-averaged spectra over the 16-year period for most EHBLs are well described by a hard power-law model, with photon indices predominantly clustered between 1.7 and 1.8. Using Fermi-LAT data in conjunction with multiwavelength observations compiled from the literature, we construct broadband SEDs for these EHBLs and fit them with a one-zone synchrotron + synchrotron-self-Compton (SSC) model. We find that this simplified theoretical framework is sufficient for modeling the observed SEDs of most of these EHBLs, albeit requiring relatively higher electron energies compared to other $\gamma$-ray emitting HBLs, and at times under-representing the UV emission. Based on the SED fitting results, we investigate the physical properties of the emission regions in these EHBLs and compare them with those of other $\gamma$-ray emitting HBLs. Consistent with other GeV--TeV $\gamma$-ray-emitting BL Lacs, the jets in these EHBLs are marked by low radiation efficiency and low magnetization.

34 pages, 7 figures, 4 tables, Published in ApJ

References in corpus (44)

• Active Galactic Nuclei: what's in a name?
• Incremental Fermi Large Area Telescope Fourth Source Catalog
• A hadronic origin for ultra-high-frequency-peaked BL Lac objects
• New constraints on the Mid-IR EBL from the HESS discovery of VHE gamma rays from 1ES 0229+200
• Multiwavelength Observations of Markarian 421 in March 2001: an Unprecedented View on the X-ray/TeV Correlated Variability
• The Spectral Energy Distributions of Fermi Blazars
• Hard TeV spectra of blazars and the constraints to the IR intergalactic background
• Discovery of VHE gamma-rays from the distant BL Lac 1ES 0347-121
• Multiwavelength observations of Mrk 501 in 2008
• Discovery of Very High Energy Gamma-Ray Emission from the BL Lac Object H2356-309 with the H.E.S.S. Cherenkov Telescopes
• Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
• A new hard X-ray selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties
• An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?
• Modeling the Extragalactic Background Light and the Cosmic Star Formation History
• Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017
• Study of the variable broadband emission of Markarian 501 during the most extreme Swift X-ray activity
• Optical spectroscopy of BL Lacertae objects. Broad lines, companion galaxies and redshift lower limits
• Investigating Broadband Variability of the TeV Blazar 1ES 1959+650
• A search for spectral hysteresis and energy-dependent time lags from X-ray and TeV gamma-ray observations of Mrk 421
• On the origin of the hard X-Ray excess of high-synchrotron-peaked BL Lac object Mrk 421
• Multi-frequency, Multi-Epoch Study of Mrk 501: Hints for a two-component nature of the emission
• Extreme Blazars Studied with Fermi-LAT and Suzaku: 1ES 0347-121 and Blazar Candidate HESS J1943+213
• Testing two-component models on very-high-energy gamma-ray emitting BL Lac objects
• Connecting steady emission and Very High Energy flaring states in blazars: the case of Mrk 421
• An intermittent extreme BL Lac: MWL study of 1ES 2344+514 in an enhanced state
• Long-term spectra of the blazars Mrk 421 and Mrk 501 at TeV energies seen by HAWC
• Hadronuclear interactions in the jet of low TeV luminosity AGN: Implications for the low-state very-high-energy gamma-ray emission
• A Steady-State Spectral Model For Electron Acceleration And Cooling In Blazar Jets: Application To 3C 279
• Blazars at Very High Energies: Emission Modelling
• Very-High-Energy $\gamma$-Ray Observations of the Blazar 1ES 2344+514 with VERITAS
• Swift XRT and UVOT deep observations of the high energy peaked BL Lac object PKS 0548-322 close to its brightest state
• Hunting extreme BL Lacertae blazars with Fermi-LAT
• Time dependent spectral modeling of Markarian 421 during a violent outburst in 2010
• A Two-Zone Model as origin of Hard TeV Spectrum in Extreme BL Lacs
• Extreme blazars: the result of unstable recollimated jets?
• The Energy Budget in the Jet of High-frequency Peaked BL Lacertae Objects
• Discovery of Very High Energy Gamma Rays from 1ES 1440+122
• Extremely High energy peaked BL Lac nature of the TeV blazar Mrk 501
• Multi-year characterisation of the broad-band emission from the intermittent extreme BL Lac 1ES~2344+514
• Exploring the Most Extreme Gamma-Ray Blazars Using Broadband Spectral Energy Distributions
• A Synchrotron Self-Compton Scenario for the Very High Energy Gamma-ray Emission of the Intermediate BL Lacertae object W Comae
• Detection of new Extreme BL Lac objects with H.E.S.S. and Swift XRT
• Prospects of additional contribution at Optical-NIR band of EBL in the light of VHE spectra
• Very high energy emission mechanism in the extreme blazar PGC 2402248