Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects
BL Lac objects are active galactic nuclei notable for a beamed nonthermal radiation, which is generated in one of the relativistic jets forming a small angle to the observer’s line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). High-energy-peaked...
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| Format: | Article |
| Language: | English |
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MDPI AG
2023-07-01
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| Series: | Universe |
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| Online Access: | https://www.mdpi.com/2218-1997/9/7/344 |
| _version_ | 1797587332033937408 |
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| author | Bidzina Kapanadze |
| author_facet | Bidzina Kapanadze |
| author_sort | Bidzina Kapanadze |
| collection | DOAJ |
| description | BL Lac objects are active galactic nuclei notable for a beamed nonthermal radiation, which is generated in one of the relativistic jets forming a small angle to the observer’s line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). High-energy-peaked BL Lacs (HBLs) exhibit their lower-energy (synchrotron) peaks at UV to X-ray frequencies. The origin of the higher-energy SED component, representing the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray range in HBLs, is still controversial and different emission scenarios (one- and multi-zone synchrotron self-Compton, hadronic etc.) are proposed. In <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-rays, HBLs show a complex flaring behavior with rapid and large-amplitude TeV-band variations on timescales down to a few minutes. This review presents a detailed characterization of the hypothetical emission mechanisms which could contribute to the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray emission, their application to the nearby TeV-detected HBLs, successes in the broadband SED modeling and difficulties in the interpretation of the observational data. I also overview the unstable processes to be responsible for the observed <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray variability and particle energization up to millions of Lorentz factors (relativistic shocks, magnetic reconnection, turbulence and jet-star interaction). Finally, the future prospects for solving the persisting problems by means of the dedicated gamma-ray observations and sophisticated simulations are also addressed. |
| first_indexed | 2024-03-11T00:35:36Z |
| format | Article |
| id | doaj.art-d4d28f793c17440bb8cceb50805c7c70 |
| institution | Directory Open Access Journal |
| issn | 2218-1997 |
| language | English |
| last_indexed | 2024-03-11T00:35:36Z |
| publishDate | 2023-07-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Universe |
| spelling | doaj.art-d4d28f793c17440bb8cceb50805c7c702023-11-18T21:40:02ZengMDPI AGUniverse2218-19972023-07-019734410.3390/universe9070344Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae ObjectsBidzina Kapanadze0Space Research Center, Department of Astronomy and Astrophysics, School of Natural Sciences and Medicine, Ilia State University, Cholokashvili Av. 3/5, Tbilisi 0162, GeorgiaBL Lac objects are active galactic nuclei notable for a beamed nonthermal radiation, which is generated in one of the relativistic jets forming a small angle to the observer’s line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). High-energy-peaked BL Lacs (HBLs) exhibit their lower-energy (synchrotron) peaks at UV to X-ray frequencies. The origin of the higher-energy SED component, representing the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray range in HBLs, is still controversial and different emission scenarios (one- and multi-zone synchrotron self-Compton, hadronic etc.) are proposed. In <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-rays, HBLs show a complex flaring behavior with rapid and large-amplitude TeV-band variations on timescales down to a few minutes. This review presents a detailed characterization of the hypothetical emission mechanisms which could contribute to the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray emission, their application to the nearby TeV-detected HBLs, successes in the broadband SED modeling and difficulties in the interpretation of the observational data. I also overview the unstable processes to be responsible for the observed <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray variability and particle energization up to millions of Lorentz factors (relativistic shocks, magnetic reconnection, turbulence and jet-star interaction). Finally, the future prospects for solving the persisting problems by means of the dedicated gamma-ray observations and sophisticated simulations are also addressed.https://www.mdpi.com/2218-1997/9/7/344galaxiesBL Lacertae objectsgeneral |
| spellingShingle | Bidzina Kapanadze Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects Universe galaxies BL Lacertae objects general |
| title | Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects |
| title_full | Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects |
| title_fullStr | Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects |
| title_full_unstemmed | Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects |
| title_short | Gamma-ray Emission and Variability Processes in High-Energy-Peaked BL Lacertae Objects |
| title_sort | gamma ray emission and variability processes in high energy peaked bl lacertae objects |
| topic | galaxies BL Lacertae objects general |
| url | https://www.mdpi.com/2218-1997/9/7/344 |
| work_keys_str_mv | AT bidzinakapanadze gammarayemissionandvariabilityprocessesinhighenergypeakedbllacertaeobjects |