Next Article in Journal
Untangling Cosmic Magnetic Fields: Faraday Tomography at Metre Wavelengths with LOFAR
Next Article in Special Issue
Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501
Previous Article in Journal
Magnetizing the Cosmic Web during Reionization
Previous Article in Special Issue
Fermi: Monitoring the Gamma-Ray Universe
Article Menu

Export Article

Open AccessReview
Galaxies 2018, 6(4), 125; https://doi.org/10.3390/galaxies6040125

The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects

1
E. KharadzeAbastumani Astrophysical Observatory, Ilia State University, Colokashvili Av. 3/5, 0162 Tbilisi, Georgia
2
INAF, OsservatorioAstronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
Received: 19 October 2018 / Revised: 15 November 2018 / Accepted: 21 November 2018 / Published: 25 November 2018
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
Full-Text   |   PDF [3738 KB, uploaded 7 December 2018]   |  

Abstract

BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the TeV energy range, is of non-thermal origin and shows a typical two-component structure. The lower-energy component, ranging from the radio to X-rays in the high-energy peaked BL Lacertae sources, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic charged particles, to be initially accelerated via the Blandford–Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should lose the energy, sufficient for the emission of the keV-GeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales only if some additional acceleration mechanisms are continuously at work. According to different studies and simulations, the particles can gain tremendous energies due to the propagation of relativistic shocks through the jet: By means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock front, in the turbulent jet medium. Our intensive X-ray spectral study of TeV-detected, bright BL Lacertae objects (Mrk 421, 1ES 1959+650, Mrk 501) often show the signatures of the stochastic acceleration, while those related to the first-order Fermi process arefound relatively rarely. The TeV-undetected sources (1H 1516+660, BZB J1341+3959, BZB J1237+6258) mostly do not show the signatures of the efficient stochastic acceleration in their jets. View Full-Text
Keywords: BL Lacertae objects; galaxies; active BL Lacertae objects; galaxies; active
Figures

Figure 1

This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
SciFeed

Share & Cite This Article

MDPI and ACS Style

Kapanadze, B. The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects. Galaxies 2018, 6, 125.

Show more citation formats Show less citations formats

Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Related Articles

Article Metrics

Article Access Statistics

1

Comments

[Return to top]
Galaxies EISSN 2075-4434 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top