Establishing a particle distribution for multi-wavelength emission from BL Lac objects
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ORIGINAL ARTICLE
Establishing a particle distribution for multi-wavelength emission from BL Lac objects X.X. Xie1 · K.R. Zhu1 · S.J. Kang2 · Y.G. Zheng1
Received: 25 April 2020 / Accepted: 3 September 2020 © Springer Nature B.V. 2020
Abstract Electrons are accelerated at the shock wave diffuse and advect outward, and subsequently drift away into the emitting region of the jet that is located in the downstream flow from the plane shock. The current work considers the acceleration of the electrons in the shock front. Assuming a proper boundary condition at the interface between the shock and the downstream zones, a novel particle distribution in the downstream flow is proposed in this work to reproduce the broadband spectral energy distribution of BL Lac objects. We find that (1) we can obtain the particle distribution downstream of the shock wave in four cases; (2) electrons with higher energy (γ > γ0 ) dominate the emission spectrum; (3) the distinctly important physical parameters assumed in our model can reasonably reproduce the multi-wavelength spectrum of the high-synchrotron-peaked BL Lac object Markarian 421 (Mrk 421). Keywords Acceleration of particles: radiation mechanisms · Non-thermal · BL Lacertae objects: individual: (Mrk 421)
1 Introduction BL Lac objects are Active Galactic Nuclei (AGNs) characterized by a polarized and highly variable nonthermal continuum emission that extends from the radio to the TeV γ -ray bands. The multi-wavelength spectral energy distributions (SEDs) of these objects have two bumps shape. It is
B Y.G. Zheng
[email protected]
1
Department of Physics, Yunnan Normal University, Kunming, Yunnan, 650092, China
2
School of Physics and Electrical Engineering, Liupanshui Normal University, Liupanshui, Guizhou, 553004, China
well known that the low-energy peak occurs at frequencies ranging from the infrared to the X-ray band, and the highenergy peak occurs at frequencies ranging from the MeV to TeV band (Konigl 1981; Fossati et al. 1998; Costamante et al. 2001). Both the original lepton and the hadron models are the currently accepted mechanisms for the emission energy spectra of BL Lac objects (Marscher and Gear 1985; Bloom and Marscher 1996; Aleksic et al. 2012). In the original lepton scenario, the low-energy peak mainly comes from the synchrotron emission of ultra-relativistic electrons in the jet with a small angle to the observer’s line of sight, and the high-energy peak comes from the inverse Compton scattering of ultra-relativistic electrons (Mannheim 1993; Sikora et al. 1994; Atoyan and Dermer 2003). Due to the different sources of the seed photons of the inverse Compton scattering, the lepton model can be divided into the External Compton (EC) model and the Synchrotron Self-Compton (SSC) model (Maraschi et al. 1992; Ghisellini and Madau 1996; Inoue and Takahara 1996; Finke et al. 2008; Kang et al. 2016; Zhu et al. 2016; Zheng et al. 2017). In the original hadron scenario, it is considered that the high-energy emission comes from the cascade or synchrotron emission pr
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