On the Origin of the Hard X-Ray Excess of High-Synchrotron-Peaked BL Lac Object Mrk 421

Abstract

For the first time, Kataoka & Stawarz reported a clear detection of a hard X-ray excess, above 20 keV, in the high-synchrotron-peaked BL Lac object Mrk 421. We find that this feature may not be produced by the low-energy part of the same electron population that produced the Fermi/LAT γ-ray. Because of that it is required that the power-law electron energy go down to γ min ≈ 19, which predicts a very strong radio emission (radio flux larger than the observed) even considering the synchrotron self-absorption effect. We investigate the possibility of this excess being produced from the spine/layer jet structure, which has been clearly detected in Mrk 421. We find that (1) similar to one-zone modeling, the spine emissions provide good modeling of the broadband spectral energy distribution, except for the hard X-ray excess; and (2) the hard X-ray excess can be well represented by the synchrotron photons (from the layer) being inverse Compton scattered by the spine electrons.

The Hard X-Ray Emission of the Blazar PKS 2155–304

“…The main reason for the choice of spine/layer model is due to the fact that PKS 2155-304 shows rapid variability and very slow radio proper motion (see Subsection 3.3.4), which indicates that its jet may structured. Also, this model is able to produce IC SEDs that are significantly different from those produced by the one-zone model (Ghisellini et al 2005;Sikora et al 2016;Chen 2017). Spine/layer model can explain some inconsistencies between observations and the AGN standard unified model.…”

“…Even up to γ min = 100, the predicted radio flux is significantly larger than the observed one. Similar to Mrk 421 (Chen 2017), it implies that the electron power-law distribution cannot extend down to such low energies (i.e., γ min 30). Hence, the observed X-ray concave feature of PKS 2155-304 cannot be produced by the low-energy part of the same electron population that produced the Fermi/LAT γ-rays.…”

“…Here, we model the broadband emission of PKS 2155-304 having concave X-ray spectra with the spine/layer model. We present very brief introduction of the model, please see Chen (2017) for its detailed description. The spine/layer model used here is same as described in Ghisellini et al (2005).…”

“…All jet parameters, except R are constrained through SED modeling. The KleinNishina (KN) and SSA effects are also considered (see Chen 2017).…”

“…We briefly describe the model here. For detailed information, please see Chen (2017). The model assumes a homogeneous and isotropic emission region, which is a sphere with radius R that has a uniform magnetic field with strength B and a uniform electron energy distribution as described by,…”