Understanding the Very High Energy {\gamma}-ray excess in nearby blazars using leptonic model

TL;DR

This paper investigates the spectral hardening in VHE gamma-ray emissions of nearby blazars Mkn 421 and Mkn 501 using leptonic models, revealing different emission mechanisms and exploring hadronic contributions.

Contribution

It provides a detailed analysis of VHE spectral hardening in blazars, proposing a broken power law electron distribution with Maxwellian pileup and considering hadronic processes.

Findings

Mkn 501's VHE spectra are steeper than X-ray spectra, indicating Klein-Nishina regime scattering.

Mkn 421's VHE spectra are harder than X-ray spectra, suggesting additional emission mechanisms.

Hard VHE spectrum of Mkn 421 can be explained by inverse Compton emission from a broken power law electron distribution.

Abstract

The availability of simultaneous X-ray and Very High Energy (VHE) observations of blazars helps to identify the plausible radiative contributors to the VHE emission. Under leptonic scenario, the VHE emission from BL Lacs are attributed to the synchrotron self Compton (SSC) emission. However, many BL Lacerate (BL Lacs) have shown significant hardening at VHE after correction for the Extra Galactic Background Light (EBL) attenuation. We study the spectral hardening of two nearby BL Lac objects, Mkn 421 and Mkn 501 having most number of simultaneous X-ray and VHE observations available among all the blazars. These BL Lacs are relatively close and the effect of EBL attenuation is relatively minimal/negligible. We study the scatter plot between the X-ray spectral indices and intrinsic VHE indices to identify the plausible origin of the VHE emission. For Mkn 501, the VHE spectral indices are steeper than X-ray spectra, suggesting the scattering process happening at extreme Klein-Nishina regime. On the other hand, for Mkn 421, the VHE spectra is remarkably harder than the X-ray spectra, which suggests an additional emission mechanism other than the SSC process. We show this hard VHE spectrum of Mkn 421 can be explained by considering the inverse Compton (IC) emission from a broken power law electron distribution with Maxwellian pileup. The possibility of the hadronic contribution at VHE {\gamma}-rays is also explored by modeling the hard spectrum under photomeson process.