The origin and emission mechanism of VHE (&gt;100GeV) emission from FSRQs

Bagmeet Behera,Anna Barnacka

doi:10.1051/epjconf/20136105007

Abstract

Flat Spectrum Radio Quasars, unlike BL Lac objects, are blazars that show prominent line-emission and strong thermal components associated with the accretion disk, the broad-line region (BLR), and/or the dusty torus. The low energy peak in the continuum is from synchrotron emission (of electrons), and the high energy peak is well explained by external-Compton emission. In these models the relativistic electrons in the jet up-scatter photons from the thermal photon fields up to GeV energies. Beyond a few tens of GeV such models predict cutoffs due to Klein-Nishina effect and internal absorption via pair production. While more than 300 FSRQs have been seen with Fermi -LAT (between 100MeV−30GeV), only three have been detected at VHE (Very High Energy, E > 100 GeV) with Cherenkov telescopes. The detection of VHE emission constrains the location of the blazar zone based on internal absorption estimates, but challenges the emission models that predict cutoffs. While a number of GeV flaring states (in various FSRQs) have been observed with Cherenkov telescopes only few have resulted in detection of a VHE signal. The broadband emission characteristics of VHE FSRQs (including the VHE-detected FSRQs) are studied and put in context to better understand their location and emission mechanism.

Highlights

Flat Spectrum Radio Quasars (FSRQs) are one of the two sub-classes of blazars that show strong line emission, have the synchrotron peak at low frequencies, and exhibit high Compton dominance 1
The low energy peak in the continuum is from synchrotron emission, and the high energy peak is well explained by external-Compton emission
There are some interesting consequences of detecting VHE emission from FSRQs

Summary

Introduction

Flat Spectrum Radio Quasars (FSRQs) are one of the two sub-classes of blazars that show strong line emission, have the synchrotron peak at low frequencies (infra-red to optical), and exhibit high Compton dominance 1. MWL SED models for Fermi-LAT FSRQs usually show an external-Compton (EC) component probably dominated by the scattering of BLR photons, i.e., the EC-BLR models [1], where the high energy component cuts off at a few 10s of GeV due to Klein-Nishina effect. In this scenario, rather large fluxes in the Fermi-LAT band are required to reproduce the VHE emission ([3, 6], among others). The MWL data during the H.E.S.S. observation has been published elsewhere (e.g. see [10])

Observational characteristics

SED modeling characteristics

Discussion and Outlook