VHE FSRQs with Fermi-LAT: VHE and even brighter states in high-z FSRQs due to an HBL-like component?

TL;DR

This study analyzes 14 years of Fermi-LAT data to understand the spectral and temporal behaviors of VHE-detected FSRQs, revealing that VHE activity often results from particle spectrum continuation or an HBL-like component, leading to brighter states.

Contribution

It demonstrates that VHE emission in FSRQs can be explained by particle spectrum extension or an HBL-like component, providing new insights into their spectral states and variability mechanisms.

Findings

VHE activity correlates with brighter, harder flux states.

Spectral energy distributions shift from power-law to log-parabola during high-flux states.

VHE emission often results from particle spectrum continuation or HBL-like components.

Abstract

Very high-energy (VHE) detected flat-spectrum radio quasars (FSRQs) are relatively few despite being the most persistent bright MeV-GeV sources. Focusing on VHE emission, we investigated the spectral and temporal properties of VHE-detected FSRQs using 14-year Fermi-LAT data. All are highly variable (flux-amplitude$>$100) with VHE detection associated with brighter flux states and relatively harder spectra. Above a flux limit, flux anti-correlates with spectral index, exhibiting a bluer-when-brighter trend. The low-flux state spectral energy distributions (SEDs) for all resembles a power-law, while high-flux and VHE-associated states resemble a log-parabola, accompanied by an almost nil (PKS0736+017, PKS1510-089) to marginal (4C+21.35, 3C279) to significant (B21420+32, TON0599, PKS1441+25, S30218+35, PKS0346-27, OP313) MeV-GeV peak-upshift -- more prominent in high-redshift sources. For no/marginal peak-upshift, the VHE emission is consistent with external Comptonization of infrared photons (EC-IR) driven primarily by a power-law continuation of the particle spectrum to higher energies. For those with a significant MeV-GeV peak-upshift, PKS0346-27 and OP313 shows peak-upshift in the synchrotron spectrum, and thus VHE is EC-IR origin, while for others without synchrotron-peak upshift, we attribute the VHE to an HBL-like component with a Compton-Dominance (CD) like FSRQs, with VHE driven primarily by particle spectrum continuation. In some, even high-state SEDs seem to require an HBL-like component. Thus, VHE activities in FSRQs mainly result from particle spectrum continuation, aided by spectral transition or a new HBL-like component with FSRQ-like CD. Such spectral changes naturally brighten the GeV-VHE flux, overcoming extragalactic background light absorption without requiring extraordinary brightening under the traditional EC-IR scenario than normally exhibited.