Leptonic and Hadronic Modeling of Fermi-LAT Hard Spectrum Quasars and Predictions for High-Energy Polarization

TL;DR

This study models the spectral energy distributions of eight FSRQs using leptonic and hadronic scenarios, predicts their polarization signatures, and assesses their detectability with future high-energy polarimeters and telescopes.

Contribution

It compares leptonic and hadronic models for FSRQ emissions, predicts polarization degrees, and estimates their detectability with upcoming high-energy observatories.

Findings

Hadronic models predict higher high-energy polarization than leptonic models.

X-ray polarization exceeds gamma-ray polarization in leptonic scenarios.

Most FSRQs could be detected by the Cherenkov Telescope Array during elevated activity.

Abstract

We present the results of a study of the time-averaged spectral energy distributions (SEDs) of eight flat spectrum radio quasars (FSRQs) present in the second catalog of high energy sources detected beyond 50 GeV by the {\it Fermi}-Large Area Telescope (2FHL). Both leptonic and hadronic scenarios are adopted to explain the multi-wavelength SEDs and we find them to be marginally consistent with the 2FHL spectra above 50 GeV. We derive the expected degree of X-ray and $\gamma$-ray polarizations both for the average and elevated activity states and note that: (i) a hadronic radiative model consistently predicts a higher degree of high energy polarization compared to leptonic ones, and (ii) the X-ray polarization degree is higher than the $\gamma$-ray polarization in the leptonic scenario, but similar to the $\gamma$-ray polarization if the observed radiation is powered by hadronic processes. From the leptonic modeling, the location of the $\gamma$-ray emitting region is found to be at the outer edge of the broad line region (BLR) and is consistent with the $\gamma\gamma$ opacity estimates for the $\gamma$-ray absorption by the BLR. We conclude that a majority of the FSRQs could be detected by the upcoming Cherenkov Telescope Array, though future high energy polarimeters will be able to detect them only during elevated activity states, which could provide supportive evidence for the hadronic origin of the X-ray and $\gamma$-ray emission.