Redshifts of distant blazar limited by Fermi and VHE $\gamma$-ray observations

TL;DR

This paper estimates upper limits on the redshifts of three blazars by analyzing their Fermi and VHE gamma-ray spectra, assuming no spectral cutoff and considering EBL absorption effects.

Contribution

It introduces a method to constrain blazar redshifts using gamma-ray observations and EBL models, providing new upper limits for three specific blazars.

Findings

Redshift upper limits: PG 1553+113 at 0.78, 3C 66A at 0.58, PKS 1424+240 at 1.19.

Assumption of no spectral cutoff across Fermi and VHE ranges.

Potential overestimation of optical depth at TeV energies by the EBL model.

Abstract

Our goal is to limit the redshifts of three blazars PG 1553+113, 3C 66A and PKS 1424+240, through the investigation of their Fermi and VHE (very high energy) $\gamma$-ray observations. We assume that the intrinsic spectra of PG 1553+113, 3C 66A, and PKS 1424+240 have not any cutoff across the Fermi and VHE $\gamma$-ray energy ranges. The intrinsic spectra of VHE $\gamma$-rays are obtained through the extrapolation of Fermi spectra. Comparing the measured and intrinsic VHE spectra due to extragalactic background light (EBL) absorption, we give the redshift upper limits of three blazars assuming a specific EBL model. The redshift upper limits of PG 1553+113, 3C 66A and PKS 1424+240 are 0.78, 0.58, and 1.19 respectively. Near the TeV energy the optical depth of VHE $\gamma$ photons might be overestimated by Franceschini (2008) EBL model, or the second emission component might be present in the VHE spectra and lead the intrinsic photon index harder than the Fermi ones.