Constraining blazar distances with combined Fermi and TeV data: an empirical approach

TL;DR

This paper presents an empirical method combining Fermi and TeV data to estimate the distances of blazars with unknown redshifts by analyzing their spectra and absorption features.

Contribution

It introduces a new empirical approach to constrain blazar distances using combined gamma-ray observations and spectral fitting, applicable to blazars with unknown redshift.

Findings

Derived an empirical relation between spectral upper limits and redshift.

Applied the method to estimate the redshift of PKS 1424+240.

Systematic variations observed with different EBL models.

Abstract

We discuss a method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes. We assume that the VHE spectrum corrected for the absorption through the interaction with the Extragalactic Background Light can not be harder than the spectrum in the Fermi/LAT band. Starting from the observed VHE spectral data we derive the EBL-corrected spectra as a function of the redshift z and fit them with power laws to be compared with power law fits to the LAT data. We apply the method to all TeV blazars detected by LAT with known distance and derive an empirical law describing the relation between the upper limits and the true redshifts that can be used to estimate the distance of unknown redshift blazars. Using different EBL models leads to systematic changes in the derived upper limits. Finally, we use this relation to infer the distance of the unknown redshift blazar PKS 1424+240.