Model-independent Redshift Estimation of BL Lac Objects through VHE Observations

TL;DR

This paper uses the correlation between VHE gamma-ray spectral indices and redshift in blazars to estimate the redshift of BL Lac objects and tests EBL models against observations.

Contribution

It introduces a model-independent method for estimating BL Lac redshifts using VHE observations and compares these with EBL model predictions.

Findings

VHE spectral indices correlate with redshift, unlike lower energy bands.

The method constrains BL Lac redshifts with improved accuracy.

Discrepancies found between EBL model predictions and observations at high redshift.

Abstract

The very high energy (VHE) gamma-ray spectral indices of blazars show strong correlation with the source redshift. Absence of any such correlation in low energy gamma rays and X-rays indicate the presence of Extragalactic Background Light (EBL) induced absorption of VHE gamma rays. By employing a linear regression analysis, this observational feature of blazars is used to constrain the redshift of BL Lac objects which was unknown/uncertain earlier. Additionally, we also compare the observed VHE spectral index-redshift correlation with the ones predicted from commonly adopted EBL models. Our study highlights the deviation of the EBL model based predictions from the observation especially at high redshifts.