Redshift constrain of BL Lac PKS 1424+240

TL;DR

This study estimates the redshift of the BL Lac object PKS 1424+240 by analyzing multiple gamma-ray spectra with different EBL models and the photohadronic model, finding consistency with spectroscopic measurements within confidence intervals.

Contribution

It introduces a method combining multiple EBL models and the photohadronic scenario to constrain the redshift of PKS 1424+240 using gamma-ray spectra.

Findings

The photohadronic model fits all observed spectra well.

The EBL model of Domínguez et al. provides the most restrictive redshift limits.

The spectroscopic redshift z=0.604 is within the 3σ confidence interval of the analysis.

Abstract

In the period between 2009 and 2015, several very high-energy (VHE $> 100$ GeV) gamma-ray flaring events from the BL Lac object PKS 1424+240 were observed by the Cerenkov telescopes VERITAS and MAGIC. It had uncertain redshift ($z$) and using spectroscopical measurement, Paiano et al. (2017) found it to be $z=0.604$. Using four different extragalactic background light (EBL) models and the photohadronic model, nine independently observed VHE gamma-ray spectra of PKS 1424+240 are analyzed and a global $\chi^2$ fit is performed on all observations to estimate the best-fit value for the redshift for each EBL model. Confidence levels (CL) intervals for the redshift are also estimated using all the EBL models. This method is tested by comparing our analysis with the observed value. It is shown that the photohadronic scenario provides an excellent description of all the observed spectra. It is found that the EBL model of Dom\'inguez et al. (2011) is the one that provides the most restrictive limits on the redshift of PKS 1424+240, but in our analysis, $z=0.604$ lies within the $3\sigma$ CL interval of the EBL model of Saldana-Lopez et al. (2021).