Constraining the redshift of BL Lac VER J0521+211

Abstract

Observation of several very high energy (VHE) flaring events of the BL Lac object VER J0521+211 were reported by the VERITAS and MAGIC collaborations between 2009 and 2014. The redshift of this source is uncertain and several analysis have derived different limits for it. In the framework of the photohadronic model, and using three different extragalactic background light (EBL) models, we analyze seven independent VHE spectra of VER J0521+211 and determine the limiting values on its redshift. It is observed that the photohadronic scenario provides excellent fits to the reported observations. It is further observed that the photohadronic scenario, along with the EBL model of Domínguez et al., puts the most restrictive limits on the redshift z of VER J0521+211: 0.29 ≤ z ≤ 0.31 from the confidence level (CL) intervals at 2σ, or a more conservative 0.28 ≤ z ≤ 0.33, at 3σ.