Redshifts of PG 1553 + 113 limited by Fermi and VHE observations

The goal of this paper is to limit the redshift of PG 1553 + 113, through the investigation of Fermi-LAT and very high energy (VHE) γγ-ray observations. We assume that the intrinsic spectra of PG 1553 + 113 do not have cutoff across the Fermi-LAT and VHE γγ-ray energy ranges. The intrinsic spectra of VHE γγ-rays are obtained through the extrapolation of Fermi-LAT spectra. Comparing the intrinsic with measured VHE spectra absorbed by extragalactic background light (EBL), we estimate the redshift upper limits of this blazar assuming a specific EBL model. The redshift upper limits of PG1553 + 113 is 0.514 at the 95% confidence level.

► SED of Fermi-LAT of PG 1553 + 113 quasi-simultaneous with VERITAS is given.
► Conservative redshift upper limit of PG 1553 + 113 is obtained.
► Lower intensity EBL model is more consistent with the observations of PG 1553 + 113.