Awakening of two $γ$-ray high redshift flat-spectrum radio quasars in the southern hemisphere

Abstract

High-redshift blazars are valuable tools to study the early Universe. So far only a handful of $γ$-ray blazars have been found at redshifts above 3. $γ$-ray signals are detected in the direction of PMN J2219-2719 ($z=3.63$) and PMN J2321-0827 ($z=3.16$) by analyzing the 10-year $Fermi$-LAT Pass 8 data. PMN J2219-2719 is not distinguished from the background in the global analysis. During the 5-month epoch, the TS value is 47.8 and the flux is more than 10 times of the 10-year averaged flux. In addition, the angular distance between the $γ$-ray position and the radio position of PMN J2219-2719 is only ${0.04}^{\circ}$. Moreover, the $γ$-ray and infrared light curves of long time scale are very similar, which support the association between the $γ$-ray source and PMN J2219-2719. The global analysis of PMN J2321-0827 suggest a new $γ$-ray source, during the flare phase, the TS value is 61.4 and the $γ$-ray flux increased significantly. The association probability suggests that PMN J2321-0827 may be the counterpart of the new $γ$-ray source. In the future, the number of high-redshift $γ$-ray sources will increase by combining $Fermi$-LAT and the upcoming Large Synoptic Survey Telescope.