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

TL;DR

This paper reports the detection of gamma-ray signals from two high-redshift flat-spectrum radio quasars, revealing their flaring activity and potential as probes of the early Universe.

Contribution

It presents the first gamma-ray detections of two high-redshift blazars, expanding the known population and demonstrating their variability and association with radio sources.

Findings

Gamma-ray signals detected in 10-year Fermi-LAT data.

Significant gamma-ray flares observed during a 5-month epoch.

Potential identification of new gamma-ray source associated with one quasar.

Abstract

High-redshift blazars are valuable tools to study the early Universe. So far only a handful of $\gamma$-ray blazars have been found at redshifts above 3. $\gamma$-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 $\gamma$-ray position and the radio position of PMN J2219-2719 is only ${0.04}^{\circ}$. Moreover, the $\gamma$-ray and infrared light curves of long time scale are very similar, which support the association between the $\gamma$-ray source and PMN J2219-2719. The global analysis of PMN J2321-0827 suggest a new $\gamma$-ray source, during the flare phase, the TS value is 61.4 and the $\gamma$-ray flux increased significantly. The association probability suggests that PMN J2321-0827 may be the counterpart of the new $\gamma$-ray source. In the future, the number of high-redshift $\gamma$-ray sources will increase by combining $Fermi$-LAT and the upcoming Large Synoptic Survey Telescope.