2020ApJ...898L..56L

GB 1508+5714 is a high-redshift blazar (z = 4.3), and a spectrally soft γ-ray source has been detected in its direction. By analyzing 11.4 yr Fermi Large Area Telescope data, significant long-term variability of the γ-ray source is confirmed. More importantly, a γ-ray emission flare appeared in an epoch of several tens of days in 2018, when the flux was about four times the value from the global fit. Meanwhile, optical flares were displayed in both the r and i bands from the Zwicky Transient Facility light curves. Detections of the simultaneous γ-ray and optical brightening provide decisive evidence to pin down the association between the γ-ray source and GB 1508+5714, which makes it the first identified γ-ray blazar beyond redshift 4. A broadband spectral energy distribution in the high flux state is constructed, and the origin of the multiwavelength brightening is also briefly discussed. Upcoming wide-deep-fast optical telescopes together with the γ-ray sky surveyors will shed light on the role that the active galactic nucleus jets play in the early cosmic time.