2022MNRAS.510.1118Z

The active galactic nucleus 4C + 28.07 is a flat-spectrum radio quasar, one of the brightest at γ-ray energies. We study its multiwavelength emission by analysing ∼12.3 yr of Fermi-LAT data in the γ-ray band and Swift-X-Ray Telescope (XRT)/Ultraviolet/Optical Telescope (UVOT) available data in X-ray and optical-to-ultraviolet bands. In the γ-ray band, five flaring periods have been detected, and quasi-simultaneously with these flaring times, the X-ray and UVOT data detected by Swift-XRT/UVOT have also been analysed. In one of the brightest flare periods (Flare 5; observed on 2018 October 12), the γ-ray flux reached (6.7 ± 0.81) x 10^–6 photon cm^–2 s^–1 (∼31 x higher than the mean flux over 12.3 yr) with detection significance of σ = 6.1. The estimated variability time (∼2 h) constrains the γ-ray emitting region size to <=9 x 10¹⁴ cm, which is close to the black hole radius. The spectral energy distributions (SEDs) in the γ-ray band for the ∼12.3 yr of data show an early cut-off at ∼14 GeV; beyond ∼60 GeV, however, the spectrum hardens and is detected up to ∼316 GeV. Similar spectral behaviour is also noticeable for the SEDs of flares, which can be linked to the photon absorption by the emitting region's internal and external narrow-band radiation fields. In the quiescent period, the γ-ray emission was described by the synchrotron self-Compton scenario, while the external photons contributions from the disc and the broad-line region were required to explain the short-term flaring γ-ray emission. Considering the significance of the obtained results from 4C + 28.07, we compared the parameters with 3C 279 and M87, to motivate further studies.