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Abstract

The active galactic nucleus 4C + 28.07 is a flat-spectrum radio quasar,
one of the brightest at gamma-ray energies. We study its multiwavelength
emission by analysing similar to 12.3 yr of Fermi-LAT data in the
gamma-ray band and Swift-X-Ray Telescope (XRD)/Ultraviolet/Optical
Telescope (UVOT) available data in X-ray and optical-to-ultraviolet
bands. In the gamma-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
gamma-ray flux reached (6.7 +/- 0.81) x 10(-6) photon cm(-2) s(-1)
(similar to 31 x higher than the mean flux over 12.3 yr) with detection
significance of sigma = 6.1. The estimated variability time (similar to
2 h) constrains the gamma-ray emitting region size to <= 9 x 10(14) cm,
which is close to the black hole radius. The spectral energy
distributions (SEDs) in the gamma-ray band for the similar to 12.3 yr of
data show an early cut-off at similar to 14 GeV; beyond similar to 60
GeV, however, the spectrum hardens and is detected up to similar 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 gamma-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 gamma-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.