2017MNRAS.470.2861S

We present the γ-ray observations of the flat-spectrum radio quasar PKS 1441+25 (z = 0.939), using the Fermi large Area Telescope data accumulated during 2015 January - December. A γ-ray flare was observed in January 24, when the flux increased up to (2.22 ± 0.38) x 10^–6 photon cm^–2 s^–1 with the flux-doubling time-scale being as short as ∼1.44 d. The spectral analysis shows that from 2015 April 13 to April 28, the MeV-to-GeV photon index has hardened and changes in the range of Γ = (1.73 - 1.79) for most of the time. The hardest photon index of Γ = 1.54 ± 0.16 has been observed on MJD 57 131.46 with 11.8σ which is not common for flat-spectrum radio quasars. For the same period the γ-ray spectrum shows a possible deviation from a simple power-law shape, indicating a spectral cutoff at E_cut = 17.7 ± 8.9 GeV. The spectral energy distributions during quiescent and flaring states are modelled using one-zone leptonic models that include the synchrotron, synchrotron self Compton and external inverse Compton processes; the model parameters are estimated using the Markov Chain Monte Carlo method. The emission in the flaring states can be modelled assuming that either the bulk Lorentz factor or the magnetic field has increased. The modelling shows that there is a hint of hardening of the low-energy index (∼1.98) of the underlying non-thermal distribution of electrons responsible for the emission in 2015 April. Such hardening agrees with the γ-ray data, which pointed out a significant γ-ray photon index hardening on 2015 April 13 to 28.