High energy properties of the flat spectrum radio quasar 4C 50.11

Abstract

We investigate the gamma-ray and X-ray properties of the flat spectrum radio quasar (FSRQ) \4c50 at redshift z= 1.517. The Fermi Large Area Telescope (LAT) data indicate that this source was in an active state since 2013 July. During the active period, the gamma-ray flux increased by more than a factor of three and two distinct flares were detected with the variability timescale as short as several hours. The gamma-ray spectra can be well fitted by a log-parabola. From the fitting, we find a correlation between the peak energy and spectral curvature for the gamma-ray spectra, which is the first time seen in gamma-ray emission from a blazar. The Swift X-ray Telescope (XRT) data show that the source was variable at X-ray energies, but no evidence shows flux or spectral changes related to the gamma-ray activity. The broad-band X-ray spectrum obtained with Swift XRT and NuSTAR is well described by a broken PL model, with an extremely hard spectrum (Γ_1 ~ 0.1) below the break energy, E_(break) ~ 2.1 keV, and Γ_2 ~ 1.5 above the break energy. The spectral steepening below ~ 3 keV is likely due to the low energy cut-off in the energy distribution of the photon-emitting electron population. Both the gamma-ray and X-ray emission appear harder when brighter. The broad-band spectral energy distribution (SED) is constructed for the source, and we provide a model fit to the SED. Our modeling suggests that the emission region should be outside the broad line region, and the properties of the region indicate a jet with 42% of the Eddington power during the active state.