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 {\it 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 {\it 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 {\it Swift} XRT and {\it NuSTAR} is well described by a broken PL model, with an extremely hard spectrum ($\Gamma_{1} \sim 0.1$) below the break energy, $E_{\rm break} \sim 2.1 {\rm keV}$, and $\Gamma_{2} \sim 1.5$ above the break energy. The spectral steepening below $\sim 3$ 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.