Study On Physical Properties Of Flat-spectrum Radio Quasars By Using The Gamma-ray Variability

Flat-spectrum radio quasars（FSRQs）have violent gamma-ray variability which associated with non-thermal radiation in relativistic jets pointing in the direction of the observer’s line of sight.Thus,the study on variability is an effective tool to understand the physical properties of jet emission region of flat-spectrum radio quasars.In this paper,we processed the long-term gamma-ray data of 3 FSRQs,PKS 1222+216,3C279,and PKS 0346-27,observed by Fermi-LAT from 2008 August to 2021.Through analyzing the light curve and the spectra of 3 FSRQs,the main results are as follows.We use the asymmetric flare model to fit the flares of 3 sources in the energy range of 0.1-500 GeV.The shortest flux doubling timescale are obtained,i.e.1.36±0.40hours for PKS 1222+216,the peak flux of（1.06±0.14）×10^-5 ph cm^-2s^-1;1.16±0.17 hours for 3C 279,with a peak flux of（3.28±0.10）×10^-5 ph cm^-2s^-1;2.00±0.67 hours for PKS 0346-27,with a peak flux of（1.93±0.33）×10^-6 ph cm^-2s^-1.By analyzing the variability in the 0.1-1 GeV and 1-500 GeV energy ranges and the hardness ratio（HR）,combined with the results of the analysis of discrete correlation function（DCF）method,it is shown that some of the flares follow the pattern of hardening of the spectrum at the peak and softening as the flux decay.There is no obvious emission delay in the light curves of the two energy ranges.We fitted spectra of 3 sources in different flares with three different models.The results show that for most of the flares,the spectra can be fitted well by the Logarithmic Parabola（LP）and the power law with an exponential cutoff（PLEC）models than the Single Power-Law（PL）model.During the long-term light curves and the flares,the photon index of 3 sources decrease with the increase of the gamma-ray flux.It follows a tendency of harder when brighter.While in the quiet state,the photon index increases with the increase of gamma-ray flux.