Study On The High-Energy Variability Of The Relativistic Jet Of Active Galactic Nuclei

The violent variability is one of the important characteristics of the high-energy e-missions from the relativistic jets of active galactic nuclei（AGN）,which carries a tremen-dous amount of physics information of the emission region.Recently,the variability properties of the AGN jets are digging out with the increasing observations,which are challenging the current theoretical models.Here,we study several crucial time-domain problems,such as theγ-ray stochastic variation or quasi-periodic oscillation,the origin of the X-ray flares,which are found in the AGN-jet observations,based on the current data and techniques.Theγ-ray variability of blazar may be driven by stochastic processes.Recently,γ-ray QPO behaviors are reported for several blazars.However,the significance of those reported QPOs are questioned.We analyze theγ-ray data of 27 blazars with possibleγ-ray QPOs,to examine their variability properties.We generate～11 yr Fermi-LAT light curves for those blazars.We fit two different Gaussian processes,CARMA and celerite,to the light curves using MCMC technique and build their intrinsic power spectral densi-ties（PSDs）.The results show that in general theγ-ray light curves can be characterized by CARMA and celerite models,indicating thatγ-ray variabilities of blazars are essen-tially Gaussian processes.The resulting PSDs are generally the red noise shapes with slopes between-0.6 and-1.7.Except some possible evidence are found for the QPOs in PKS 0537-441 and PG 1553+113,no QPO signal is found in our analysis for the other25 blazars.The energy dissipation mechanism in AGN jet is unknown.The flares observed in the jets may be the probe to figure out this question.We analyze 50 XMM-Newton X-ray observations for blazar Mrk 421,and extract the spectra and light curves.The result of the spectral analysis shows that the X-ray are dominatingly emitted by synchrotron radiation.In general,the 50 light curves show high-amplitude X-ray flares,and their PSDs indicate they are about red noise.We fit the light curves,and obtain the time scales,flaring duration time and the peak flux from the flares.We find that the distributions of the flaring duration time and the peak flux obey a power-law distribution,with the index of～1.0.These statistic properties are consistent with the predictions of the self-organized criticality（SOC）model,indicating that the X-ray flares of Mrk 421 are possibly triggered by magnetic reconnection.Additionally,there is a magnetic field of 2.1δ_D^-1/3in the emission region calculated via the time scales of～1000 s.Such a large magnetic field suggests that magnetic reconnection may naturally happen in the jet,and the accelerated particles dissipate the energy through synchrotron radiation.The emission region of blazar is believed to locate near the black hole,but the other emission region that is far from the central engine also shows the X-ray flares.To figure out the origin of those flares,we analyze the X-ray emission from the core and the knot,HST-1,in the M87 jet.We process 105 Chandra X-ray observations for M87,and extract the light curves of core and HST-1 respectively.We read out the flares in the light curves,and obtain the flaring duration time and peak intensity.We find that the distributions of flaring duration time and peak intensity for both core and HST-1 obey the power-law distribution,which is consistent with the predictions of SOC model.It indicates that the X-ray flares in core and HST-1 may triggered by magnetic reconnection.The statistic properties of the flares in core and HST-1 are similar,suggesting that those regions have gone through the similar physical process.The strong correlation found in their light curves also supports this point.All in all,magnetic reconnection may not only happen in the region near the black hole,but also may occur in the outer part of the AGN jet.The magnetic reconnection may be the main dissipation mechanism in the AGN jets.