X-ray Variability Studies Of Active Galactic Nuclei

Studying the variability properties of active galactic nuclei(AGNs)in the X-rays can help us understand the physical processes around the black hole(BH).The spectral variability in the X-ray bands has been observed in many AGNs,which seems to be different between blazars and Seyfert galaxies.Besides,flux distribution and rms-flux relation(the dependence of flux standard deviation on the mean flux)are two important tools to study the temporal variability in AGNs.However,few works have studied these two temporal characteristics in the X-ray bands based on sample research,which play an important role in helping us understand the general variability properties of AGNs.Except the intrinsic variability from the physical processes,when obscuring clouds or gas clumps move cross the line of sight,they will obscure the radiation from the central region,and then will produce the observed X-ray variability.In order to investigate the aforementioned phenomena and questions,this doctoral dissertation mainly carries out on four works on the X-ray variability studies of AGNs.In the first work,utilizing all the 16-yr RXT E observations,we perform a system-atic investigation of X-ray spectral variability for the five brightest sources during their major flares that lasted several days.We fit the spectra with the exponential cutoff power law model to obtain the photon spectral index(?)and flux,with the log-parabolic model to obtain the synchrotron radiation peak energy(Ep),peak luminosity(Lp)and the cur-vature parameter(b),and with the theoretical synchrotron radiation model to obtain the electron spectral index(p).We find that both a and p generally display a harder-when-brighter trend.We see apparent electron spectral hysteresis in many but not all p-flux plots that takes a form of "loop" or oblique "8." We obtain a tight p-hardness ratio(HR)relation and a tighter p-? relation using the spectra of flaring periods,both of which are also applicable to the stacked data of quiescent periods.We demonstrate that these two empirical relations can be used efficiently to estimate p from HR or a that is readily achieved.Finally,we find that,when considering TeV blazars as a whole,? and X-ray luminosity are positively correlated,Ep is negatively correlated with p and ?.However,after correcting for the Doppler boosting effect,? and intrinsic X-ray luminosity follow an anti-correlation.It has been suggested that Ep shows relations with Lp and b in several sources,which can be used to constrain the physical properties of the emitting region and/or acceleration processes of the emitting particles.In the second work,we systemati-cally study the Ep-Lp and Ep-(1/b)relations for 14 BL Lac objects using the 3-25 keV RXTE/PCA and 0.3-10 keV Swift/XRT data.Most objects(9/14)exhibit positive Ep-Lp correlations,three sources show no correlation,and two sources display nega-tive correlations.In addition,most targets(7/14)present no correlation between Ep and 1/b,five sources pose negative correlations,and two sources demonstrate positive cor-relations.1ES 1959+650 displays two different Ep-Lp relations in 2002 and 2016.The Ep-Lp relation does not exhibit significant differences between flares,while the Ep-(1/b)relation varies from flare to flare.For the total sample,the Ep-Lp correlation is related to the luminosity.Lp and the slope of Ep-Lp relation present an anti-correlation,which indicates that the causes of spectral variations might be different between luminous and faint sources.Ep shows a positive correlation with the black hole mass.To systemically study the X-ray flux distribution and rms-flux relation of AGNs,in the third work,we select a sample of 27 AGNs that consist of 17 Seyfert galaxies,6 narrow-line Seyfert 1(NLSyl)galaxies,and 4 blazars from the RXT E AGN database.We focus on the 2-10 keV variability at a minimum baseline timescale of 3 years with a cadence of 10 days.We use the log-normal,normal,and double-components model to the flux distribution.Blazars,Seyfert,and NLSy1 galaxies have different flux distribu-tion,which imply that their variability processes might be different.Some objects show two peaks in the flux distribution,some of which might be due to the obscuration in the X-ray band.Due to the limitation of the data quality,we only investigate the rms-flux correlation(timescale is 2-10 days)for 15 objects in our sample.Most of the objects(12/15)present a positive linear correlation,while 3 objects show a non-linear correla-tion.The fractional variability amplitude(Fvar)shows a significant negative correlation with both black hole mass and bolometric luminosity,which indicates that the objects with smaller black holes might have larger variability amplitudes.In the fourth work,we use SPEX software and a broadband spectral energy distri-bution model to re-analyze XMM-Newton and NuSTAR data of this source which were taken in 2006 and 2016.We find four WA components with different ionization states(log ??-1.0,2.0,2.5,3.0 erg cm s-1)outflowing with velocities ranging from 100 to 1300 km s-1.The highest-ionized WA component has a much higher hydrogen col-umn density(?1022 cm-2)than the other three components(?1021 cm-2).Moreover,their outflow velocity shows a positive correlation with the ionization parameter,which does not follow the scaling relation predicted by either the radiatively driven or magneto hydrodynamically driven outflowing mechanism.These WA components appear to be distributed from the outer part of the broad-line region(BLR)to the narrow-line region(NLR)and even further.We find a rapid occultation event in 2006,which was missed by previous studies.We also re-analyze the previously published obscuration event cap-tured in 2016.One and two obscurer components are required for the obscured events in 2006 and 2016,respectively.The high-ionized obscurer component(log ??2.8)only appears in the 2016 observation,which has a higher column density(?1023 cm-2),while both the 2006 and 2016 observations show a low-ionized obscurer component(log ??1.1-1.9),which has a lower column density(?1.3-2.0×1022 cm-2).The obscurer components likely resides within the BLR.