Analyses On Hard X-ray Spectra Variability Of AGNs

Astrophysicists believe that high-energy radiation of the active galactic nucleus(AGNs)originates from a hot plasma-corona.The disk with corona model shows that photons from the optical/UV band of the accretion disk are converted into higher-energy photons by thermoelectric Compton scattering in the corona,so the X-ray spectrum of AGNs is usually dominated by a powerlaw spectrum,thanks to more and more high-quality observations,the high-energy cut-off around 100 keV have been observed in a number of sources,which is a key evidence of thermal Comptonization.The X-ray radiation of the AGNs usually changes,and this change may occur in a very short time.The study of variability is an important way to discover the basic prop-erties of the center of a galaxy.For example,by measuring the time scale of variability,we can understand the size of the AGNs,and the Reverberation Mapping can help us weigh the black hole.The research of the variabilites in different bands can help us to obtain the connection between various regions in the AGNs.The X-ray spectra model of the AGNs is extremely complex,including not only the power spectrum,but also the relativistic reflection of accretion disk,distant cold reflection,and emission lines.Absorption adds difficulties in measuring parameters.Therefore,we proposed a spectra ratio method that is not based on model fitting to find spectral variations and assist spectral fitting.We demonstrated that using spectra ratio to quickly analyze the spectral difference between two observations is reliable and model independent.Using spectra ratio to find spectra variability is convenient and effective,and can also help find the best model.Assisted with spectraratio technique,werevisit the 5 AGNs in literature(MCG-5-23-16,3C 382,NGC 4593,NGC 5548 and Mrk 335)for which Ec(Te)variations have been claimed with NuSTAR observations.Strikingly,their cut-off energy variations revealed by the spectra ratios contradict published measurements in 3 of them(MCG-5-23-16,3C 382,NGC 4593).We present thereby improved spectra fitting results.We also report a new source with Ec variations based on NuSTAR observations(radio galaxy 4C +74.26).We find that all 6 sources in this small sample appear softer-when-brighter.And the corona tends to be hotter when it brightens(hotter-when-brighter)in 3C 382,NGC 5548,Mrk 335 and 4C +74.27.Changes in corona geometry are required to explain the more common hotter-when-brighter trends.