Sequence Study Of Blazar

Active galactic nuclei(AGNs)are special galaxies with violent activity in the Universe,and blazars are a special subclass of AGNs with extreme observational properties.Based on the behavior of emission lines,blazars are classified into two subclasses,namely BL Lacertae objects(BL Lacs)and flat spectrum radio quasars(FSRQs).The SEDs of blazars consist of two bumps;the lower-energy bump(or synchrotron bump),whose peak frequency is located in the infrared to X-ray,is due to synchrotron emission while the higher-energy bump with peak frequency located from the X-rays bands through MeV bands,originates from inverse Compton emission.It is found that the synchrontron peak frequency is anti-correlated with luminosity,and FSRQs have low peak frequency and high luminosity,while BL Lacs have high peak frequency and low luminosity,and this tendency is called blazars sequence.It is well known that strong jet in blazars can cause discrepancies between observed and intrinsic data.The origin of this sequence has been discussed.Some authors think it is intrinsic,while others think it is the selection result caused by beaming effect.In addition,the isolate sources released by Fermi-LAT are mainly active galactic nuclei,which in turn are dominated by blazars.The newly released 4FGL-DR3 contains 3743 blazars sources,of which 1,517 are not classified as BL Lacs or FSRQS and are called as blazars candidates unclassified(BCUs).The released Fermi-LAT catalogues provide a very important observational data for the study of high-energy gamma-ray radiation mechanism of blazars.In addition,the classification of BCU is of great significance for us to expand the analytical sample of BL Lacs and FSRQ.The research part consists of two works:1.The”blazar sequence”investigation:A sample of 255 Fermi blazars with doppler factor were matched with 4FGL,and observation data of their low energy bands were collected.The relationship between observed luminosity and observed peak frequency,that between intrinsic luminosity and intrinsic peak frequency,and those between Compton Dominant Parameter and other physical quantities were studied.Our analysis comes to the following conclusions.(1)The observed radio,X-ray,γ-ray,and synchrotron peak luminosity are all anti-correlated with the synchrotron peak frequency,but the debeamed luminosity is positively correlated with the debeamed peak frequency.The anti-correlation is due to a selection effect or a beaming effect.(2)The Compton Dominance Parameter is correlated with both the bolometric luminosity and doppler factor,implying that the more highly compton dominated sources are more luminous.(3)The bolometric luminosity can be represented by the γ-ray luminosity for Fermi blazars.2.The relationship between brightness variability and spectrum of Fermi-LAT source and the classification of BCUs:By studying the relationship between photon number(logF),photon spectral index(αph)and optical variability index(log V.I.,here we only consider the source with V.I.>18.48),a machine learning is used to separate BCUs,BL Lacs and FSRQs in these correlation graphs,and the two boundaries obtained are used to separate BCUs.Our calculations and analyses reach following results:(1)the averaged luminosity,spectral index,and variability index of FSRQs are higher than those of BL Lacs for the whole sample.(2)It is found that the spectral index variation is closely anti-correlated with the luminosity variation implying that the spectrum becomes harder when the source becomes brighter in the γ-ray band.(3)Positive correlations are found between the photon spectral index and bothγ-ray luminosity and variability index for the whole sample,but anti-correlations are found in the two correlations for FSRQs.For BL Lac subclass,there is a marginal anti-correlation between the photon spectral index and both γ-ray luminosity,and a positive correlation between the photon spectral index and the variability index.We think those two positive correlations found for the whole sample are apparent.(4)We adopted the SVM machine learning method to separate BL Lacs and FSRQs in theαph—logLγ and αph—log(V.I.)plots and proposed that a BCUs is an FSRQs candidate if it satisfies log(V.I.)>-10.119α+24.855,or αph>-0.048 logLγ+4.498,otherwise,it is a BL Lac candidate.Our classification results are quite consistent with those by Kang et al(2019).