| An Active Galactic Nucleus(AGN)is a compact region at the centre of a galaxy which has a much higher than normal luminosity over some or all of the electromagnetic spectrum.Quasi-Stellar Objects(QSOs)are some AGNs with higher luminosity than Seyfert galaxies.The standard paradigm for AGNs posits an accretion disk surrounding a central super-massive black hole(SMBH),along with other components,such as the broad-line regions(BLRs),narrow-line regions(NLRs),torus,and jet.AGNs are believed to be powered by accretion onto their central super-massive black holes(with masses between 105 and 1010times that of the Sun).The orientation-based unified model was suggested to interpret the spectral difference for AGNs.It is suggested that the emission line regions in AGNs are stratified.The broad line fluxes are observed to vary in response to the continuum variations,with a short time delay(～days up to years)that is attributable to the light travel time across the BLR(it is called the reverberation mapping method).The time lag reflects the BLRs size.So far there are more than 30 AGNs were studied by this method,and their central SMBH masses are calculated.This thesis investigates the origin of optical FeⅡemission by carefully analysis the strong FeⅡemission lines in a QSO of PG1700+518.For 39 spectra of PG1700+518 from 7.5 years observations in global AGN watch,the light curves of FeⅡ,the broad component Hβ,and narrow component of Hβare obtained.Through the cross-correlation centroid distribution(CCCD),we calculate the time lags for light curves of FeⅡ,the broad component Hβ,and the narrow component of Hβrespect to the continuum light curve.For the first time,we find that the time lag of FeⅡin PG1700+518 is 263±68 days,Hβtime lag is 170±170 days,which is smaller than 251.8-38.8+45.9days that found by Peterson et al.(2004).And we also find that the mean value of FeⅡFWHM is 1554±110 km/s,which is also consistent with the HβFWHM(1846±682 km/s, Peterson et al.2004).It implies that the region emitting FeⅡis located outside of the region emitting the broad Hβlines.The BLRs are stratified for these two regions.
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