Optical Variability Of Mrk421

Active Galactic Nuclei (AGNs) have been one of the most interesting topic in astronomy since Quasi-Stellar Objects (QSOs), blazars and other types of AGNs were found in1960’s. Researches in AGNs play an important role in explaining some basic questions related to energy generating, radiation mechanism and cos-mology. Blazars are the most extreme class of AGN. They exhibit rapid, large amplitude variability in all the observed wavelengths from radio to y-ray. Variabil-ity is one of the most important characters of blazars, and variability analysis is a powerful method in understanding their properties. Mrk421is one of the closest blazars with a redshift of z=0.031. It is classified to be a high frequency peaked blazar (HBL) because its synchrotron peak is higher than0.1keV. Studying vari-ability of Mrk421could provide us with useful information in modelling AGNs.Mrk421has been monitored extensively since2009by the1.0meter telescope at Weihai Observatory (WHO) of Shandong University. For processing those data efficiently, an Interactive Data Language (IDL) procedure was developed from the NASA IDL Astronomy Library, including image pre-processing and aperture pho-tometry. Tests for the procedure were performed and the results derived from the IDL procedure are in agreement with results derived from IRAF.Mrk421was observed on40nights from WHO in R and V bands. C-test and F-test were performed to detect intra-day variability (IDV) on39nights in which at least5frames were taken in at least one band. However, no IDV was detected. Previous works indicated that HBLs show less variability than LBLs, which was supported by our results. The scenario of stronger magnetic fields in HBLs (Romero et al.,1999) may be responsible for explaining this phenomenon.Long-term light curves of Mrk421in B and V bands were constructed with data from literatures, Steward Observatory, American Association of Variable Star Observers Database and WHO. Discrete correlation function and structure function were used to analyse its long periodic variability and a possible period of about1.37years was found. This period could be explained by a close binary black hole system model, with a primary black hole mass of1.68×108M(·) and the secondary6.80×104M(·)Long-term B-V color index and quasi-simultaneous V-R color index of M-rk421were analysed. Results showed that Mrk421behaved a bluer when brighter behavior during its faint state, which may suggest that the contribution of the ther-mal radiation from the host galaxy is relatively strong in the faint state.