Study On Variability Properties Of Blazars With Multi-band Observations

Blazars,including BL Lac objects and flat spectrum radio quasars(FSRQs),are the most violently variable subclass of active galactic nuclei(AGNs).They are characterized by rapid and violently variability across the entire electromagnetic waveband.These extreme properties are generally interpreted as a consequence of non-thermal emission from the relativistic jet oriented at small angle from the line of sight.Thus,variability has been the most powerful tools to reveal the nature of blazars.The amplitude of variability,timescale,multi-band correlation and spectral properties can shed light on the location,size,structure,and dynamics of the non-thermal emitting regions and on the radiation mechanisms.Meanwhile,the blazars are thought to be the main sources of very high energy emission,it is very useful to investigate the very high energy(VHE)gamma-ray from blazars combining with optical multi-band monitoring.This could constrain the density and evolution of the extragalactic background light(EBL),intergalactic magnetic field,as well as study the origin of ultra high energy cosmic rays(UHECRS).We performed a long-term optical photometric monitoring campaign to ob-serve 3 BL Lacs S5 0716+7141,OJ 287,Mrk 501 and one FSRQs 3C 273.Large higher quality data obtained in optical and high energy gamma-ray data were re-duced.The analysis methods of structure function,discrete correlation function and wavelet were employed to study the properties of multi-band variability and spectrum evolution.The main results are in the following.Three BL Lacs S5 0716+714,OJ 287,Mrk 501 were always in active state during the whole monitoring campaign.The overall variability amplitudes were?B = 1m.834,?V=2m.167,?R = 2m.148,and ?I = 1m.912 for S5 0716+714.?V = 1.m335,?R = L.m237 and ?I = 1.m594 for OJ 287.?B = 0.m566.?V=0.m582,?R=0.m698,and ?I = 0.m713 for Mrk 501.A short-term variability timescale of 10-day period was found in the optical bands for S5 0716+714,Causality arguments constrain the mass of the central black hole are MBH = 4.4 × 1010M?(Kerr black hole)and MBH=1.1×1010M?(Schwarzschild black hole).The correlation analysis between gamma-ray and op-tical light curves tends to support a leptonic model.In our observation,the major outbursts of OJ 287 occurred in January 2016,as predicted by the binary black holes model.For Mrk 501,the minimal optical variability timescale was about 2 minutes.This fast variability requires the co-moving magnetic field strength in the jet to be above 52 G(?D?3),which supports a hadronic model.We analyzed the gamma-ray light curve with the wavelet analysis,and a quasi-periodicity with a period of 25-day was found at the significance level larger than 95%,which is similar with the 23-day quasi-periodic oscillation period of Mrk 501 in X-ray and TeV gamma-ray.FSRQs 3C 273 was in a steady state during the whole monitoring campaign,the overall variability amplitudes were ?B =0.m799,?V=0.m816,?R =0.m818 and ?I = 1.m129.The fractional variability amplitude in each band was below 0.5%(Fvar-<0.5%).We found two exceptionally luminous gamma-ray outbursts observed by Fermi-LAT.The minimal variability timescale was 4.9-hour.This timescale constrains the size of the emission region to be 2.19 × 1015 cm with Doppler factor ?? ? 4.784.By analyzing color-magnitude behaviors in different timescales,we found that the four blazars have different optical spectral behavior.Our observations showed bluer-when-brighter behavior on long,short,and intraday timescales for blazar S5 0716+714.While O J 287 and Mrk 501 presented bluer-when-fainter and redder-when-brighter behavior,respectively.The correlation between the color index and magnitude variation of 3C 273 is complex.The spectrum displayed a bluer-when-brighter behavior when the source is fainter than Fv = 30.201 ± 0.026 mJy,while the spectrum had redder-when-brighter trend when the source is brighter than critical value.The spectrum analysis results indicate that the optical emission consists of two variable components.This can be explained by a geometrically-thick accretion disk model.The color-index trends of Mrk 501 suggest the presence of a strong host galaxy component.In high energy gamma-ray region 100MeV-300GeV,the BL Lacs have much harder spectrum than that of FSRQs,which means that the spectrum of BL Lacs can extend to higher energy region.The gamma-ray spectral properties implied that the BL Lacs are more likely to be candidates of TeV gamrma-ray emission.