Optical monitoring and IDV analysis of the blazars S50716+714 and 3C 273

S50716+714 and 3C 273 are frequently studied blazars,which show obvious optical variabilities with different timescales.Using the 1.0 m telescope at the Yunnan Observatory,we monitored the two sources.For S50716+714,we report 990 observations during the monitoring duration from JD 2458536 to JD 2458540.For 3C 273,there are 884 observations during the monitoring duration from JD 2458539 to JD2458542.Based on those observations,we obtain the following results.(1)For S50716+714,there lie intraday optical variabilities(IDVs),with timescales from 0.31 hours to 2.64 hours.For 3C 273,it is possible that there lie IDVs,with timescales from 0.36 hours to 0.49 hours.(2)The time delay of S50716+714 isτRI=3.46 min between R and I bands,and the time delay of 3C273 isτIV=6.42 min between I and V bands.(3)We find that,for S50716+714,there lies a suspected intra-day period,P≈185.78 min;for 3C273,there lie intra-day periods,which are about~60 min and~80 min.

Chaotic Feature in the Light Curve of 3C 273

Some nonlinear dynamical techniques, including state-space reconstruction and correlation integral, are used to analyze the light curve of 3C 273. The result is compared with a chaotic model. The similarities between them suggest there is a low-dimension chaotic attractor in the light curve of 3C 273.

Periodic Variations of the Jet Flow Lorentz Factor in 3C 273

C 273 has been observed with VLBI for more than 30 years. The entire data have shown that the position angle of the superluminal knots ejected from the core varies periodically with a period of ~15 years. Moreover, their apparent velocity observed during the period 1963 -- 1997 has systematically decreased by a factor of 2. These remarkable properties are explained in terms of a precessing jet model, in which the ejection Lorentz factor of the superluminal knots has been decreasing during the last thirty years and has superposed on it a short-term (~5 year) oscillation. The periodic variations derived by the model-fitting are compared with the variations in the optical flux density. Binary black hole models are briefly discussed to show possible relations of the observed periods to the periods involved in a binary system (orbital motion, spin of black hole, accretion-disk rotation and Newtonian-driven precession etc.).

Spectral Reversal and Stratification of the Jet in 3C 273

Quasi-simultaneous VLBI observations at 15-86 GHz have shown that in the classical superluminal radio source 3C273, the spectral index a (Sv ∝ vα) has a systematic variation along the jet. For epoch 1995.15, a spectral reversal was observed at core distance - 1.5 mas, where the superluminal knot C12 located. Similarly, for epoch 1997.18, two spectral reversals were observed at core distances of -1.8 mas and -4.2 mas, where superluminal knots C11 and C14 werel respectively. These spectral reversals are associated with local maxim of the jet width. We suggest that this phenomenon may be related to a stratification of the jet structure, i.e., its physical parameters (flow velocity, Doppler factor, electron density and energy, magnetic field strength, etc.) are substantially dependent on the distance from the jet axis. These properties may be naturally formed through gasdynamic processes when the jet expands into a lower pressure ambient medium.

A modified stratified model for the 3C 273 jet

We present a modified stratified jet model to interpret the observed spectral energy distributions of knots in the 3C 273 jet. Based on the hypothesis of the single index of the particle energy spectrum at injection and identical emission processes among all the knots, the observed difference of spectral shape among different 3C 273 knots can be understood as a manifestation of the deviation of the equivalent Doppler factor of stratified emission regions in an individual knot from a characteristic one. The summed spectral energy distributions of all ten knots in the 3C 273 jet can be well fitted by two components; a low-energy component （radio to optical） dominated by synchrotron radiation and a high- energy component （UV, X-ray and γ-ray） dominated by inverse Compton scattering of the cosmic microwave background. This gives a consistent spectral index of α = 0.88 （Sv ∝ v^-α） and a characteristic Doppler factor of 7.4. Assuming the average of the summed spectrum as the characteristic spectrum of each knot in the 3C 273 jet, we further get a distribution of Doppler factors. We discuss the possible implications of these results for the physical properties in the 3C 273 jet. Future GeV observations with GLAST could separate the 7-ray emission of 3C 273 from the large scale jet and the small scale jet （i.e. the core） through measuring the GeV spectrum.

The Optical Variability of 3C 273

B-band measurements of 3C273 over some 110 years are compiled and used in a search for periodicities using the Jurkevich method. Periods of 2.0, 13.65±0.20 and 22.5±0.2yr are found. If the long-term periodicity is from the instability of a slim disk, then the periodicity (~ 13yr or ~ 22-yr) suggests masses of 107M for the central black holes.

Simultaneous optical g, r, i monitoring and IDV periodic analysis for quasar 3C 454.3

With the 1.26 m National Astronomical Observatory-Guangzhou University Infrared/Optical Telescope(NAGIOT) at Xinglong Station of National Astronomical Observatories, Chinese Academy of Sciences, we obtained 419 groups of simultaneous observations at g, r and i bands, for the first time,targeting quasar 3 C 454.3 during 15 nights from 2016 October 23 to 2016 December 15. Based on our observations, we investigate the optical variabilities, the relation between brightness and color index, and the periodicity variability. The presented analyses demonstrate that: 1. The maximum variations at the g, r and i bands are △mg|max= 1.015 ± 0.042 mag,△mr|max= 1.188 ± 0.050 mag and △mi|max= 1.305 ± 0.057 mag respectively. 2. During our 15 night monitoring program, intra-day variability was detected on one night(Nov. 2). Also, the brightness increased by A = 15.86% over 50.8 min,then decreased by A = 22.42% over 40.1 min. After a small bright state, its brightness increased again by18.1% over 55 min at the g band. Similar phenomena happened at r and i bands. The intra-day variabilities at the three bands on 2016 November 2 indicate a period of 105 min, which implies a black hole mass of MBH =(0.3 ~ 1.85)× 10^9 M⊙. 3. There is an anti-correlation between color index and magnitude,suggesting the source becomes redder when it brightens.

A Possible Periodicity in the Radio Light Curves of 3C 454.3

During the period 1966.5-2006.2 the 15 GHz and 8 GHz light curves of 3C 454.3 （z = 0.859） show a quasi-periodicity of ,-12.8 yr （-6.9 yr in the rest frame of the source） with a double-bump structure. This periodic behaviour is interpreted in terms of a rotating double-jet model in which the two jets are created from the black holes of a binary system and rotating with the period of the orbital motion. The periodic variations in the radio fluxes of 3C 454.3 are suggested to be mainly due to the lighthouse effects （or the variation in Doppler boosting） of the precessing jets caused by the orbital motion. In addition, variations in the rate of mass accreting onto the black holes may be also involved.

Long-term multi-wavelength variations of Fermi blazar 3C 279

Long-term optical,X-ray and γ-ray data of blazar 3 C 279 have been compiled from Swift-XRT,RXTE-PCA,Fermi-LAT,SMARTS and literature.The source exhibits strong variability on long timescales.From the 1980 s until now,the optical R band light curve spans more than 32 yr,and a possible 5.6-yr-long quasi-periodic variation component has been found in it.The optical spectral behavior has been investigated.In the optical band,the mean spectral index is –1.71.The source exhibits an obvious special spectral behavior.In the low state,the source manifests a clear bluer-when-brighter behavior in the sense that the optical spectrum turns harder(flatter) when the brightness increases.While in the high state,the optical spectrum is stable,which means the source spectral index does not vary with brightness.The correlation analysis has been performed among optical,X-ray and γ-ray energy bands.The result indicates that the variations of γ-ray and X-ray bands are well correlated without time delay on the timescale of days,and their variations exhibit weak correlations with those of the optical band.The variations,especially outbursts,are simultaneous,but the magnitude of variations is disproportionate.The detailed analysis reveals that the main outbursts exhibit strong correlations in different γ-ray,X-ray and optical bands.

Periodicity of the ejection of superluminal components in 3C345

The search for periodic behavior in Blazars has been an important subject, which is helpful for providing significant clues to the structure and physical processes of their central energy engine. A binary black hole system has recently been suggested for causing precession of relativistic jets and rotation of the ejection position angle of VLBI knots in superluminal sources. It has been suggested that in QSO 3C345, the ejection direction of the superluminal knots rotates due to the precession of the central engine and thus the ejection position angle of the successive knots shows a periodic behavior. Some authors argue for a period of precession being ～5.6 yr （Abraham ＆ Caproni）, ～8-10 yr （Klare et al.） and ～9.5 yr （Lobanov ＆ Roland）. Applying the helical model proposed by Qian et al. and selecting appropriate parameters to fit the initial trajectories （within 0.3 mas） of all the components （C4 to C10）, we derive the relation between the ejection position angle of the components and their precession phase, and thus find a 6.9-year precession period （4.3 yr in the source frame）, which can fit the ejection position angle of all these superluminal knots well. Since the VLBI observations have covered more than two precession periods, confirmation in one or more future periods would be important. In addition, we emphasize that the initial parts of the trajectories of these knots can be fitted by a common helical pattern （channel） through a precessing of its initial phase. This scenario （or helical precessing model） is different from the usual ballistic precessing model in which the individual superluminal knots move along straight-lines after ejection （Tateyama ＆ Kingham）.

Interpretation of the"Catastrophe"of Quasars on the Basis of New Decisions of the Problem of the Reverse Compton-Effect

In a model based on a search with increased accuracy of calculations-four times-of all the roots of the equations for the velocities of electrons in the inverse Compton effect,it is established that for new values of the roots of the equations(corresponding to the laws of conservation of momentum and energy),the recoil electrons remain ultra-relativistic,according to the authors,allows to eliminate the reverse Compton catastrophe of quasars.The maximum value of the scattering angle of the formed particles was found to explain the thin quasar jets.

A possible precession period for superluminal ejection in QSO 3C 279

The search for periodic features in fiux variability and superluminal ejection in blazars has been an important subject, which is helpful for providing significant clues to the understanding of the structure and kinematics of relativistic jets （physical processes of acceleration and collimation） and their association with central energy engines （black hole/accretion disk systems）. The wobbling of the ejection position angle of VLBI knots in superluminal sources has been interpreted in terms of the precession of the jet ejection nozzle as one of the alternative interpretations. We study the change with time of the initial position angle of superluminal knots in QSO 3C 279, using VLBI-data collected from the literature spanning more than ～30 yr and propose a jet-nozzle precession model which has very small viewing angles （less than 2°） to fit the long-term trends in both variations in the inner-jet position angle （Chatterjee et al.） and the ejection position angle of the VLBI components. It is shown that an ejection-nozzle precession period of ～25 yr could be appropriate to fit the long-term trend in the variation of the ejection position angle. However, the short-term swings and fluctuations in the ejection position angle cause some uncertainties. We also fit a model to the trajectory of component C4, correcting its non-ballistic motion near the core.

Flux and spectral variation characteristics of 3C 454.3 at the GeV band

We analyze the long-term lightcurve of 3C 454.3 observed with Fermi/LAT and investigate its relation to flux in the radio,optical and X-ray bands.By fitting the 1-day binned Ge V lightcurve with multiple Gaussian functions（MGF）,we propose that the typical variability timescale in the Ge V band is 1–10 d.The Ge V flux variation is accompanied by the spectral variation characterized as fluxtracking,i.e.,＂harder when brighter.＂The Ge V flux is correlated with the optical and X-ray fluxes,and a weak correlation betweenγ-ray flux and radio flux is also observed.Theγ-ray flux is not correlated with the optical linear polarization degree for the global lightcurves,but they show a correlation for the lightcurves before MJD 56000.The power density spectrum of the global lightcurve shows an obvious turnover at^7.7 d,which may indicate a typical variability timescale of 3C 454.3 in theγ-ray band.This is also consistent with the derived timescales by fitting the global lightcurve with MGF.The spectral evolution and an increase in the optical linear polarization degree along with the increase inγ-ray flux may indicate that the radiation particles are accelerated and the magnetic field is ordered by the shock processes during the outbursts.In addition,the nature of 3C 454.3 may be consistent with a self-organized criticality system,similar to Sagittariusand thus the outbursts could be from plasmoid ejections driven by magnetic reconnection.This may further support the idea that the jet radiation regions are magnetized.

The nature of the γ-ray flare associated with blazar 3C 454.3

We have studied the simultaneous spectral energy distributions （SEDs） of the 2009 December flare and those of the quiescent state of blazar 3C 454.3 by con- structing a multi-component model We find that all six SEDs can be explained by a one-zone leptonic model involving synchrotron self-Compton （SSC） plus external Compton emission from an accretion disk （ECD） and that from a broad-line region （ECC）. X-ray emission is dominated by the SSC mechanism, and the γ-ray spectrum is well represented by a combination of ECD and ECC processes. Our results indicate that the energy density of the magnetic field and electrons decrease with distance from the central engine, and the Doppler factor increases with the blob moving outward in the development of the 2009 December flare. The increase in the observed flux density is possibly due to the increase in the Doppler factor of the blob. The relation between the Doppler factor σb and the distance from the central black hole suggests the magnetically driven jets span a sub-pc scale, and the relation between the magnetic field Bt and the dimension of the emission region R＇b is in good agreement with what is required by conservation of magnetic flux. The weak ＂harder-when-brighter＂ behavior of the γ-ray spectrum could be a result of the increase in Doppler factor during the outward motion of the blob. The parameters during the quiescent state obviously deviate from those during the flare state. We propose that the flare was likely caused by the ejection of a new blob. The gamma-ray emissions in different states are associated with the evolution of the blob.

The periodicity of 3C 273's radio light curve at 15 GHz found by the wavelet method

We analyze the radio light curve of 3C 273 at 15 GHz from 1963 to 2006 taken from the database of the literature,and find evidence of quasi-periodic activity.Using the wavelet analysis method to analyze these data,our results indicate that:(1) There is one main outburst period of P1=8.1±0.1 year in 3C 273.This period is in a good agreement with Ozernoi's analysis in optical bands.(2) Based on the possible periods,we expect the next burst in 2014 October.

Time Lags between the 22 and 37 GHz Bursts of 48 Radio-loud AGNs

Based on the light curves at 22 and 37 GHz from the Metsahovi monitoring program, we investigate the time lags between the two radio bands for 48 radio-loud AGNs. DCF and ZDCF analyses are applied to the data. Our results show that there is a strong correlation between the two radio frequencies for all the sources, with the variations in the light curves at 37 GHz leading the ones at 22 GHz in general. There is no obvious differences between different sub-class AGNs as regards the time lag. In two sources, it was found that the bursts at the lower frequency lead the ones at the higher frequency. One possible explanation is that electron acceleration dominates the light curve until the radiation reaches the maximum. Some sources, such as 3C 273, 3C 279, 3C 345 and 3C 454.3, have good enough data, so we can calculate their lags burst-by-burst. Our calculations show that different outbursts have dif- ferent lags. Some bursts have positive lags, most of bursts have no clear lags, and a few have negative lags. This result means that different bursts are triggered by different mechanisms, and the interpretation for the result involves both an intrinsic and a geometric mechanism. The positive lags are well consistent with the shock model, and we use these lags to calculate the typical magnetic field strength of the radiating region.

Broadband spectral fitting of blazars using XSPEC

The broadband spectral energy distribution（SED） of blazars is generally interpreted as radiation arising from synchrotron and inverse Compton mechanisms. Traditionally,the underlying source parameters responsible for these emission processes,like particle energy density,magnetic field,etc.,are obtained through simple visual reproduction of the observed fluxes. However,this procedure is incapable of providing confidence ranges for the estimated parameters. In this work,we propose an efficient algorithm to perform a statistical fit of the observed broadband spectrum of blazars using different emission models. Moreover,we use the observable quantities as the fit parameters,rather than the direct source parameters which govern the resultant SED. This significantly improves the convergence time and eliminates the uncertainty regarding initial guess parameters. This approach also has an added advantage of identifying the degenerate parameters,which can be removed by including more observable information and/or additional constraints. A computer code developed based on this algorithm is implemented as a user-defined routine in the standard X-ray spectral fitting package,XSPEC. Further,we demonstrate the efficacy of the algorithm by fitting the well sampled SED of blazar 3 C 279 during its gamma ray flare in 2014.