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We present the historical light curve of NGC 4151 for 1906–2016. The light curve (Oknyanskij and Lyuty, 2007) is primarily based on our published photoelectric data (1968–2007, about 1040 nightly mean measurements (Oknyanskij and Lyuty, 2007)) and photographic estimates (mostlyOdessaandMoscowplates taken in 1906 – 1982 (Oknyanskij, 1978, 1983), about 350 measurements). Additionally, we include all data obtained prior to 1968 (de Vaucouleurs and de Vaucouleurs, 1968; Barnes 1968; Sandage, 1967; Wisniewski and Kleinmann, 1968; Fitch et al., 1967) in total, 19 photoelectric observations from 1958–1967, were reduced by us to the same diaphragm aperture as that used in our measurements) as well as photographic data (Pacholczyk et al., 1983) (Harvard and Steward observatories' patrol plates taken in 1910–1968, about 210 measurements). The light curve includes our old and new photometrical data obtained during last years at SAI, ShAO and Weihai Observatory as well as other published data (Roberts and Rumstey, 2012; Schnülle et al., 2015). All these data were reduced to an uniform photometric system. Applying Fourier (CLEAN algorithm) we have found periodic component ~16 years in the 110 years light curve. 40 years ago about the same "period" was firstly reviled fromOdessa's photometrical data (Oknyanskij, 1977; 1978). This “period” seen in the light curve was then found independently in the spectral variability and interpreted as a case of the supermassive binary black hole (Bon et al., 2012). We interpret these circles as some accretion dynamic time.

We present the results of time delay measurements between the A and B components of the gravitationally lensed quasar UM673. The measurements are based on the brightness variations of the quasar components observed with the 1.5-m telescope of the Maidanak Observatory in the V, R and I bands during the 2003-2005 observational period. The amplitudes of variations in the R band are 0.046 and 0.122 mag for Component A and B, respectively. Due to the short visibility period of UM673, about 3 months in a year, the light curves of the quasar components have large annual gaps. It complicates the time delay estimation. From the cross-correlation analysis, we find that the brightness variations in component B follow the brightness variations in Component A by about 150 days. We also explore the possibility of measuring the wavelength-dependent time delay between the brightness variations in the V, R and I bands. We find that in agreement with reprocessing model of quasar variability, the brightness variation at longer wavelengths follows the brightness variations at shorter wavelengths. Finally, we show that the combination of the multiband data corrected for the wavelength-dependent time delay can help to improve the cross-correlation analysis of the light curves with large annual gaps.

The time delays between brightness variations in different optical bands have been measured for a large number of Seyfert galaxies and low-redshift quasars. These time delays represent the travel time of light between different emission regions of the source and can be used to test variability scenarios. Based on multiwavelength observations of the lensed quasars: Q2237+0305, SBS1520+530, HE2149-2745, HE1104-1805 and UM673, we explore a variability mechanism in the redshift range 1.4z2.7 . From cross-correlation analysis of the light curves of the quasars we find that the brightness variations at longer wavelengths may follow the brightness variations at shorter wavelengths by a few days. An independent analysis of the multiband light curves of nonlensed quasars from the same redshift interval confirms the evidence of a time delay. A comparison of the observed time delays with the delays expected from a model of an accretion disc irradiated by the central variable source shows that reprocessing might be the primary mechanism responsible for the observed variability on the considered timescales.

Aims. We considered the possibility of measuring time delays between components of the multiplied quasar QSO2237+0305 and between V and R band variations. Methods. The analysis is based on the flux variations of four components observed by the OGLE collaboration and Maidanak group during the last quarter of 2003. The observed gradients of the brightness variations in this period for OGLE data in V band are 3.4 mmag/day, 2.2 mmag/day, 2.4 mmag/day, and 1.1 mmag/day for the components A, B, C, and D, respectively. The variations are probably intrinsic source variations. The basic method used for time-delay measurements is the cross-correlation technique. To estimate the uncertainty of the time-delay measurements, Monte Carlo simulations were carried out. Results. The calculations showed the impossibility of unambiguously measuring the differential time delays between the components. The observations at shorter wavelengths seem to be the only way to achieve robust time delay measurements in the system. The wavelength-dependent time delays can be used to provide the evidence of an accretion-disk structure of the central optical variable source. The observed V and R band variations of component C show good correlation with the correlation coefficient of 0.83. However, the obtained time delay, about 16.2 days, and its accuracy are far from reliable.

We present the results of CCD photometry of the first gravitational lenssystem QSO 0957+561, obtained on the base of La Palma archive. Usingcross-correlation analysis we have found the time delay between A and Bquasar images to be 430 ± 15 days. Our photometry of QSO 0957+561 A & Bgives additional independent confirmation of the recent results on the time delay about 425 days.

At the previous publication (Oknyanskij, 1997) we considered and used a new algorithm for time-delay investigations in case when the time delay was a linear function of time and the echo response intensity was a power-low function of the delay. We applied this method to investigate optical-to-radio delay in the double quasar 0957 + 561 (generally accepted to be a case of gravitational lensing). It was found in this way that the radio variations (5 MHz) followed the optical ones, but the time delay was a linear function of time with the mean value being about 2370 day and with the rate of increase V≈ 110 days/year. Here we use Monte-Carlo simulations to estimate the significance of the results. We estimate (with the 95% confidence level) that the probability of getting the same result by chance (if real V value was equal to 0 days/year) is less then 5%. We also show that the method can determinate the actual rate of increase Va of the time delay in the artificial light curves, which have the same data spacing, power spectrum and noise level as real ones. We briefly consider some other possible fields for using the method.

We use the intensive optical spectral monitoring of NGC 4151 obtained during the LAG campaign in January—June 1990, to investigate the profile and intensity variations of Hα and Hβ emission lines. The continuum light curve values were supplemented with photoelectrical photometry (11 dates) obtained at the Crimean observatory of the Sternberg Astronomical Institute by V. M. Lyuty. Cross-correlation analysis shows that the time delay between continuum changes and variability of Hβ and Hα is about 4 ± 2 days. The error in the delay was found by applying cross-correlation analysis to a model series, with the same temporal distribution, power spectrum, and signal-to-noise ratio as the actual data. This value of time delay is much lower than was found in other papers for other epochs. We can note two variabilly components in these changes in the lines: one fast and one slow. The amplitude of the slow component relative to the fast component is higher for Hα then for Hβ. The year 1990 was peculiar for N...

Time delay determinations in astrophysics are used most often to find time delays between flux density variations of different spectral bands and/or spectral lines in AGNs and different images of gravitationally lensed QSOs. Here we consider a new algorithm for a complex case, when the time delay is itself a linear function of time and the intensity of echo response is power function of the delay. We apply this method to investigate optical-to-radio delay in the double quasar 0957+561, which is a generally accepted case of gravitational lensing.

The time delay in Q0957+561 remains indeterminate due to differences between statistical methods, irregular data spacings, and (possibly) microlensing.

We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. In the paper we combine our previously published observational data of UM673 obtained during the 2003 - 2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyze the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyze the time evolution of the difference in magnitudes between images A and B of UM673 over more than ten years. We find that the quasar exhibits both short-term (with amplitude of \sim 0.1 mag in the R band) and high-amplitude (\sim 0.3 mag) long-term variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From cross-correlation analysis of the A and B quasar light curves and error analysis we measure the mean time delay and its error of 89 \pm11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673 is 95{+5/-16}{+14/-29} days (68 and 95 % confidence intervals). Analysis of the V - I color variations and V, R and I-band magnitude differences of the quasar images does not show clear evidence of the microlensing variations between 1998 and 2010., Comment: Submitted to A&A, 11 pages, 9 figures

We have used long light curves in the X-ray (ARIEL V, EXOSAT, Ginga and others), UV (IUE), and optical (data of intensive ground-based monitoring) ranges of NGC 4151 to investigate connections between variations in these spectral fields. Applying Fourier (CLEAN algorithm) and modern cross-correlation analysis, we revealed the different character of the variability and the cross-correlation between “high” and “low” states of the nucleus. In contrast to earlier results, we found that X-ray and optical fluxes correlate in the “active” state without any apparent delay. In the power spectrum of the optical and X-ray light curves in the “high state” we found the existence of the same “period” of about 65 days, which disappeared in the “low state”. We confirmed previous data that X-ray variations correlate more strongly with UV and optical continuum variability during the “low state”, but it is possible that the optical variations appear to lag behind those involving X-rays by several tens of days. Strong variations in Hβ line profile and [Fe X]λ6374 line intensity variations correlate with the observed X-ray covering dip in 1990 (Yaqoobet al., 1993). We discuss briefly the implications of these results.

1. We proposed a new special method for the estimation of the time delay. On the basis of the new analysis of the optical data carried out by us we can assume that the true delay is τ = 530 ± 15 days. The confidence level of this result is better than 99%. 2. The value of the time delay found by us leads to a essentially smaller value of the constant H than in the case with τ ≈ 415 days. On the base of the model by Falco et al.(1985) in conformity with the value of τ0 found by us, H0 < 68kms−1Mpc−1.

Time delay determinations in astrophysics are used most often to find time shifts between variations of different spectral bands and or spectral lines in AGNs as well as time delays between different images of gravitationally lensed QSOs. Most often are used two different methods: CCF (Gaskell &Spark 1986) and DCF (Edelson & Krolik 1988), which are based respectively on line interpolation of data sets or binning of correlation coefficients. We have introduced several simple improvements to the CCF (Oknyanskij, 1994) and this modernized method MCCF combines best properties of these CCF and DCF methods. In addition we calculate in the MCCF line regression coefficients as functions of time shift. Here we use the same method, but generalized for the more complex case when the time delay is a linear function of time and a portion of response flux density is itself a power-low function of the delay. We apply this method to investigate optical-to-radio time delay in the double quasar 0957+561, which is a generally accepted case of gravitational lensing. Possibility for this correlation in Q0957+561 was first reported by Oknyanskij & Beskin (1993, here after OB) on the basis of radio observations made in the years 1979 to 1990. OB used an idea to take into account the known gravitational lensing time delay to get combined radio and optical light curves and then to use them for determination of the possible radio-from-optical time delay. It was found this way that radio variations (5 MHz) followed optical ones by about 6.4 years with high level of correlation (≈0.87). Here we use new data sets which were obtained during 1979–1994 to determine the gravitational lensing time delay τo (Haarsa et al. 1996). We will base our discussion below on the τo = 425 days which is preferred today, since we find that our results are virtually independent of which value in the interval of 410–550 days we take as. τo

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short time-scale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the {\it International Ultraviolet Explorer} (IUE) for 9.3 days, yielding a pair of LWP and SWP spectra every $\sim$70 minutes, and during four-hour periods for 4 days prior to and 5 days after the continuous monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 \AA\ to 2688 \AA\ went through four significant and well-defined ``events'' of duration 2 -- 3 days during the continuous monitoring period. We find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller time scales (i.e., a few days). The continuum variations in all of the UV bands are {\it simultaneous} to within an accuracy of about 0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip), and do not correlate well with continuum light curves over the (short) duration of the campaign, because the time scale for continuum variations is apparently smaller than the response times of the emission lines., Comment: 39 pages, LaTeX, including 7 PostScript figures; To appear in the ApJ (October 20, 1996) Vol. 470

After a minimum in the spring of 1984, when the broad permitted lines were found to become much weaker, the spectrum of NGC 4151 became much more like a Sy 2 (Lyutyi et al., 1984; Chuvaev and Oknyanskij, 1989; Penston and Perez, 1984). Recently, this object returned to the high state after about 8 yr in the semiquiescent phase (Oknyanskij et al., 1991).

This paper combines data from the three preceding papers in order to analyze the multi-waveband variability and spectral energy distribution of the Seyfert~1 galaxy NGC~4151 during the December 1993 monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium energy ($\sim$1.5~keV) X-rays, with a normalized variability amplitude (NVA) of 24\%. Weaker (NVA = 6\%) variations (uncorrelated with those at lower energies) were seen at soft $\gamma$-ray energies of $\sim$100~keV. No significant variability was seen in softer (0.1--1~keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9\% to 1\% as the wavelength increased from 1275~\AA\ to 6900~\AA. These data do not probe extreme ultraviolet (1200~\AA\ to 0.1~keV) or hard X-ray (2--50~keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of $\ls$0.15~day between 1275~\AA\ and the other ultraviolet bands, $\ls$0.3~day between 1275~\AA\ and 1.5~keV, and $\ls$1~day between 1275~\AA\ and 5125~\AA. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-waveband AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well-fitted with a very steep, red power-law ($ a = -2.5 $). If photons emitted at a ``primary" waveband are absorbed by nearby material and ``reprocessed" to produce emission at a secondary waveband, causality arguments require that variations in the secondary band follow those in the primary band. The tight interband correlation and limits on the ultraviolet and, Comment: 35 pages, LaTeX (including aaspp4), including 7 PostScript figures; To appear in the ApJ (October 20, 1996) Vol. 470