Flicker-noise model for optical fluctuations of NGC 4151

P. K. Shternberg State Astronomical Institute; Crimean Astrophysical Observatory. Translated from Astrofizika, Vol. 31, No. 1, pp. 75–86, July–August, 1989.     

Connection between X-ray and optical variability in NGC 4151

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 H line profile and [Fe X]6374 line intensity variations correlate with the observed X-ray covering dip in 1990 (Yaqoobet al., 1993).We discuss briefly the implications of these results.     

A model for X-ray emission from loop prominences

A study is made of X-ray line emission observed during the developing stages of a set of post-flare loop prominences. The time behaviour of the line emission can be described by a model consisting of two flux tubes containing plasma heated impulsively at the flash phase; the plasma cools by radiation and by conduction to the chromosphere. These ideas are extended to the possible formation of H prominences from low-lying hot loops.     

NGC 4151 entering in an active phase

Photometric observations of the nucleus of NGC 4151 obtained in a period of 16 nights in April 1990 in the 5672 Å continuum and theU-band have clearly shown that the nucleus has entered into an active phase and its energy output is still rising.     

Dust extinction and X-ray emission from the starburst galaxy NGC 1482

We present the results based on multiwavelength imaging observations of the prominent dust lane starburst galaxy NGC 1482 aimed to investigate the extinction properties of dust existing in the extreme environment. (B-V) colour-index map derived for the starburst galaxy NGC 1482 confirms two prominent dust lanes running along its optical major axis and are found to extend up to ∼11 kpc. In addition to the main lanes, several filamentary structures of dust originating from the central starburst are also evident. Though, the dust is surrounded by exotic environment, the average extinction curve derived for this target galaxy is compatible with the Galactic curve, with R_V = 3.05, and imply that the dust grains responsible for the optical extinction in the target galaxy are not really different than the canonical grains in the Milky Way. Our estimate of total dust content of NGC 1482 assuming screening effect of dust is ∼2.7 × 10⁵ M_⊙, and provide lower limit due to the fact that our method is not sensitive to the intermix component of dust. Comparison of the observed dust in the galaxy with that supplied by the SNe to the ISM, imply that this supply is not sufficient to account for the observed dust and hence point towards the origin of dust in this galaxy through a merger like event.Our multiband imaging analysis reveals a qualitative physical correspondence between the morphologies of the dust and Hα emission lines as well as diffuse X-ray emission in this galaxy. Spatially resolved spectral analysis of the hot gas along outflows exhibit a gradient in the temperature. Similar gradient was also noticed in the measured values of metallicity, indicating that the gas in the halo is not yet enriched. High resolution, 2-8 keV Chandra image reveals a pair of point sources in the nuclear region with their luminosities equal to 2.27 × 10³⁹ erg s⁻¹ and 9.34 × 10³⁹ erg s⁻¹, and are in excess of the Eddington-limit of 1.5 M_⊙ accreting source. Spectral analysis of these sources exhibit an absorbed-power law with the hydrogen column density higher than that derived from the optical measurements.     

The complex narrow line region in NGC 4151

We present new narrow band images of NGC 4151, taken in excellent seeing conditions with the 4.2-m William Herschel Telescope on La Palma. These reveal a wealth of detail that can be used to test models for the anisotropy of the narrow-line region.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Optical observations of the nucleus of NGC 4151

Photographic observations of the nucleus of the Seyfert galaxy NGC 4151, carried out during the last seven years, are reported. The object shows irregular variations between photographic magnitudes 11.2 and 13.0.     

Nature of the optical and ultraviolet variability of the nucleus of NGC 4151: A new concept

We have analyzed the optical (U BV) and ultraviolet (λ1000–2700 Å) observations of the nuclear variability of the Seyfert galaxy NGC 4151 in the period 1987–2001 (the second cycle of activity). The fast (tens of days) and slow (～10 years) components of the nuclear variability, F and S, respectively, are shown to be completely different, but thermal in nature. We associate the S component with the formation and evolution of an accretion disk and the F component (flares) with instabilities in the accretion disk and their propagation over the disk in the form of a shock wave. The S component is present not only in the optical, but also in the ultraviolet range, with its amplitude being comparable over the entire range λ1000–5500 Å under study. The amplitude of the average flare (the F component) doubles as the wavelength decreases from 5500 to 1000 Å, while the rise time of the brightness to its maximum Δt (the variability time scale) decreases from

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to 6^d ± 2^d. The brightness decline (flare decay) time decreases by a factor of 16. The extinction in the ultraviolet is shown to have been grossly underestimated: beginning from the first IUE data, only the extinction in our Galaxy,

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, has been taken into account. A proper allowance for the total extinction, i.e., for the extinction in the nucleus of NGC 4151 as well

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leads to a large increase in the luminosity of the variable source in the nucleus of NGC 4151: L = (6–8) × 10⁴⁶ erg s^?1. The spectral energy distribution for the variable source (λ950–5500 Å) agrees well with two Planck distributions: T_e = 65 000 (λ_max = 450 Å) and 8000 K. The radiation with T_e = 8000 K is the reprocessing of the bulk of the ultraviolet radiation by the accretion disk with a lag of 0.5–0.6 days in the V band. The lag in the U-B variability of the slow component revealed the existence of an extended broad line region (EBLR) at an effective distance of 1.5 lt-years, as confirmed by spectroscopic data obtained at the Crimean Astrophysical Observatory. This yields the following mass of the central object in NGC 4151: M_c = (1–3) × 10⁹M_⊙. The luminosity of the variable source then accounts for 50–60% of L_Edd rather than 1–2%, as has been thought previously. In general, the pattern of ultraviolet and optical variability in NGC 4151 agrees excellently with the theory of disk accretion instability for a supermassive black hole suggested by N. Shakura and R. Sunyaev 30 years ago: the energy release is at a maximumin the ultraviolet (in the case under consideration, at λ450 Å), the luminosity is ～10⁴⁷ erg s^?1 for M_c ～ 10⁹M_⊙ (several tens of percent of L_Edd), and the variability time scale ranges from several days to many years.

     

NGC4151的长期光变行为

本文收集了NGC4151近90年的光学资料,并用Jurkevich分析方法对其进行了光变周期分析其光变存在一个14．04±0．8年的长周期,且该天体的谱型与源的亮度强烈相关。     

NGC4151的光变周期分析(英)

本文利用地面光学望远镜对NGC4151进行国际联测所得的结果,通过周期日方法和逐步回归方法得到了该目标的一些较为明显的光变周期．结果表明,NGC4151存在两个主要光变周期．一个是所有光学波段存在约14天的周期,另一个是连续谱存在约30天周期,而对于Hα和Hβ则分别为24和38天的周期．     

The nature of the luminous X-ray emission of NGC 6240

We briefly review and extend our discussion of the ROSAT detectionof the extraordinarily luminous (>10⁴² erg/s) partly extended(>30 kpc diameter) X-ray emission from the ultraluminous infrared galaxy NGC 6240.The `standard'-model of starburst outflow is contrasted with alternatives and a comparison with the X-ray properties of ellipticals is performed.     

CCD surface photometry of the Seyfert 1 galaxy NGC 4151

We present CCD surface photometry of the central region of the bright Seyfert 1 galaxy NGC 4151. The observations were obtained under excellent seeing conditions (0.75 FWHM). Morphological parameters for the bulge of the galaxy are estimated from the observed broad bandV, R, andI surface brightness profiles.From the analysis of the colour maps two distinct nuclear structures emerge perpendicular to each other. One of them is elliptically elongated at PA angle 50°. This direction is close to the direction of the radio jets and coincides with the Extended Narrow Emission Line Region (ENELR). Its extension is about 7 arc sec and its colours are bluer than the surronding bulge. The second structure is smaller in extension (about 1 arc sec) and is elongated at PA130°. Its colours are redder than those of the surrounding bulge. It is suggested that this might constitute areal extension of the Broad Line Region (BLR).Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Optical and X-ray variability of Seyfert galaxies NGC 5548, NGC 7469, NGC 3227, NGC 4051, NGC 4151, Mrk 509, Mrk 79, and Akn 564 and quasar 1E 0754

The results of a cross-correlation analysis of the optical and X-ray light curves for eight Seyfert galaxies, NGC 5548, NGC 7469, NGC 3227, NGC 4051, NGC 4151, Mrk 509, Mrk 79, and Akn 564 and for the optical spectra of the quasar 1E 0754 are presented. In the case of the galaxies NGC 5548 and NGC 7469, the maximum values of cross-correlation coefficients for optical and X-ray variations proved to be high (0.73 and 0.79, respectively). The lag time, determined from the maximum of cross-correlation function, is 2.800 _?1.58 ^+3.12 days for NGC 5548 and 0.6 _?4.3 ^+0.9 days for NGC 7469. This result favors downscattering of the X-ray emission into the optical range (direct Compton effect) for NGC 5548 and NGC 7469. In addition to the main maximum, which corresponds to the lag of the optical flux variations behind the X-ray flux, six objects (excluding Akn 564 and NGC 4151) show the wings in the intervals of cross-correlation functions that correspond to the time lags of X-ray emission behind the optical emission of approximately 10 days. A method of determining the masses of central black holes in AGNs through spectral line widths is presented; with this method the mass of the central black hole in the quasar 1E 0754 was found (M _BH = 1.01 × 10⁸ M _sun). The position of the quasar 1E 0754 in the mass-luminosity diagram meets the position of other NLS 1 galaxies.     

The Massive Black Hole at the Center of NGC 4151

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