The Seyfert Galaxy NGC 1275: Superfine Structure

Astronomy Letters - Based on VLBA observations (NRAO archive) at λ = 2 cm, we have constructed radio maps of the Seyfert galaxy NGC 1275 with a resolution of 25 µas, the epochs of...     

Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273

We present spectrophotometric results of the Seyfert 2 galaxy NGC 2273. The presence of high-order Balmer absorption lines (H8, H9, H10) and weak equivalent widths of Call K A3933, CN A4200, G-band A4300 and MgIb 5173 clearly indicate recent star-forming activity in the nuclear region. Using a simple stellar population synthesis model, we find that for the best fit, the contributions of a power-law featureless continuum, an intermediate-age (~ 108 yr) and an old (> 109yr) stellar population to the total light at the reference normalization wavelength are 10.0%, 33.4% and 56.6%, respectively. The existence of recent starburst activity is also consistent with its high far-infrared luminosity (log LFIR/L = 9.9), its infrared color indexes [a(25,60) = -1.81 and a(60,100) = -0.79, typical values for Seyfert galaxies with circumnuclear starburst], and its q-value (2.23, ratio of infrared to radio flux, very similar to that of normal spirals and starburst galaxies). Byrd et al. have suggested that NGC 2273 mig     

Long-Term Variability Of The Optical Emission Lines In The Nuclear Spectrum Of The Seyfert Galaxy Ngc 3227

Fifty-three spectrograms in the optical region (3700–7300 Å) with the spectral resolution ～8 Å have been obtained for the Seyfert nucleus of the galaxy NGC 3227 with the 6-m telescope on January 1977, while the nucleus was in the historically important epoch of its extreme maximum brightness. Width of the slit was 1″, length of the box during the spectra measurements was 1.5″. Data obtained by us and those compiled from literature showed that profiles of the Balmer lines H_α, H_β and H_γ are different, evidencing that the gas emitting these lines is highly self-absorbed. It was shown that narrow components of the profiles revealed by Rubin and Ford kept their positions (radial velocities) over 25 years. The components showed intensity variations compared to the central one from minimum to maximum of the nucleus brightness. The same variations were observed by us earlier in the emission line profiles of the NGC 7469 nucleus spectrum. Narrow profile components can reflect long-lived flows or jets in the broad line region (BLR). Obtained facts evidenced that long-lived gas streams and flows causing narrow components of broad line profiles presented not only when BLR of accretion disc is strong but when BLR of accretion disc declined. Blue bump at radial velocity of ?5000 km/s in H_γ profile was revealed in spectra of high states of the nucleus, which disappeared in low state. One of the interpretations of this event can be in the framework of a model of one-sided or two-sided gas ejection during the high state of the nucleus, positive radial velocities of which being screened out by a circumnuclear disk.     

The Origin of Infrared Emission from the Infrared Luminous Galaxy NGC 4418

We present a study of the origin of infrared (IR) emission in the optically normal, infrared luminous galaxy NGC 4418. By decomposing the stellar absorption features and continua in the range of 3600-8000 A from the Sloan Digital Sky Survey into a set of simple stellar populations, we derive the stellar properties for the nuclear region of NGC 4418. We compare the observed infrared luminosity with the one derived from the starburst model, and find that star-forming activity contributes only 7% to the total IR emission, that as the IR emission region is spatially very compact, the most possible source for the greater part of the IR emission is a deeply embedded AGN, though an AGN component is found to be unnecessary for fitting the optical spectrum.     

The Correlation between Optical Spectral Index and Continuum Luminosity Variation in the Seyfert 1 Galaxy NGC 5548

Using the archived optical spectra of NGC 5548 between 1989 and 2001, we derived the optical spectral index by fitting the spectra in wavelength windows unaffected by strong emission lines. We found that the index is anti-correlated with the continuum luminosity at 5100 A with a correlation coefficient of -0.8. Based on the standard thin accretion disk model, we investigated whether the correlation is related to the variations of the dimensionless accretion rate m (mass accretion rate in Eddington unit), or the inner radius of the accretion disk Rin, or both. The correlation can be modeled well using a co-variable mode of Rin/Rs = 12.5m-0.8 (Rs is Schwarzschild radius). As luminosity increases, m increases from 0.05 to 0.16 and at the same time Rin decreases from 133.9.RS to 55.5.RS, consistent with the prediction for a transition radius within which an ADAF structure exists. We concluded that the change of both inner accretion radius and the dimensionless accretion rate are key factors for the variation     

Correlation and time delays of the X-ray and optical emission of the Seyfert Galaxy NGC 3783

We present simultaneous X-ray and optical B - and V -band light curves of the Seyfert Galaxy NGC 3783 spanning 2 years. The flux in all bands is highly variable and the fluctuations are significantly correlated. As shown before by Stirpe et al. the optical bands vary simultaneously, with a delay of less than 1.5 d but both B and V bands lag the X-ray fluctuations by 3–9 d. This delay points at optical variability produced by X-ray reprocessing and the value of the lag places the reprocessor close to the broad-line region. A power spectrum analysis of the light curve, however, shows that the X-ray variability has a power-law shape bending to a steeper slope at a time-scale of ∼2.9 d while the variability amplitude in the optical bands continues to grow towards the longest time-scale covered, ∼300 d. We show that the power spectra together with the small value of the time delay are inconsistent with a picture where all the optical variability is produced by X-ray reprocessing, though the small amplitude, rapid optical fluctuations might be produced in this way. We detect larger variability amplitudes on long time-scales in the optical bands than in the X-rays. This behaviour adds to similar results recently obtained for at least three other active galactic nuclei and indicates a separate source of long-term optical variability, possibly accretion rate or thermal fluctuations, in the optically emitting accretion disc.     

Differences in the emission line spectrum at maximum and minimum of the nuclear brightness of the Seyfert galaxy NGC 7469

We have compiled data on relative intensties, fluxes and profiles of emission lines of the NGC 7469 nuclear spectrum observed in the optical region by many authors during 1966–1986. The aim of our investigation is to reveal differences in the characteristics of the emission lines in maxima and minima of the nuclear brightness. The distinction was revealed for relative intensities, fluxes and profiles. During the extreme maxima profiles ofH ,H andH lines, colour indices of the nucleus and its high level of accretion rate resemble those of QSOs. On the other hand, during the deep minimum 1989 the nucleus had the properties of a Sy2 type. There is a relation between variable fluxes ofH and [OIII] 5007 Å lines which is different for the minima and maxima of the nuclear brightness. The fluxes of the [OIII] 5007 Å line during the minima of the nuclear brightness are systematically higher than those in maxima. The above mentioned peculiarities of the emission lines in the NGC 7469 nucleus can be understood in terms of the increase of electron temperature, electron density and velocities of the nuclear gas from minimum to maximum.     

The optical variability of the Seyfert galaxies NGC 4051 and NGC 1068

The optical (BV) monitoring data for the Seyfert galaxies NGC 4051 and NGC 1068 are presented in this paper. NGC 4051 showed a variation of 0.43 mag in 21 minutes in B band from its bright nucleus, and the nucleus of NGC 1068 exhibited a variability of 0.46 mag in B band in a timescale of 1.8 hours. These results argued that the optical emission from the nuclei of the two Seyfert galaxies could be dominated by the nonthermal radiation. The data in this paper, however, are only marginal evidence on the rapid optical variability of these two Seyferts, and further monitoring is needed to confirm this type of variability.     

Variability of Seyfert galaxies in the near-infrared — II. The unusual starburst Seyfert NGC 7469

The bright Seyfert 1 and starburst galaxy NGC 7469 has been monitored at JHKL for 13 yr. Its variations have been small, amounting to less than 0.3 mag, except for an event in 1989 when the flux from its active nucleus declined to about one quarter of its normal level.   Using the technique of flux variation gradients (FVGs), the variations have been analysed, taking into account the known JHK fluxes from the star-forming ring surrounding the nucleus. By comparing the FVGs of NGC 7469 with those of other Seyfert galaxies, the nucleus of NGC 7469 is found to suffer very little reddening.   The active nucleus normally contributes more than 50 per cent of the flux measured in the K and L bands through an aperture of 12 arcsec diameter.   Infrared excesses (above the fluxes expected from stars) are observed from the circumnuclear ring at both K and L and it is suggested that they arise from warm, small particles that also emit the aromatic hydrocarbon features seen in this galaxy.     

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.     

Spectrophotometric observations of the nuclei of the seyfert galaxies NGC 4051, NGC 7469, and NGC 1275

A digital television complex, equipped with an original slitless spectrograph with transpaarent diffraction gratings, and operating on the 0.5-meter telescope of the Crimean Astrophysical Observatory, was used for spectrophotometric observations of the nuclei of Seyfert galaxies. The absolute energy distributions in the spectra of the nuclei of the Seyfert galaxies NGC 4501, NGC 7469, and NGC 1275 in the wavelength range 4000–7000 Å were obtained. Synthetic stellar magnitudes in the V band were calculated. The apparatus can be used to investigate the spectral variability of emission from the nuclei of Seyfert galaxies on a time scale of tens of minutes or more.Translated from Astrofizika, Vol. 39, No. 1, pp. 101–110, January–March, 1996.     

Photometry and Spectrophotometry of the Seyfert Galaxy MRK 509 over 23 Years Photometry And Spectrophotometry Of The Seyfert Galaxy Mrk 509

In the time period from 1972-1993, the Seyfert 1 galaxy Mrk 509 showed large variations of its optical continuum and broad emission line fluxes. The broad H and H emission lines have approximately the same profiles, which retain their shape during flux variations. The relationship between the continuum and emission line fluxes varies with time. The spectral energy distribution of the variable continuum based on UBVRI data has the form lgF() lg, with the dereddened = -0.60.     

X-ray and optical variability of the seyfert galaxy NGC 7469

Based on our UBV RI observations and X-ray data from the RXTE satellite, we have investigated the variability of the galaxy NGC 7469 over the period 1995–2009. In 1995–2000, the optical brightness of the galactic nucleus changed almost by 1 ^m in the U band. In 2000–2009, the amplitude of the optical variations was considerably lower. Regular X-ray observations began only in 2003. The X-ray fractional variability amplitude is higher than the optical one. The optical variability amplitude decreases with increasing wavelength. The full width at half maximum of the X-ray and B-band autocorrelation functions is about 8 and 62 days, respectively. The structure functions (SF) in the X-ray range on time scales up to 7 days and in the optical range on time scales up to 100 days have the form of a power law SF(τ) ∼ τ ^b , where τ is the time shift. On time scales of more than a day, where both structure functions have been determined rather reliably, their slopes differ markedly: b = 1.34 ± 0.06 and b = 0.25 ± 0.05 for the optical and X-ray ranges, respectively. The X-ray and B-band structure functions begin to flatten, respectively, near 6–8 days and on time scales of about 90 days. The observed structure functions can be described by the model of a superposition of independent Gaussian flares whose number changes with duration ω as n(ω) ∼ ω ^α and whose amplitudes depend on duration as A(ω) ∼ ω ^β. The flux distribution and the flux-amplitude relation are consistent with the model of a light curve in the form of a superposition of random flares. Once the fast intensity variations have been filtered out on long time scales, the X-ray light curve correlates well with the optical one. No lag of the X-ray variations relative to those in the B band is detected. The light variations in the R and I bands lag behind those in the B band calculated from the centroid of the cross-correlation function by 2.6 and 3.5 days, respectively, at a 3σ confidence level.     

The exceptional X-ray variability of the dwarf Seyfert nucleus NGC 4395

An analysis of the X-ray variability of the low-luminosity Seyfert nucleus NGC 4395, based on a long XMM–Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum  (α≈ 1)  to a steeper spectrum  (α≈ 2)  at a frequency   f _br= 0.5–3.0 × 10⁻³ Hz  , comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured   M _BH− f _br  values to lower M _BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among the most violent seen in an active galactic nuclei to date, with the fractional rms amplitude  ( F _var)  exceeding 100 per cent in the softest band. The amplitude of the variations seems intrinsically higher in NGC 4395 than most other Seyfert galaxies, even after accounting for the differences in characteristic frequencies. The origin of this difference is not clear, but it is unlikely to be a high accretion rate (   L / L _Edd≲ 20  per cent for NGC 4395). The variations clearly follow the linear rms–flux relation, further supporting the idea that this is a ubiquitous characteristics of accreting black holes. The variations are highly coherent between different energy bands with any frequency-dependent time delay limited to ≲1 per cent.