Broad-line Seyfert 1 Galaxies Research Articles

Probing the Ionizing Continuum of Narrow‐Line Seyfert 1 Galaxies. I. Observational Results

We present optical spectra and emission-line ratios of 12 narrow-line Seyfert 1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A common feature in the EUV continuum of active galactic nuclei is the big blue bump (BBB), generally associated with thermal accretion disk emission. While Galactic absorption prevents direct access to the EUV range, it can be mapped by measuring the strength of a variety of forbidden optical emission lines that respond to different EUV continuum regions. We find that narrow emission line ratios involving [O II] λ3727, Hβ, [O III] λ5007, [O I] λ6300, Hα, [N II] λ6583, and [S II] λλ6716, 6731 indicate no significant difference between NLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy distributions of their ionizing EUV-soft X-ray continua are similar. The relative strength of important forbidden high-ionization lines like [Ne V] λ3426 compared to He II λ4686 and the relative strength of [Fe X] λ6374 appear to show the same range as in BLS1 galaxies. However, a trend of weaker F([O I] λ6300)/F(Hα) emission-line ratios is indicated for NLS1 galaxies compared to BLS1 galaxies. To recover the broad emission line profiles, we used Gaussian components. This approach indicates that the broad Hβ profile can be well described with a broad component (FWHM 3275 ± 800 km s-1) and an intermediate broad component (FWHM 1200 ± 300 km s-1). The width of the broad component is in the typical range of normal BLS1 galaxies. The emission-line flux that is associated with the broad component in these NLS1 galaxies amounts to at least 60% of the total flux. Thus, it dominates the total line flux, similar to BLS1 galaxies.

The optical to X-ray spectrum of the broad-line ultrasoft AGN RE J2248-511

We present a series of monitoring observations of the ultrasoft broad-line Seyfert galaxy RE J2248-511 with XMM-Newton. Previous X-ray observations showed a transition from a very soft state to a harder state five years later. We find that the ultrasoft X-ray excess has re-emerged, yet there is no change in the hard power-law. Reflection models with a reflection fraction of ⩾15, and Comptonisation models with two components of different temperatures and optical depths ( kT 1 = 83 keV, T 1 = 30 eV, τ 1 = 0.8; KT 2 = 3.5 keV, T 2 = 60 eV, τ 2 = 2.8) can be fit to the spectrum, but cannot be constrained. The best representation of the spectrum is a model consisting of two blackbodies ( kT 1 = 0.09 ± 0.01 keV, kT 2 = 0.21 ± 0.03 keV) plus a power-law ( Γ = 1.8 ± 0.08). We also present simultaneous optical and infrared data showing that the optical spectral slope also changes dramatically on timescales of years. If the optical to X-ray flux comes primarily from a Comptonised accretion disk we obtain estimates for the black hole mass ∼ 10 8 M ˙ ⊙ , accretion rate ∼ 0.8 M ˙ Edd and inclination cos( i) ⩾ 0.8 of the disk.

A study of a complete soft X-ray selected sample of AGN

We present preliminary results from analyses of hard X-ray and optical observations of a soft X-ray selected sample. We created a small but complete sample with 20 of the softest and brightest objects with low Galactic absorption from the ROSAT bright soft X-ray selected radio-quiet AGN sample. This sample consists of 10 narrow-line Seyfert 1 galaxies and 10 broad-line Seyfert galaxies. We analyze ASCA data in the 0.6–10 keV band and optical spectra from ground-based telescopes. We investigate the photon indices in the hard X-ray band, soft excesses in the ASCA band, and optical emission line properties. The photon indices in the 2–10 keV band are nominal for both narrow-line Seyfert 1 galaxies and broad-line Seyfert 1 galaxies in each class compared with other heterogeneous samples. All of the narrow-line Seyfert 1 galaxies show soft excesses, but this component seems to be less significant for broad-line Seyfert 1 galaxies. There seems to be a trend of steeper X-ray spectra to be accompanied by narrower Hβ for narrow-line Seyfert 1 galaxies, but this is not extended to the larger velocity width regime of broad-line Seyfert 1 galaxies, and no clear trend is seen among them.

X-Ray Power Density Spectrum of the Narrow-Line Seyfert 1 Galaxy Arakelian 564

Beginning in 1999 January, the bright, strongly variable narrow-line Seyfert 1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ~4.3 days. It was also monitored every ~3.2 hr throughout 2000 July. These evenly sampled observations have allowed the first quantitative comparison of long- and short-timescale X-ray variability in an NLS1 galaxy and the derivation of an X-ray power density spectrum (PDS). The variability amplitude in the short-timescale light curve is very similar to that in the long-timescale light curve, in marked contrast to the stronger variability on longer timescales, which is characteristic of normal broad-line Seyfert 1 (BLS1) galaxies. Furthermore, the Akn 564 PDS power law cuts off at a frequency of 8.7 × 10-7 Hz, corresponding to a timescale of ~13 days, significantly shorter than that seen in the PDS of NGC 3516, a BLS1 galaxy of comparable luminosity. This result is consistent with NLS1 galaxies showing faster (as opposed to larger amplitude) variations than BLS1 galaxies, providing further evidence that NLS1 galaxies harbor lower mass black holes than BLS1 galaxies of similar luminosity, accreting at a correspondingly higher relative rate.

A Comprehensive Spectral and Variability Study of Narrow‐Line Seyfert 1 Galaxies Observed by ASCA . II. Spectral Analysis and Correlations

I present a comprehensive and uniform analysis of 25 ASCA observations from 23 narrow-line Seyfert 1 galaxies (NLS1s). The spectral analysis and correlations are presented in this paper; the reduction and time series analysis are presented in the companion paper. A maximum likelihood analysis confirms that the hard X-ray photon index is significantly steeper at more than 90% confidence in this sample of NLS1s compared with a random sample of Seyfert galaxies with broad optical lines. Soft excess emission was detected in 17 of the 19 objects that had no significant absorption, a result that demonstrates that soft excesses appear considerably more frequently in NLS1s than in Seyfert 1 galaxies with broad optical lines. The strength of the soft excess, parameterized using a flux ratio obtained from the blackbody plus power-law model, has a wide range of values in these objects (a factor of 50). The photon index was found to be correlated with the Hβ FWHM, despite the small range of the latter parameter; however, neither parameter is correlated with the strength of the soft excess or ROSAT slope. Therefore, assuming that an excess of soft photons causes the steep photon index and narrow Hβ FWHM, that excess may lie primarily in the unobservable EUV. The strength of the soft excess is correlated with the variability parameters, so that objects with strong soft excesses show higher amplitude variability; this potentially important result is not easily explained. While a range of 2 orders of magnitude in luminosity is represented, the temperature of the soft excess is approximately consistent throughout the sample, in contrast with expectations of simple accretion disk models. The presence of ionized absorption was sought using a two-edge model. It was found that this component appears to be typically less common in NLS1s and some evidence was found that the typical ionization state is lower compared with broad-line Seyfert galaxies. This fact, plus evidence for a correlation between the presence of the warm absorber and significant optical polarization, may imply that the inner warm absorber is missing or is too highly ionized to be detected, and only the outer, dusty, less ionized warm absorber is present in many cases. The iron line equivalent width appears to be similar among narrow and broad-line Seyfert galaxies. This could mean that reprocessing occurs with similar geometry in both classes of objects; however, the detection of ionized iron lines in a few objects, implying possibly altered fluorescence yields, and poor statistics, makes this conclusion tentative. Constraints on physical processes and models based on extreme values of orientation and accretion rate for NLS1s are examined in light of the observational results.

Arakelian 564: An Unusual Component in the X‐Ray Spectra of Narrow‐Line Seyfert 1 Galaxies

We present an ASCA observation of the narrow-line Seyfert 1 (NLSy1) galaxy Arakelian 564. The X-ray light curve shows rapid variability, but no evidence for energy dependence to these variations, within the 0.6-10 keV bandpass. A strong (EW ~ 70 eV) spectral feature is observed close to 1 keV. A similar feature has been observed in Ton S180, another member of the NLSy1 class of objects, but it has not been observed in broad-line Seyfert galaxies. The feature energy suggests a large contribution from Fe L-shell lines, but its intensity is difficult to explain in terms of emission and/or absorption from photoionized gas. The models that predict most emission at 1 keV are characterized by extreme values of column density, Fe abundance, and ionization parameter. Models based on gas in thermal equilibrium with kT ~ 1 keV provide an alternative parameterization of the soft spectrum. The latter may be interpreted as the hot intercloud medium, undergoing rapid cooling and producing strong Fe L-shell recombination lines. In all cases the physical conditions are rather different from those observed in broad-line Seyfert galaxies. The hard X-ray spectrum shows a broad and asymmetric Fe Kα line of large equivalent width (~550 eV), suggestive of significant emission from the inner accretion disk. The profile can be explained by a neutral disk viewed at ~60° to the line of sight, contrary to the hypothesis that NLSy1's are viewed pole-on. The large EW of this line, the strong 1 keV emission, and the strong optical Fe emission lines all suggest an extreme Fe abundance in this and perhaps other NLSy1's.

The extinction, flux distribution and luminosity of Seyfert 1 nuclei derived from UBV(RI)C aperture photometry

UBV(RI)c aperture photometry was carried out over a 4-yr period for 92 broad-line Seyfert galaxies. Where possible, the observations were repeated at different epochs in order to try to detect brightness variations. Significant variations were found in the majority of the objects observed at more than one epoch. Plots of the flux measurements through different bands against each other closely resemble linear relationships. Error contours of the data points in these plots are shown to be covariant ellipses. A statistical method is developed that enables one to obtain the best linear fit taking into account the error geometry peculiar to these data. The nuclear colours are determined for 50 significantly variable objects by calculating the best straight-line fits according to this statistical method. Chi-squared tests are used to show that, contrary to claims made in some recent studies, there is no significant deviation of the data from the straight-line fit in the vast majority of cases. This confirms the result of a previous paper that the shape of the nuclear flux distribution remains constant despite variations of the flux. The dereddened intrinsic colours for Seyfert nuclei are derived from the bluest nuclei, and these are then used to determine the nuclear reddening and extinction. It is shown that there are strong reasons to believe that the extinction values thus obtained are more accurate than those obtained though other methods, such as Balmer line ratio measurements. The relationship between galaxy inclination and nuclear extinction is found to be very poorly defined, showing that the extinction largely depends on amount of dust in the circumnuclear region rather than interstellar dust in the galaxy itself. Nuclear fluxes are determined after subtracting the background flux. These and the intrinsic colours are combined with infrared and ultraviolet data to construct the typical Seyfert nuclear flux distribution. The significance of the derived flux distribution for theoretical accretion disc models is discussed. Extinction corrected nuclear absolute magnitudes are then calculated and the effect of reddening and improved background subtraction on the Seyfert luminosity function determinations is briefly described.

Extreme spectral variations of the Seyfert galaxy Markarian 993

The nuclear spectrum of the broad-line Seyfert galaxy Mrk 993 was observed to display strong variations in the strengths of the broad Hα and Hβ emission lines, changing its classification from a Seyfert 1 to a Seyfert 1.9. Moderate to high resolution spectra were obtained with the IDS and CCD spectrograph on the 3 m Shane telescope at Lick Observatory in four different epochs spanning a period of approximately eight years. Using a spectral decomposition method which isolates the pure emission component of the active nucleus from the contaminating underlying host galaxy, our analysis indicates that the broad-line fluxes varied such that the Balmer decrement steepens whenever broad Hα and Hβ decrease in strength, and that the ionizing continuum correspondingly reddens in slope