T. J. Turner

ASCA observations of Seyfert 1 galaxies. 2. Relativistic iron K-alpha emission

We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA, 14 show an iron K? line that is resolved, with mean width ?K? = 0.43 ? 0.12 keV for a Gaussian profile (full width at half-maximum, FWHM ~ 50,000 km s-1). However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion disk models provide an excellent description of the data. The peak energy of the line is 6.4 keV, which indicates that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30?, consistent with orientation-dependent unification schemes. Differences in the line profiles from source to source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and nonrotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of ?3-10 = 1.91 ? 0.07 after accounting for the effects of reflection. Such observations probe the innermost regions of active galactic nuclei and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.

ASCA Observations of Seyfert 1 Galaxies. I. Data Analysis, Imaging, and Timing

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the active galactic nuclei. All but one of the Seyferts show evidence for variability on timescales of minutes to hours, with the amplitude anticorrelated with the source luminosity, confirming previous results. In at least eight sources there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power law in the soft band. Ultrarapid variability, implying significant power at frequencies greater than 10-3 Hz is detected in at least five sources but is difficult to detect in most cases, because of the sampling and signal-to-noise ratio. In Mrk 766 and MCG -6-30-15 there is also an indication that the high-frequency power spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151 but on longer timescales.

ASCA Observations of Seyfert 1 Galaxies. III. The Evidence for Absorption and Emission Due to Photoionized Gas

We present the results from a detailed analysis of the 0.6-10 keV spectra of 23 ASCA observations of 18 objects. We find that in most cases the underlying continuum can be well represented by a power law with a photon index Γ ~ 2. However, we find strong evidence for photoionized gas in the line of sight to 13/18 objects. We present detailed modeling of this gas based upon the ION photoionization code. Other studies have been made of the warm absorber phenomenon, but this paper contains the first consideration of the importance of the covering fraction of the ionized gas and a direct comparison between models of attenuation by ionized versus neutral material. We find the X-ray ionization parameter for the ionized material is strongly peaked at UX ~ 0.1. The column densities of ionized material are typically in the range NH, z ~ 1021-1023 cm-2, although highly ionized (and hence pseudotransparent) column densities up to 1024 cm-2 cannot be excluded in some cases. We also investigate the importance of the emission spectrum from the ionized gas, finding that it significantly improves the fit to many sources with an intensity consistent with material subtending a large solid angle at the central source. Allowing a fraction of the continuum to be observed without attenuation also improves the fit to many sources and is definitely required in the case of NGC 4151. A deficit of counts is observed at ~1 keV in the sources exhibiting the strongest absorption features. We suggest this is likely to be the signature of a second zone of (more highly) ionized gas, which might have been seen previously in the deep Fe K-shell edges observed in some Ginga observations. We find evidence that the ionized material in NGC 3227 and MCG -6-30-15 contains embedded dust, while there is no such evidence in the other sources We discuss these results in the context of previous studies and briefly explore the implications in other wave bands.

ASCA Observations of Type 2 Seyfert Galaxies. I. Data Analysis Results

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by ASCA. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the AGN. All but one of the Seyferts show evidence for variability on timescales of minutes-hours, with the amplitude anti-correlated with the source luminosity, confirming previous results. In at least 8 sources, there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power-law in the soft band. Ultra-rapid variability, implying significant power at frequencies > 10 −3 Hz is detected in at least 5 sources, but is difficult to detect in most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and MCG-6-30-15 there is also an indication that the high–frequency power–spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151, but on longer time scales.We present ASCA spatial, temporal, and spectral data for a sample of 26 observations of 25 type 2 active galactic nuclei (AGNs), composed of 17 Seyfert 2 galaxies and eight narrow emission line galaxies (NELGs). Twenty-four of the 25 sources were detected. The ASCA images are generally consistent with emission from point sources at energies above ~3 keV. We use archival ROSAT data to examine each field at high spatial resolution and to check for the presence of sources that would contaminate the ASCA data. Of the five sources bright enough for 128 s temporal analysis, three are variable at >99% confidence, with characteristics consistent with those observed in Seyfert 1 galaxies. Analysis on a timescale of 5760 s reveals six sources variable at >99% confidence, and comparison with previous X-ray results shows most of the sample to be variable in hard X-ray flux on timescales of years. Simple continuum models are fitted to the sample spectra to characterize the variety of spectral forms and hence to determine the fundamental nature of the X-ray spectrum of each source. No single spectral model provides an adequate fit to all the sample sources. Thirty-six percent of the sample cannot be adequately fitted by any of our test models (all rejected at >95% confidence). Approximately half of the sample have an iron Kα line with an equivalent width consistent with an origin in the line-of-sight absorber; the remaining lines must be produced in material out of the line of sight. Absorbing columns up to 1024 atoms cm-2 are detected, and even larger columns are inferred for some sources. The mean underlying hard X-ray power-law index is Γ ~ 2. Many X-ray emission lines were detected at high levels of confidence. The iron K-shell regime is dominated by emission from neutral material. Many data sets also show evidence for complexity in the iron Kα profile, which may be interpreted as evidence for broad-line profiles, including flux both redward and blueward of the line peak, and/or for the existence of hydrogen-like and helium-like iron K lines. Hydrogen-like and helium-like lines are detected from Fe, Ne, Si, S, and Ar in addition to Mg lines. While almost half of the sample have an estimated starburst contribution of >30% in the 0.5-4.5 keV bandpass, the soft X-ray emission lines are not solely associated with a strong starburst component.

ASCA Observations of Type 2 Seyfert Galaxies. II. The Importance of X-Ray Scattering and Reflection

We discuss the importance of X-ray scattering and Compton reflection in type 2 Seyfert Galaxies, based upon the analysis of ASCA observations of 25 such sources. Consideration of the iron Kα complex, [O III] line, and X-ray variability suggest that NGC 1068, NGC 4945, NGC 2992, Mrk 3, Mrk 463E, and Mrk 273 are dominated by reprocessed X-rays. We examine the properties of these sources in more detail. We find that the iron Kα complex contains significant contributions from neutral and high-ionization species of iron. Compton reflection, hot gas and starburst emission all appear to make significant contributions to the observed X-ray spectra. Mrk 3 is the only source in this subsample that does not have a significant starburst contamination. The ASCA spectrum below 3 keV is dominated by hot scattering gas with UX ~ 5, NH ~ 4 × 1023 cm-2. This material is more highly ionized than the zone of material comprising the warm absorber seen in Seyfert 1 galaxies, but may contain a contribution from shock-heated gas associated with the jet. Estimates of the X-ray scattering fraction cover 0.02%-5%. The spectrum above 3 keV appears to be dominated by a Compton reflection component, although there is evidence that the primary continuum component becomes visible close to ~10 keV.

On the Dependence of the Iron K-Line Profiles with Luminosity in Active Galactic Nuclei

We present evidence for changes in the strength and profile of the iron Kα line in active galactic nuclei (AGNs), based on X-ray observations with ASCA. There is a clear decrease in the strength of the line with increasing luminosity. This relation is not due solely to radio power, as it persists when only radio-quiet AGNs are considered and therefore cannot be fully explained by relativistic beaming. In addition to the change in strength, the line profile also appears to be different in higher luminosity sources. We discuss these results in terms of a model where the accretion disk becomes ionized as a function of the accretion rate.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XI. Intensive monitoring of the ultraviolet spectrum of NGC 7469

From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays, one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 23-31 for Lyα, 27 for C IV λ1549, 19-24 for N V λ1240, 17-18 for Si IV λ1400, and 07-10 for He II λ1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 A, the flux at 1485 A, 1740 A, and 1825 A lags with time delays of 021, 035, and 028, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of ~007 for the 1 σ uncertainties in the interband continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.

New constraints on the continuum emission mechanism of active galactic nuclei: Intensive monitoring of NGC 7469 in the x-ray and ultraviolet

We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC 7469 in the X-ray with RXTE over a ∼30 day baseline. The source shows strong variability with a root mean square (rms) amplitude of ∼16% and peak-to-peak variations of a factor of order 2. Simultaneous data over this period were obtained in the ultraviolet (UV) using IUE, making this the most intensive X-ray UV/X-ray variability campaign performed for any active galaxy. Comparison of the continuum light curves reveals very similar amplitudes of variability but different variability characteristics, with the X-rays showing much more rapid variations. The data are not strongly correlated at zero lag. The largest absolute value of the correlation coefficient occurs for an anticorrelation between the two bands, with the X-ray variations leading the UV by ∼4 days. The largest positive correlation is for the ultraviolet to lead the X-rays by ∼4 days. Neither option appears to be compatible with any simple interband transfer function. The peak positive correlation at ∼4 days occurs because the more prominent peaks in the UV light curve appear to lead those in the X-rays by this amount. However, the minima of the light curves are near simultaneous. These observations provide new constraints on theoretical models of the central regions of active galactic nuclei. Models in which the observed UV emission is produced solely by reradiation of absorber X-rays are ruled out by our data, as are those in which the X-rays are produced solely by Compton upscattering of the observed UV component by a constant distribution of particles. New or more complex models must be sought to explain the data. We require at least two variability mechanisms, which have no simple relationship. We briefly explore means by which these observations could be reconciled with theoretical models.

Steps toward Determination of the Size and Structure of the Broad-Line Region in Active Galactic Nuclei. XIII. Ultraviolet Observations of the Broad-Line Radio Galaxy 3C 390.3

As part of an extensive multiwavelength monitoring campaign, the International Ultraviolet Explorer satellite was used to observe the broad-line radio galaxy 3C 390.3 during the period 1994 December 31E1996 March 5. Spectra were obtained every 6E10 days. The UV continuum varied by a factor of 7 through the campaign, while the broad emission lines varied by factors of 2E5. Unlike previously moni- tored Seyfert 1 galaxies, in which the X-ray continuum generally varies with a larger amplitude than the UV, in 3C 390.3 the UV continuum light curve is similar in both amplitude and shape to the X-ray light curve observed by ROSAT . The UV broad emission-line variability lags that of the UV continuum by 35E70 days for Lya and C IV, values larger than those found for Seyfert 1 galaxies of comparable UV luminosity. These lags are also larger than those found for the Balmer lines in 3C 390.3 over the same period. The red and blue wings of C IV and Lya vary in phase, suggesting that radial motion does not dominate the kinematics of the UV line-emitting gas. Comparison with archival data provides evidence for velocity-dependent changes in the Lya and C IV line pro-les, indicating evolution in the detailed properties and/or distribution of the broad-line emitting gas. Subject headings: galaxies: active E galaxies: individual (3C 390.3) E ultraviolet: galaxies

Deconvolution of the X-Ray Emission and Absorption Components in Centaurus A

We present ROSAT HRI and PSPC plus ASCA X-ray observations of the radio galaxy Centaurus A. The HRI image reveals that the X-ray jet has remained constant in flux since the Einstein observation 15 years ago, while the nuclear flux has decreased by ~60%. The PSPC data allows the first detailed spatially resolved X-ray spectroscopy of the nucleus, jet, and diffuse emission in Cen A. The 0.1-2 keV spectrum of the nucleus is heavily absorbed, as expected. The jet can be described by a power law of photon index Γ ~ 2.3, although thermal models cannot be ruled out. The presence of faint diffuse emission is confirmed, extending out to ~6 kpc from the nucleus. The spectrum of the diffuse emission can be modeled as two Raymond-Smith plasmas. A low-temperature component, kT = 0.29 keV, is consistent with emission from gas heated by stars, while the harder component may represent the contribution of galactic X-ray binary systems; the diffuse emission increases in intensity close to the galactic disk. The identification of these extended components makes it possible to separate the nuclear and diffuse contributions to the ASCA spectrum. The 0.6-10 keV nuclear spectrum is well modeled by a power-law continuum of photon index Γ ~ 1.96, in good agreement with the canonical value for Seyfert 1 galaxies. Forty percent of the nuclear continuum is absorbed by a column density ~4 × 1023 cm-2, 59% is absorbed by a column ~1 × 1023 cm-2, and 1% is absorbed by a value close to the 21 cm Galactic line-of-sight column. The high degree of nuclear absorption is consistent with the circumnuclear molecular clouds whose existence is suggested by absorption measurements in a number of wave bands. The iron Kα emission line observed appears broad; the best-fit Gaussian width implies an origin in material with velocities ~11,000 km s-1, while the equivalent width is 114 ± 18 eV. Both quantities are in good agreement with an origin in the circumnuclear absorbing material.