I. M. George

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.

An XMM–Newton survey of broad iron lines in Seyfert galaxies

We present an analysis of the X-ray spectra of a sample of 37 observations of 26 Seyfert galaxies observed by XMM–Newton in order to characterize their iron Kα emission. All objects show evidence for iron line emission in the 6–7 keV band. A narrow ‘core’ at 6.4 keV is seen almost universally in the spectra, and we model this using a neutral Compton reflection component, assumed to be associated with distant, optically thick material such as the molecular torus. Once this, and absorption by a zone of ionized gas in the line of sight is accounted for, less than half of the sample observations show an acceptable fit. Approximately two-thirds of the sample shows evidence for further, broadened emission in the iron K band. When modelled with a Gaussian, the inferred energy is close to that expected for neutral iron, with a slight redshift, and an average velocity width of ∼0.1c. The mean parameters are consistent with previous ASCA results and support the idea that the broad components can be associated with the accretion disc. Before proceeding to that conclusion, we test an alternative model comprising a blend of three to four narrow, unshifted emission lines (including the 6.4-keV core), together with one to two zones of highly ionized gas in the line of sight. Around one-third of the objects are not adequately fitted by this model, and in general better fits are obtained with a relativistic disc line model, which has fewer free parameters. None the less we find that absorption by ionized gas affects the spectrum above 2.5 keV in approximately half the sample. There is evidence for multiple ionized zones in at least three objects, but in all those cases a blurred reflector is required in addition to the complex absorption. We also identify a number of narrow emission and absorption features around the Fe complex, and the significance and interpretation of these lines is discussed. After accounting for these additional complexities, we determine the typical parameters for the broad reflection. The emission is found to come, on average, from a characteristic radius ∼15 rg and the average disc inclination is ∼ 40°. The broad reflection is on average significantly weaker, by a factor of ∼2, than that expected from a flat disc illuminated by a point source. Notwithstanding these average properties, the objects exhibit a significant and wide range of reflection parameters. We find that 30 per cent of the sample observations can be explained solely with narrow-line components, with no evidence for broadened emission at all. A further 25 per cent show evidence for significant broad emission, but at a characteristic radius relatively far from the black hole. The remaining ∼45 per cent are best fitted with a relativistically blurred reflection model. In 12/37 observations the characteristic emission radius is constrained to be <50 rg, where the gravitational redshift is measurable. For at least this subsample, our observations verify the potential for X-ray spectroscopy to diagnose the strong-gravity regime of supermassive black holes.

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 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.

X-Ray Observations of Optically Selected, Radio-quiet Quasars. I. The ASCA Results

We present the result of 27 ASCA observations of 26 radio-quiet quasars (RQQs) from the Palomar-Green (PG) survey. The sample is not statistically complete, but it is reasonably representative of RQQs in the PG survey. For many of the sources, the ASCA data are presented here for the first time. All the RQQs were detected except for two objects, both of which contain broad absorption lines in the optical band. We find the variability characteristics of the sources to be consistent with Seyfert 1 galaxies. A power law offers an acceptable description of the time-averaged spectra in the 2-10 keV (quasar frame) band for all but one data set. The best-fitting values of the photon index vary from object to object over the range 1.5 ?2-10 3, with a mean ?2-10 2 and dispersion ?(?2-10) 0.25. The distribution of ?2-10 is therefore similar to that observed in other RQ active galactic nuclei (AGNs) and seems to be unrelated to X-ray luminosity. No single model adequately describes the full 0.6-10 keV (observed frame) continuum of all the RQQs. Approximately 50% of the sources can be adequately described by a single power law or by a power law with only very subtle deviations. All but one of the remaining data sets were found to have convex spectra (flattening as one moves to higher energies). The exception is PG 1411+442, in which a substantial column density (NH,z ~ 2 ? 1023 cm-2) obscures ~98% of the continuum. We find only five (maybe six) of 14 objects with z 0.25 to have soft excesses at energies 1 keV, but we find no universal shape for these spectral components. The spectrum of PG 1244+026 contains a rather narrow emission feature centered at an energy ~1 keV (quasar frame). The detection rate of absorption due to ionized material in these RQQs is lower than that seen in Seyfert 1 galaxies. In part, this may be due to selection effects. However, when detected, the absorbers in the RQQs exhibit a similar range of column density and ionization parameter as Seyfert 1 galaxies. We find evidence of Fe K-shell emission in at least eight RQQs. These are all low-luminosity objects, and the line parameters are consistent with those of other low-luminosity RQ AGNs. However the construction of the mean data/model ratios for various luminosity ranges reveals a trend whereby the profile and strength of the Fe K-shell emission changes as a function of luminosity.

On x-ray variability in seyfert galaxies

This paper presents a quantification of the X-ray variability amplitude for 79 ASCA observations of 36 Seyfert 1 galaxies. We find that consideration of sources with the narrowest permitted lines in the optical band introduces scatter into the established correlation between X-ray variability and nuclear luminosity. Consideration of the X-ray spectral index and variability properties together shows distinct groupings in parameter space for broad- and narrow-line Seyfert 1 galaxies, confirming previous studies. A strong correlation is found between hard X-ray variability and FWHM Hβ. A range of nuclear mass and accretion rate across the Seyfert population can explain the differences observed in X-ray and optical properties. An attractive alternative model, which does not depend on any systematic difference in central mass, is that the circumnuclear gas of narrow-line Seyfert 1 galaxies is different from broad-line Seyfert 1 galaxies in temperature, optical depth, density, or geometry.

Physical Diagnostics from a Narrow Fe Kα Emission Line Detected by Chandra in the Seyfert 1 Galaxy NGC 5548

We report the detection of a narrow Fe K? emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 keV, a FWHM of 4515 km s-1, an intensity of 3.6 ? 10-5 photons cm-2 s-1, with an equivalent width of 133 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but, assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light month from the central X-ray source. We cannot rule out some contribution to the narrow Fe K? line from a putative, parsec-scale, obscuring torus that is postulated to be a key component of active galactic nuclei (AGNs) unification models. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe K? intensity had not yet responded to such a change, then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe K? line profile in terms of a rotating disk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple disk model, even when the narrow-line component is accounted for. It will be critical to ascertain the importance of a similar nondisk Fe K? line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe K? component with large collecting area and high spectral resolution will enable reverberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.

High-Resolution X-Ray Spectroscopy and Modeling of the Absorbing and Emitting Outflow in NGC 3783

The high-resolution X-ray spectrum of NGC 3783 shows several dozen absorption lines and a few emission lines from the H-like and He-like ions of O, Ne, Mg, Si, and S, as well as from Fe XVII-Fe XXIII L-shell transitions. We have reanalyzed the Chandra HETGS spectrum using better flux and wavelength calibrations, along with more robust methods. Combining several lines from each element, we clearly demonstrate the existence of the absorption lines and determine that they are blueshifted relative to the systemic velocity by -610 ? 130 km s-1. We find the Ne absorption lines in the High-Energy Grating spectrum to be resolved with FWHM = 840 km s-1; no other lines are resolved. The emission lines are consistent with being at the systemic velocity. We have used regions in the spectrum where no lines are expected to determine the X-ray continuum, and we model the absorption and emission lines using photoionized-plasma calculations. The model consists of two absorption components, with different covering factors, which have an order-of-magnitude difference in their ionization parameters. The two components are spherically outflowing from the active galactic nucleus, and thus contribute to both the absorption and the emission via P Cygni profiles. The model also clearly requires O VII and O VIII absorption edges. The low-ionization component of our model can plausibly produce UV absorption lines with equivalent widths consistent with those observed from NGC 3783. However, we note that this result is highly sensitive to the unobservable UV to X-ray continuum, and the available UV and X-ray observations cannot firmly establish the relationship between the UV and X-ray absorbers. We find good agreement between the Chandra spectrum and simultaneous ASCA and RXTE observations. The 1 keV deficit previously found when modeling ASCA data probably arises from iron L-shell absorption lines not included in previous models. We also set an upper limit on the FWHM of the narrow Fe K? emission line of 3250 km s-1. This is consistent with this line originating outside the broad-line region, possibly from a torus.

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.

The XMM-Newton Iron Line Profile of NGC 3783

We report on observations of the iron K line in the nearby Seyfert 1 galaxy, NGC 3783, obtained in a long, two- orbit (� 240 ks) XMM-Newton observation. The line profile obtained exhibits two strong narrow peaks at 6.4 and 7.0 keV, with measured line equivalent widths of 120 and 35 eV, respectively. The 6.4 keVemission is the Kline from near neutral Fe, while the 7.0 keV feature probably originates from a blend of the neutral Fe Kline and the hydrogen-like line of Fe at 6.97 keV. The relatively narrow velocity width of the Kline (P5000 km s � 1 ), its lack of response to the continuum emission on short timescales, and the detection of a neutral Compton reflection component are all consistent with a distant origin in Compton-thick matter such as the putative molecular torus. A strong absorption line from highly ionized iron (at 6.67 keV) is detected in the time-averaged iron line profile, while the depth of the feature appears to vary with time, being strongest when the continuum flux is higher. The iron absorption line probably arises from the highest ionization component of the known warm absorber in NGC 3783, with an ionization of log � � 3 and column density of NH � 5 � 10 22 cm � 2 and may originate from within 0.1 pc of the nucleus. Aweak red wing to the iron K line profile is also detected below 6.4 keV. However, when the effect of the highly ionized warm absorber on the underlying continuum is taken into account, the requirement for a relativistic iron line component from the inner disk is reduced. Subject headings: galaxies: active — galaxies: individual (NGC 3783) — galaxies: Seyfert — X-rays: galaxies