I. M. George

We report the detection of redshifted iron Kα absorption lines in the Chandra LETG spectrum of the narrow-line quasar PG 1211+143. The absorption lines are observed at 4.22 and 4.93 keV in the quasar spectrum, corresponding to 4.56 and 5.33 keV in the rest frame of PG 1211+143. From Monte Carlo simulations, the chance probability of both lines being false detections is low at 1.36 × 10^-4. Highly redshifted ionized iron Kα (Fe XXV or Fe XXVI) is the most plausible identification for the lines at their observed energies. If identified with H-like iron Kα at 6.97 keV, then the relativistic velocity shifts required are 0.40c and 0.26c. The extreme velocities can be explained by pure gravitational redshift if the matter exists in a stable orbit within 6 gravitational radii of the black hole. This would require a Kerr metric for the black hole. Alternatively, the absorption may be the result of matter infalling directly onto the black hole, with a maximum observed velocity of 0.38c at 6R_g in the Schwarzschild metric. This matter may originate in a failed outflow or jet, which does not escape the gravitational potential of the black hole.

We report the detection of redshifted iron K-alpha absorption lines in the Chandra LETG spectrum of the narrow-line quasar, PG 1211+143. The absorption lines are observed at 4.22 keV and 4.93 keV in the quasar spectrum, corresponding to 4.56 keV and 5.33 keV in the rest frame of PG 1211+143. From Monte Carlo simulations, the chance probability of both lines being false detections is low at 1.36e-04. Highly redshifted ionized iron K-alpha (Fe XXV or Fe XXVI) is the most plausible identification for the lines at their observed energies. If identified with H-like iron K-alpha at 6.97 keV, then the relativistic velocity shifts required are 0.40c and 0.26c. The extreme velocities can be explained by pure gravitational redshift if the matter exists in a stable orbit within 6 gravitational radii of the black hole. This would require a Kerr metric for the black hole. Alternatively the absorption may be the result of matter infalling directly onto the black hole, with a maximum observed velocity of 0.38c at 6Rg in the Schwarzschild metric. This matter may originate in a failed outflow or jet, which does not escape the gravitational potential of the black hole.

We present a new XMM-Newton observation of the ultraluminous infrared galaxy (ULIRG) NGC 6240. We analyze the reflecting grating spectrometer (RGS) data, as well as data from the other instruments, and find a starburst-dominated 0.5-3 keV spectrum with global properties resembling those observed in M82 but with a much higher luminosity. We show that the starburst region can be divided into an outer zone, beyond a radius of about 2.1 kpc, with a gas temperature of about 10⁷ K, and a central region with temperatures in the range (2-6) × 10⁷ K. The gas in the outer region emits most of the observed O VIII Lyα line, and the gas in the inner region the emission lines of higher ionization ions, including a strong Fe XXV line. We also identify a small inner part, very close to the active nuclei, with typical Seyfert 2 properties, including a large amount of photoionized gas producing a strong Fe Kα 6.4 keV line. The combined abundance, temperature, and emission measure analysis indicates supersolar Ne/O, Mg/O, Si/O, S/O, and possibly also Fe/O. The analysis suggests densities in the range of (0.07-0.28)^-1/2 cm^-3 and a total thermal gas mass of ~4 × 10^81/2 M_☉, where is the volume filling factor. We used a simple model to argue that a massive starburst with an age of 2 × 10⁷ yr can explain most of the observed properties of the source. NGC 6240 is perhaps the clearest case of an X-ray-bright luminous AGN that is in a merger and whose soft X-ray spectrum is dominated by a powerful starburst.

We present the intrinsic spectral energy distribution (SED) of the narrow-line Seyfert 1 galaxy (NLS1) Ark 564, constructed with contemporaneous data obtained during a multiwavelength, multisatellite observing campaign in 2000 and 2001. We compare this SED with that of the NLS1 Ton S180 and with those obtained for broad-line Seyfert 1 galaxies to infer how the relative accretion rates vary among the Seyfert 1 population. Although the peak of the SED is not well constrained, in our parameterization most of the energy of this object is emitted in the 10-100 eV regime, constituting roughly half of the emitted energy in the optical/X-ray ranges. This is consistent with a primary spectral component peaking in the extreme-UV/soft X-ray band and with disk-corona models, hence high accretion rates. Indeed, we estimate that ≈ 1. We also address the issue of the energy budget in this source by examining the emission lines observed in its spectrum, and we constrain the physical properties of the line-emitting gas through photoionization modeling. The available data suggest that the line-emitting gas is characterized by log n ≈ 11 and log U ≈ 0 and is stratified around log U ≈ 0. Our estimate of the radius of the Hβ-emitting region R ≈ 10 ± 2 lt-days is consistent with the R-luminosity relationships found for Seyfert 1 galaxies, which indicates that the narrowness of the emission lines in this NLS1 is not due to the broad-line region being relatively farther away from the central mass than in broad-line Seyfert 1 galaxies of comparable luminosity. We also find evidence for supersolar metallicity in this NLS1. We show that the emission lines are not good diagnostics for the underlying SEDs and that the absorption line studies offer a far more powerful tool to determine the ionizing continuum of active galactic nuclei, especially if one is comparing the lower and higher ionization lines.

We present Reflection Grating Spectrometer data from an XMM-Newton observation of the Seyfert 1 galaxy NGC 3516, taken while the continuum source was in an extremely low flux state. This observation offers a rare opportunity for a detailed study of emission from a Seyfert 1 galaxy, as these are usually dominated by high nuclear continuum levels and heavy absorption. The spectrum shows numerous narrow emission lines (FWHM 1300 km s^-1) in the 0.3-2 keV range, including the H-like lines of C, N, and O and the He-like lines of N, O, and Ne. The emission-line ratios and the narrow width of the radiative recombination continuum of C VI indicate that the gas is photoionized and of fairly low temperature (kT 0.01 keV). The availability of emission lines from different elements for two isoelectronic sequences allows us to constrain the element abundances. These data show that the N lines are far stronger than would be expected from gas of solar abundances. Based on our photoionization models, we find that nitrogen is overabundant in the central regions of the galaxy, compared to carbon, oxygen, and neon, by at least a factor of 2.5. We suggest that this is the result of secondary production of nitrogen in intermediate-mass stars and indicative of the history of star formation in NGC 3516.

We present observations of the UV absorption lines in the luminous Seyfert 1 galaxy Mrk 509, obtained with the medium-resolution (λ/Δλ ≈ 40,000) echelle gratings of the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The spectra reveal the presence of eight kinematic components of absorption in Lyα, C IV, and N V, at radial velocities of -422, -328, -259, -62, -22, +34, +124, and +210 km s^-1 with respect to an emission-line redshift of z = 0.03440, seven of which were detected in an earlier Far Ultraviolet Spectrographic Explorer (FUSE) spectrum. The component at -22 km s^-1 also shows absorption by Si IV. The covering factor and velocity width of the Si IV lines were lower than those of the higher ionization lines for this component, which is evidence for two separate absorbers at this velocity. We have calculated photoionization models to match the UV column densities in each of these components. Using the predicted O VI column densities, we were able to match the O VI profiles observed in the FUSE spectrum. Based on our results, none of the UV absorbers can produce the X-ray absorption seen in simultaneous Chandra observations; therefore, there must be more highly ionized gas in the radial velocity ranges covered by the UV kinematic components.

We present the first high-resolution, X-ray image of the circumnuclear regions of the Seyfert 1 galaxy NGC 3516, using the Chandra X-ray Observatory (CXO). All three of the CXO observations reported were performed with one of the two grating assemblies in place, and here we restrict our analysis to undispersed photons (i.e., those detected in the zeroth-order). A previously unknown X-ray source is detected ~6'' (1.1 h kpc) NNE of the nucleus (position angle ~29°) which we designate CXOU 110648.1+723412. Its spectrum can be characterized as a power law with a photon index Γ ~ 1.8-2.6, or as thermal emission with a temperature kT ~ 0.7-3 keV. Assuming a location within NGC 3516, isotropic emission implies a luminosity L ~ 2-8 × 10³⁹ h ergs s^-1 in the 0.4-2 keV band. If due to a single point source, the object is super-Eddington for a 1.4 M_☉ neutron star. However, multiple sources or a small, extended source cannot be excluded using the current data. Large-scale extended X-ray emission is also detected out to ~10'' (~1.7 h kpc) from the nucleus to the NE and SW and is approximately aligned with the morphologies of the radio emission and extended narrow emission line region (ENLR). The mean luminosity of this emission is 1-5 × 10³⁷ h ergs s^-1 arcsec^-2, in the 0.4-2 keV band. Unfortunately, the current data cannot usefully constrain its spectrum. These results are consistent with earlier suggestions of circumnuclear X-ray emission in NGC 3516. If the extended emission is due to scattering of the nuclear X-ray continuum, then the pressure in the X-ray-emitting gas is at least 2 orders of magnitude too small to provide the confining medium for the ENLR clouds.

We present spectral results from a multisatellite, broadband campaign on the narrow-line Seyfert 1 galaxy Ton S180 (PHL 912) performed at the end of 1999. We discuss the spectral energy distribution (SED) of the source, combining simultaneous Chandra, ASCA, and Extreme Ultraviolet Explorer data with contemporaneous Far Ultraviolet Spectroscopic Explorer (FUSE), Hubble Space Telescope, and ground-based optical and infrared data. The resulting SED shows that most of the energy is emitted in the 10-100 eV regime, which must be dominated by the primary energy source. No spectral turnover is evident in the UV regime. This, the strong soft X-ray emission, and the overall shape of the SED indicate that emission from the accretion disk peaks between 15 and 100 eV. High-resolution FUSE spectra showing UV absorption due to O VI and the lack of detectable X-ray absorption in the Chandra spectrum demonstrate the presence of a low column density of highly ionized gas along our line of sight. The highly ionized state of the circumnuclear gas is most likely linked to the high luminosity and steep spectrum of the active nucleus. Given the strong ionizing flux in Ton S180, it is possible that the clouds within a few tens of light days of the central source are too highly ionized to produce much line emission. Thus, the narrow width of the emission lines in Ton S180 is due to the emission arising from large radii.

We use Hubble Space Telescope UV and optical spectra of the narrow-line Seyfert 1 (NLS1) galaxy Ark 564 to investigate its internal reddening and properties of its emission-line and intrinsic UV absorption gas. We find that the extinction curve of Ark 564, derived from a comparison of its UV/optical continuum to that of an unreddened NLS1, lacks a 2200 Å bump and turns up toward the UV at a longer wavelength (4000 Å) than the standard Galactic, LMC, and SMC curves. However, it does not show the extremely steep rise to 1200 Å that characterizes the extinction curve of the Seyfert 1 galaxy NGC 3227. The emission lines and continuum experience the same amount of reddening, indicating the presence of a dust screen that is external to the narrow-line region. Echelle spectra from the Space Telescope Imaging Spectrograph show intrinsic UV absorption lines due to Lyα, N V, C IV, Si IV, and Si III, centered at a radial velocity of -190 km s^-1 (relative to the host galaxy). Photoionization models of the UV absorber indicate that it has a sufficient column (N_H = 1.6 × 10²¹ cm^-2) and is at a sufficient distance from the nucleus (D > 95 pc) to be the source of the dust screen. Thus, Ark 564 contains a dusty "lukewarm absorber" similar to that seen in NGC 3227.

We present the results from ASCA observations of NGC 3227 performed during 1993 and 1995, along with those from a ROSAT observation performed pseudosimultaneously with the former. We find the 0.6-10 keV continuum to be consistent with a power law with a photon index Γ ~ 1.6, flatter than that typically observed in Seyfert galaxies confirming previous results. Significant Fe K-shell emission is observed during both epochs, with an equivalent width and profile typical of Seyfert 1 galaxies. The ASCA observations in 1993 reveal absorption by a screen N 3×10²¹ cm^-2 of ionized material with an X-ray ionization parameter U_X 0.01. Both the column and ionization state of this material are at the low end of the distribution of parameters observed for Seyfert 1 galaxies. Joint analysis of the ASCA and ROSAT data at this epoch shows an additional screen of neutral material instrinsic to NGC 3227 with N~few×10²⁰ cm^-2. We find NGC 3227 to exhibit significant spectral variability both within and between the observations. The most likely explanation involves short-term variability in the continuum emission and longer term variability in the column density of the ionized material. Time-resolved spectroscopy and color-color analysis indicate that the slope of the continuum steepened by ΔΓ 0.1 during a flare of duration ~10⁴ s, within the 1993 observation. However, we were unable to distinguish between a steepening of the "primary" continuum and a change in the relative strengths of the power law and a putative Compton-reflection component. The absorbing column increased by a factor of ~10 by the 1995 epoch, while the continuum is consistent with that observed in 1993. The 1995 data also show evidence that the warm absorber allows ~10% of the nuclear emission to escape without attenuation. We review our findings in the context of the previous results from this and similar objects and discuss the prospects of future observations.