X-ray Warm Absorber Research Articles

Multiwavelength studies of the Seyfert 1 galaxy NGC 7469 II. X-ray and UV observations with XMM-Newton

We present an XMM-Newton observation of NGC 7469, including studies of the X-ray and UV variability, 0.2-10 keV spectral continuum, Fe K-alpha emission line and the first-ever high-resolution X-ray spectrum of the soft X-ray warm absorber. We compare the properties of this X-ray warm absorber with the UV warm absorber as seen in a FUSE observation one year previously. The 0.2-10 keV spectral continuum is best fitted by a power-law plus two blackbody model. An Fe K-alpha emission line is visible which consists of a single narrow component and is well-modelled by a simple gaussian. Narrow absorption and emission lines in the soft X-ray RGS spectrum demonstrate the existence of a multi-phase warm absorber with a range in log xi of ~ 2 to ~ -2 where xi is in erg cm s^-1. The warm absorber is blueshifted by several hundred km s^-1. The highest-ionisation phase of the absorber is the best constrained and has an overall equivalent Hydrogen column of order 10^20 cm^-2; we find that its ionisation parameter is consistent with that of the warm emitter which generates the narrow emission lines. We identify this high ionisation absorber with the low-velocity phase of the UV absorber observed by FUSE.

Multi-wavelength study of the Seyfert 1 galaxy NGC 3783 with XMM-Newton

We present the analysis of multi-wavelength XMM-Newton data from the Seyfert galaxy NGC 3783, including UV imaging, X-ray and UV lightcurves, the 0.2-10 keV X-ray continuum, the iron K-alpha emission line, and high-resolution spectroscopy and modelling of the soft X-ray warm absorber. The 0.2-10 keV spectral continuum can be well reproduced by a power-law at higher energies; we detect a prominent Fe K-alpha emission line, with both broad and narrow components, and a weaker emission line at 6.9 keV which is probably a combination of Fe K-beta and Fe XXVI. We interpret the significant deficit of counts in the soft X-ray region as being due to absorption by ionised gas in the line of sight. This is demonstrated by the large number of narrow absorption lines in the RGS spectrum from iron, oxygen, nitrogen, carbon, neon, argon, magnesium, silicon and sulphur. The wide range of iron states present in the spectrum enables us to deduce the ionisation structure of the absorbing medium. We find that our spectrum contains evidence of absorption by at least two phases of gas: a hotter phase containing plasma with a log ionisation parameter xi (where xi is in erg cm/s) of 2.4 and greater, and a cooler phase with log xi centred around 0.3. The gas in both phases is outflowing at speeds of around 800 km/s. The main spectral signature of the cold phase is the Unresolved Transition Array (UTA) of M-shell iron, which is the deepest yet observed; its depth requires either that the abundance of iron, in the cold phase, is several times that of oxygen, with respect to solar abundances, or that the absorption lines associated with this phase are highly saturated. The cold phase is associated with ionisation states that would also absorb in the UV.

Associated Absorption Lines in the Radio‐loud Quasar 3C 351: Far‐Ultraviolet Echelle Spectroscopy from theHubble Space Telescope

As one of the most luminous radio-loud quasars showing intrinsic ultraviolet (UV) and X-ray absorption, 3C 351 provides a laboratory for studying the kinematics and physical conditions of such ionized absorbers. We present an analysis of the intrinsic absorption lines in the high-resolution (~7 km s-1) far-UV spectrum that was obtained from observations with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The spectrum spans wavelengths from 1150 to 1710 A and shows strong emission lines from O VI and Lyα. Associated absorption lines are present on the blue wings of the high-ionization emission doublets O VI λλ1032, 1038 and N V λλ1238, 1242, as well as the Lyman lines through Ly. These intrinsic absorption features are resolved into several distinct kinematic components, covering rest-frame velocities from -40 to -2800 km s-1, with respect to the systemic redshift of zem = 0.3721. For the majority of these absorption-line regions, strong evidence of partial covering of both the background continuum source and the broad emission line region is found, which supports the intrinsic absorption origin and rules out the possibility that the absorption arises in some associated cluster of galaxies. The relationship between the far-UV absorbers and X-ray warm absorbers is studied with the assistance of photoionization models. Most of the UV-associated absorption components have low values of the ionization parameter and total hydrogen column densities, which is inconsistent with previous claims that the UV and X-ray absorption arises in the same material. Analysis of these components supports a picture with a wide range of ionization parameters, temperatures, and column densities in active galactic nucleus outflows.

Discovery of Associated Absorption Lines in an X‐Ray Warm Absorber:Hubble Space TelescopeFaint Object Spectrograph Observations of MR 2251−178

The presence of a 'warm absorber' was first suggested to explain spectral variability in an X-ray spectrum of the radio-quiet quasi-stellar object (QSO) MR 2251-178. A unified picture, in which X-ray warm absorbers and 'intrinsic' UV absorbers are the same, offers the opportunity to probe the nuclear environment of active galactic nuclei. To test this scenario and understand the physical properties of the absorber, we obtained a UV spectrum of MR 2251-178 with the Faint Object Spectrograph on board the Hubble Space Telescope (HST). The HST spectrum clearly shows absorption due to Lyalpha, N v, and C IV, blueshifted by 300 km s(exp -1) from the emission redshift of the QSO. The rarity of both X-ray and UV absorbers in radio-quiet QSOs suggests these absorbers are physically related, if not identical. Assuming the unified scenario, we place constraints on the physical parameters of the absorber and conclude the mass outflow rate is essentially the same as the accretion rate in MR 2251-178.

A High Signal‐to‐Noise Ultraviolet Spectrum of NGC 7469: New Support for Reprocessing of Continuum Radiation

From 1996 June 10 to 1996 July 29, the International AGN Watch monitored the Seyfert 1 galaxy NGC 7469 using the International Ultraviolet Explorer, the Rossi X-Ray Timing Explorer, and a network of ground-based observatories. On 1996 June 18, in the midst of this intensive monitoring period, we obtained a high signal-to-noise snapshot of the UV spectrum from 1150 to 3300 Å using the Faint Object Spectrograph (FOS) on the Hubble Space Telescope. This spectrum allows us to disentangle the UV continuum more accurately from the broad wings of the emission lines, to identify clean continuum windows free of contaminating emission and absorption, and to deblend line complexes such as Lyα + N V, C IV + He II + O III], Si III] + C III], and Mg II + Fe II. Using the FOS spectrum as a template, we have fitted and extracted line and continuum fluxes from the IUE monitoring data. The cleaner continuum extractions confirm the discovery of time delays between the different UV continuum bands by Wanders et al. Our new measurements show delays increasing with wavelength for continuum bands centered at 1485, 1740, and 1825 Å relative to 1315 Å with delays of 0.09, 0.28, and 0.36 days, respectively. Like many other Seyfert 1 galaxies, the UV spectrum of NGC 7469 shows intrinsic, blueshifted absorption in Lyα, N V, and C IV. Soft X-ray absorption is also visible in archival ASCA X-ray spectra. The strength of the UV absorption, however, is not compatible with a single-zone model in which the same material absorbs both the UV and X-ray light. Similar to other Seyfert galaxies, such as NGC 3516, the UV-absorbing gas in NGC 7469 has a lower ionization parameter and column density than the X-ray-absorbing material. While the UV and X-ray absorption does not arise in the same material, the frequent occurrence of both associated UV absorption and X-ray warm absorbers in the same galaxies suggests that the gas supply for each has a common origin.

On the Nature of Soft X‐Ray Weak Quasi‐stellar Objects

Recent studies of quasi-stellar objects (QSOs) with ROSAT suggest the existence of a significant population of soft X-ray weak QSOs (SXW QSOs) where the soft X-ray flux is ~10-30 times smaller than in typical QSOs. Why are these QSOs soft X-ray weak, and what is their relationship to broad absorption line QSOs (BAL QSOs) and X-ray warm absorber QSOs? As a first step in a systematic study of these objects, we establish a well-defined sample of SXW QSOs that includes all αox ≤ -2 QSOs from the Boroson & Green sample of 87 Bright Quasar Survey QSOs. SXW QSOs comprise ≈11% of this optically selected QSO sample, and we find soft X-ray weakness in both radio-quiet and radio-loud QSOs. From an analysis of C IV absorption in the 55 Boroson & Green QSOs with available C IV data, we find a remarkably strong correlation between αox and the C IV absorption equivalent width. This correlation suggests that absorption is the primary cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs, and BAL QSOs. Many of our SXW QSOs have ultraviolet absorption that is intermediate in strength between that of X-ray warm absorber QSOs and that of BAL QSOs, and their X-ray absorption is also likely to be of intermediate strength. From a practical point of view, our correlation demonstrates that selection by soft X-ray weakness is an effective (≳80% successful) and observationally inexpensive way to find low-redshift QSOs with strong and interesting ultraviolet absorption.

Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from theHubble Space Telescope

We present a study of the intrinsic absorption lines in the ultraviolet spectra of Seyfert 1 galaxies. The study is based on spectra from the Hubble Space Telescope and includes the Seyfert 1 galaxies observed with the Faint Object Spectrograph and Goddard High-Resolution Spectrograph at spectral resolutions of λ/Δλ ≈1000-20,000 with good signal-to-noise ratios. We find that the fraction of Seyfert 1 galaxies that show intrinsic absorption associated with their active nuclei is more than one-half (10 of 17), which is much higher than previous estimates (3%-10%) based on IUE data. There is a one-to-one correspondence between Seyfert galaxies that show intrinsic UV absorption and X-ray warm absorbers, indicating that these two phenomena are related. Although our sample is not complete, we conclude that intrinsic absorption represents an important component that needs to be integrated into our overall physical picture of active galaxies. The intrinsic UV absorption is generally characterized by high ionization: C IV and N V are seen in all 10 Seyfert galaxies with detected absorption (in addition to Lyα), whereas Si IV is present in only four of these Seyfert galaxies, and Mg II absorption is detected only in NGC 4151. The absorption lines are blueshifted (or in a few cases at rest) with respect to the narrow emission lines, indicating that the absorbing gas is undergoing net radial outflow. At high resolution, the absorption often splits into distinct kinematic components that show a wide range in widths (20-400 km s-1 FWHM), indicating macroscopic motions (e.g., radial velocity subcomponents or turbulence) within a component. The strong absorption components have cores that are much deeper than the continuum flux levels, indicating that the regions responsible for these components lie completely outside of the broad emission-line regions. Additional information on the covering factors and column densities can be derived from the absorption profiles in the high-resolution spectra. The covering factor of the absorbing gas in the line of sight, relative to the total underlying emission, is Clos ≥ 0.86, on average. The global covering factor, which is the fraction of emission intercepted by the absorber averaged over all lines of sight, is Cglobal ≥ 0.5. Thus, structures covering large solid angles as seen by the central continuum source (e.g., spherical shells, sheets, or cones with large opening angles) are required. The individual absorptioncomponents show a wide range in C IV column densities (0.1-14 × 1014 cm-2), and the ratio of N V to C IV column density varies significantly from one absorption component to the next, even in the same Seyfert galaxy. Thus, the intrinsic absorption in a Seyfert 1 galaxy is typically comprised of distinct kinematic components that are characterized by a range in physical conditions (e.g., ionization parameter and hydrogen column density). Finally, we show evidence for extreme variability in the intrinsic absorption lines of NGC 3783. In addition to our earlier report of the appearance of a C IV absorption doublet at -560 km s-1 (relative to the emission lines) over 11 months, we have detected the appearance of another C IV doublet at -1420 km s-1 over 15 months. On the other hand, the C IV absorption lines of NGC 3516 and NGC 4151 were very stable over periods of 6 months and 4 years, respectively. Monitoring observations of individual Seyfert galaxies at higher time resolution are needed to distinguish between different sources of variability (variable ionization, motion of gas across the line of sight) and to determine the densities and radial locations of the absorption components.

The Very Highly Ionized Broad Absorption Line System of the QSO SBS 1542+541

We have analyzed the broad absorption line system of the bright (V ~ 16.5) high-redshift (z = 2.361) QSO SBS 1542+541 using UV spectra from the Hubble Space Telescope Faint Object Spectrograph, along with optical data from the Multiple Mirror Telescope and the Steward Observatory 2.3 m telescope. These spectra offer continuous wavelength coverage from 1200 to 8000 A, which corresponds to ~340-2480 A in the QSO rest frame. The line of sight to the object contains only three identified intervening Lyman-limit absorption systems. Only one of these is optically thick at the Lyman edge, a low-redshift (z = 0.156) system with a strong Lyman edge observed at 1055 A (314 A in the rest frame) in a Hopkins Ultraviolet Telescope spectrum from the Astro-2 mission. The spectra therefore offer a rare opportunity to study broad absorption lines in the rest-frame extreme UV. We find that the broad absorption line system is lacking in species of relatively low ionization often seen in broad absorption systems, such as C III, O III, and Si IV. Instead, the system is dominated by very high ionization species. The strongest features correspond to O VI, Ne VIII, and Si XII. In addition to other high-ionization lines, we identify apparently saturated broad Lyman-series lines of order Lyγ and higher. There is strong evidence for partial occultation of the QSO emission source, particularly from the higher order Lyman lines that indicate a covered fraction less than 0.2. With the exception of C IV and N V, which are low-ionization species in the context of this system, all of the other lines depress the flux by more than 20%. Absorption from Lyα also depresses the flux more than 20%, indicating that there are at least two different regions contributing to H I absorption. Overall, the data suggest a correlation between a larger covered fraction and a higher state of ionization. These observations reveal inhomogeneity in the ionization structure of the broad absorption line gas. We have used photoionization models to constrain the total column density and ionization state of the system. A single-slab model consistent with our observational limits on the column densities requires NH ≈ 5 × 1022 cm-2 and an incident ionization parameter U ≈ 2. Since the observed covered fractions suggest multiple zones, we also produced a two-slab model and find 1021 cm-2 < NH < 1023 cm-2 and 0.08 < U < 4 for the smaller zone, NH 3 × 1021 cm-2 and U 2 for the larger zone. We suggest that the different covered fractions can be explained either by a special line of sight through a disklike geometry or by the existence of density fluctuations of a factor of 2 in the broad absorption line gas. The large column density and high state of ionization suggest that the system is likely associated with an X-ray warm absorber.

Resolving the Intrinsic CivAbsorption in the Seyfert 1 Galaxy NGC 3516

We observed the Seyfert 1 galaxy NGC 3516 with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, and obtained UV spectra at a resolution of about 20,000 in the redshifted C IV 1549 region. The intrinsic C IV absorption in the core of the broad emission line is resolved for the first time into four distinct kinematic components, which are all blue-shifted relative to the systemic radial velocity of the host galaxy. Two components are narrow (20 and 30 km/sec FWHM) and at small radial velocities (-90 and -30 km/sec respectively), and could arise from the interstellar medium or halo of the host galaxy. The two broad components are centered at radial velocities of -380 and -150 km/sec, have widths of 130 and 210 km/sec (FWHM) respectively, and most likely arise in outflowing gas near the active nucleus. The cores of the broad C IV absorption components are very close to zero intensity, indicating that the absorption regions are extended enough to completely occult the broad C IV emitting region (which is ~9 light days in extent). The total column density of the C IV absorption is substantially larger than that measured by Kriss et al. (1996a) from contemporaneous HUT spectra, but we agree with their conclusion that the UV absorption is too strong to arise primarily from the X-ray warm absorber region. The GHRS observations separated by six months reveal no appreciable changes in the equivalent widths or radial velocities of any of the absorption components. At present, we know of two Seyfert galaxies, NGC 3516 and NGC 4151, with complex multiple absorption zones that can remain stable in column density and velocity field over time scales of months to years.

Broad Neviiiλ774 Emission from Quasars

Ne VIII λ774 is an important tracer of the high-ionization gas in QSOs. We examine the Ne VIII emission-line properties using new Hubble Space Telescope (HST)-FOS spectra of four sources, mean spectra derived from two QSO samples in the HST archives, and new photoionization calculations. The results support our previous claim that broad Ne VIII lines are common in QSOs, with an average flux of ~42% of O VI λ1034 and velocity widths that are ~2 to 5 times larger than O VI, C IV λ1549 and other broad lines in the same spectra. The strongest and most reliably measured Ne VIII λ774 lines (in two sources) have FWHM ~14,500 km s-1. Line profile fits in these cases show that the unusually large widths might be caused by blending with emission from N IV λ765 and O IV λ789. However, standard photoionization calculations indicate that N IV, O IV, and all other lines near this wavelength should be too weak, leaving (very broad) Ne VIII as the only viable identification for the ~774 A feature. (This conclusion might be avoided if there are large radial velocity dispersions [ 1000 km s-1] in the emitting region, and the resonant absorption of continuum photons enhances the flux in weaker lines.) The calculations also indicate that the Ne VIII-emitting regions have ionization parameters in the range 5 U 30, total hydrogen column densities of 1022 NH 3 × 1023 cm-2, and an average covering factor of 30% (for solar abundances and a nominal QSO continuum shape). The Ne VIII-emitting region is therefore more extensive, more highly ionized, and has much higher velocities than the rest of the broad emission line region (BELR). This highly ionized BELR component would be a strong X-ray warm absorber if it lies along our line of sight to the X-ray continuum source.