Absorber In NGC Research Articles

Timing the warm absorber in NGC 4051

We investigated, using spectral-timing analysis, the characterization of highly ionized outflows in Seyfert galaxies, the so-called warm absorbers. Here, we present our results on the extensive ~ 600 ks of XMM-Newton archival observations of the bright and highly variable Seyfert 1 galaxy NGC 4051, whose spectrum has revealed a complex multicomponent wind. Making use of both RGS and EPIC-pn data, we performed a detailed analysis through a time-dependent photoionization code in combination with spectral and Fourier spectral-timing techniques. The source light curves and the warm absorber parameters obtained from the data were used to simulate the response of the gas due to variations in the ionizing flux of the central source. The resulting time variable spectra were employed to predict the effects of the warm absorber on the time lags and coherence of the energy dependent light curves. We have found that, in the absence of any other lag mechanisms, a warm absorber with the characteristics of the one observed in NGC 4051, is able to produce soft lags, up to 100 s, on timescales of ~ hours. The time delay is associated with the response of the gas to changes in the ionizing source, either by photoionization or radiative recombination, which is dependent on its density. The range of radial distances that, under our assumptions, yield longer time delays are comparable to the existing estimates of the location of the warm absorber in NGC 4051. For this reason, we suggest that it is likely that the observed X-ray time lags may carry a signature of the warm absorber response time, to changes in the ionizing continuum. These results highlight the importance of understanding the contribution of the warm absorber to the AGN X-ray time lags, since it is also vital information for interpreting the lags associated with propagation and reverberation effects in the inner emitting regions.

The thermal instability of the warm absorber in NGC 3783

We model the observed X-ray spectral continuum shape, ionic column densities, and absorption measure distribution (AMD) of the warm absorber in the Seyfert galaxy NGC 3783. We assume a photo-ionized medium with a uniform total (gas+radiation) pressure. The irradiation causes the wind to be radiation pressure compressed (RPC). We compare the observational characteristics derived from the 900 ksec Chandra observation to radiative transfer computations in pressure equilibrium using the radiative transfer code TITAN. We explore different values of the ionization parameter xi of the incident flux and adjust the hydrogen-equivalent column density, N_H0 of the warm absorber to match the observed soft X-ray continuum. We derive theoretical column densities for a broad range of ionic species of iron and neon and an AMD that we compare to the observations. We find an extension of the degeneracy between xi and N_H0 for the constant pressure models previously discussed for NGC 3783. Including the ionic column densities of iron and neon in the comparison between observations and data we conclude that a range of ionization parameters between 4000 and 8000 ergs cm/s is preferred. For the first time, we present theoretical AMD for a constant pressure wind in NGC 3783 that correctly reproduces the observed level and is in approximate agreement with the observational appearance of an instability region. Using a variety of observational indicators, we confirm that the X-ray outflow of NGC 3783 can be described as an RPC medium in pressure equilibrium. The observed AMD agrees with a uniformly hot or a uniformly cold thermal state. The measured ionic column densities suggest that the wind tends to the uniformly cold thermal state. The occurrence of thermal instability in the warm absorber model may depend on the computational method and the spatial scale on which the radiative transfer is solved.

NGC 1365: A LOW COLUMN DENSITY STATE UNVEILING A LOW IONIZATION DISK WIND

We present the time-resolved spectral analysis of the XMM-Newton data of NGC 1365, collected during one XMM-Newton observation, which caught this "changing-look" AGN in a high flux state characterized also by a low column density ($N_{\mathrm{H}}\sim 10^{22}$ cm $^{-2}$) of the X-ray absorber. During this observation the low energy photoelectric cut-off is at about $\sim 1$ keV and the primary continuum can be investigated with the XMM-Newton-RGS data, which show strong spectral variability that can be explained as a variable low $N_{\mathrm{H}}$, which decreased from $N_{\mathrm{H}} \sim10^{23}$ cm $^{-2}$ to $10^{22}$ cm $^{-2}$ in a 100 ks time-scale. The spectral analysis of the last segment of the observation revealed the presence of several absorption features that can be associated with an ionized (log $\xi \sim 2$ erg cm s$^{-1}$) outflowing wind ($v_{\mathrm{out}} \sim 2000$ km s$^{-1}$). We detected for the first time a possible P-Cygni profile of the Mg\,\textsc{xii} Ly$\alpha$ line associated with this mildly ionized absorber indicative of a wide angle outflowing wind. We suggest that this wind is a low ionization zone of the highly ionized wind present in NGC 1365, which is responsible for the iron K absorption lines and is located within the variable X-ray absorber. At the end of the observation, we detected a strong absorption line at $E\sim 0.76$ keV most likely associated with a lower ionization zone of the absorber (log $\xi \sim 0.2$ erg cm s$^{-1}$, $N_{\mathrm{H}} \sim 10^{22}$ cm $^{-2}$), which suggests that the variable absorber in NGC 1365 could be a low ionization zone of the disk wind.

The X-ray/UV absorber in NGC 4593

We present the results of a recent (March 2011) 160 ks Chandra-LETGS observation of the Seyfert galaxy NGC 4593, and the analysis of archival X-ray and UV spectra taken with XMM-Newton and HST/STIS in 2002. We find evidence of a multi-component warm absorber (WA) in the X-rays with four distinct ionisation degrees (log xi = 1.0, log xi = 1.7, log xi = 2.4, and log xi = 3.0) outflowing at several hundreds of km/s. In the UV we detect 15 kinematic components in the absorbers, blueshifted with respect to the systemic velocity of the source, ranging from -60 km/s to -1520 km/s. Although the predicted CIV and NV column densities from the low-ionisation X-ray outflow are in agreement with those measured for some components in the STIS spectrum, there are kinematic discrepancies that may prevent both the X-ray and UV absorbers from originating in the same intervening gas. We derive upper limits on the location of the absorbers finding that the high-ionisation gas lie within ~6 - 29 pc from the central ionising source, while the low-ionisation gas is located at several hundreds of pc. This is consistent with our line of sight passing through different parts of a stratified wind. The total kinetic energy of the outflows injected into the surroundings of the host galaxy only accounts for a tiny fraction of the bolometric luminosity of the source, and it is therefore unlikely that they may cause a significant impact in the interstellar medium of NGC 4593 in a given single episode of activity.

The warm absorber and X-ray variability of the Seyfert 1 galaxy NGC 3516 as seen by theXMM-NewtonRGS

We present a new analysis of the soft and medium energy X-ray spectrum of the Seyfert 1 galaxy NGC 3516 taken with the Reflection Grating Spectrometer (RGS) and European Photon Imaging Camera (EPIC) on board the XMM-Newton observatory. We examine four observations made in October 2006. We investigate whether the observed variability is due to absorption by the warm absorber and/or is intrinsic to the source emission. We analyse in detail the EPIC-pn and RGS spectra of each observation separately. The warm absorber in NGC 3516 is found to consist of three phases of ionisation, two of which have outflow velocities of more than 1000 km/s. The third phase (the least ionised one) is much slower at 100 km/s. One of the high ionisation phases, with log xi of 2.4, is found to have a partial covering fraction of about 60%. It has previously been suggested that the passage of a cloud, part of a disk wind, in front of the source (producing a change in the covering fraction) was the cause of a significant dip in the lightcurve during one of the observations. From our modelling of the EPIC-pn and RGS spectra, we find that variation in the covering fraction cannot be solely responsible for this. We show that intrinsic change in the source continuum plays a much more significant role in explaining the observed flux and spectral variability than originally thought.

The warm absorber in NGC 5548

We study the variability of the warm absorber and the gas responsible for the emission lines in the Seyfert 1 galaxy NGC 5548 to constrain the location and physical properties of these components. Using X-ray spectra acquired using the Chandra-LETGS in 2002 and 2005, we study the variability of the ionic column densities and line intensities. We measure a lower forbidden emission line flux in 2005, while the Fe Kα line flux remains constant. The warm absorber is less ionized in 2005, which enbles its location to be constrained to within 7 pc of the central source. Using both the observed variability and the limit on the FWHM of the f line, we constrain the location of the narrow line region to a distance of 1 pc from the central source. The apparent lack of variability in the Fe Kα line flux does enable a unique explanation to be derived.

A new model for the Warm Absorber in NGC 3783: a single medium in total pressure equilibrium

Context. Many active galactic nuclei exhibit X-ray features typical of the highly ionized gas called “Warm Absorber” (WA). Such a material appears to be stratified, displaying zones of di fferent density, temperature, and ionization. In this paper, we investigate the possibility of modelling the WA gas in NGC 3783 as a single medium in total pressure equilibrium. Aims. Our goal is to demonstrate that the WA can be well modelled assuming constant total pressure, in contrast to the current de scriptions that are based on the presence of multiple regions, each in constant density. The assumption of total pressure equilibrium yields a more physical description of the WA, resulting in the natural stratificati on of the ionized gas, and providing an explanation for the presence of lines from different ionization states, as observed in WA spectra. Methods. We have used the photoionization code TITAN, developed by our team, to compute a grid of constant total pressure models with the purpose of fitting the WA in NGC 3783. We have compared our models to the 900 ks Chandra spectrum of NGC 3783 and to previous studies where the WA was described by multiple zones of constant density. Results. In the case of NGC 3783, the WA features can be well reproduced by a clumpy, ionized gas with cosmic abondances, ionization parameter� = 2500 erg cm s −1 , column density NH = 4 10 22 cm −2 , and constant total pressure. Conclusions. We have shown that the WA in NGC 3783 can be modelled by a single medium in total pressure equilibrium; this is probably the case for other WAs currently described by multi-zone, constant density models. In addition, our work demonstrates that the TITAN code is well adapted to the study of the WA in active galactic nuclei, opening new prospects for the use of TITAN by a larger community.

TheXMM‐NewtonIron 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, 2 orbit (approx. 240 ks) XMM-Newton observation. The line profile obtained exhibits two strong narrow peaks at 6.4 keV and at 7.0 keV, with measured line equivalent widths of 120 and 35 eV respectively. The 6.4 keV emission is the K(alpha) line from near neutral Fe, whilst the 7.0 keV feature probably originates from a blend of the neutral Fe K(beta) line and the Hydrogen-like line of Fe at 6.97 keV. The relatively narrow velocity width of the K(alpha) line (approx. less than 5000 km/s), 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, whilst 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 xi approx 3 and column density of N(sub H) approx. 5 x 10(exp 22)/sq cm and may originate from within 0.1 pc of the nucleus. A weak 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.

Toward a Self‐Consistent Model of the Ionized Absorber in NGC 3783

We present a detailed model for the ionized absorbing gas evident in the 900 ksec Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Gamma=1.53) and a soft black-body component (kT=10 keV). The spectrum contains over 100 features, which are well fit by PHASE with just six free parameters. The model consists of a simple two phase absorber with difference of 35 in the ionization parameter and difference of 4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium, and are consistent with a single outflow (750 kms-1) an a single turbulent velocity (300 km s-1), and with solar elemental abundances. The main features of the low ionization phase are an Fe M-shell unresolved transition array (UTA) and the OVII lines. The OVII features, usualy identified with the OVIII and a warm absorber, are instead produced in a cooler medium also producing OVI lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high ionization phase is required by the OVII and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. The low ionizaton phase can be decomposed into three subcomponents based on the outflow velocity, FWHM, and H column densities found for three out of the four UV absorbers detected in NGC 3783. However, the ionization parametes are systematically smaller in our model than derived from UV data, indicating a lower degree of ionization.

Evidence for an intermediate-mass black hole and a multi-zone warm absorber in NGC 4395

We report on the results of an analysis in the X-ray band of a recent long ASCA observation of NGC 4395, the most variable low-luminosity active galactic nucleus known. A relativistically broadened iron line at ∼6.4 keV is clearly resolved in the time-averaged spectrum, with an equivalent width of 310 + 7 0 - 9 0 eV. Time-resolved spectral analysis of the heavily absorbed soft X-ray band confirms the existence of a variable, multi-zone warm absorber in this source, as proposed in a previous analysis of a shorter ASCA observation. The light curve of the source is wildly variable on time-scales of hours or less, and a factor of nearly 10 change in count rate was recorded in a period of less than 2000 s. The long observation and variability of the source allowed the power density spectrum (PDS) to be constructed to an unprecedented level of detail. There is evidence for a break in the PDS from a slope of a ∼ 1 to a ∼ 1.8 at a frequency of around 3 x 10 - 4 Hz. The central black hole mass of NGC 4395 is estimated to be approximately 10 4 -10 5 M O . using the break in the PDS, a result consistent with previous analyses using optical and kinematical techniques.

XMM‐NewtonReflection Grating Spectrometer Observations of Discrete Soft X‐Ray Emission Features from NGC 1068

We present the first high-resolution, soft X-ray spectrum of the prototypical Seyfert 2 galaxy, NGC 1068. This spectrum was obtained with the XMM-Newton Reflection Grating Spectrometer (RGS). Emission lines from H-like and He-like low-Z ions (from C to Si) and Fe L-shell ions dominate the spectrum. Strong, nar- row radiative recombination continua (RRCs) for several ions are also present, implying that most of the observed soft X-ray emission arises in low-temperature plasma (kTea few eV). This plasma is photoion- ized by the inferred nuclear continuum (obscured along our line of sight), as expected in the unified model of active galactic nuclei (AGNs). We find excess emission (compared to pure recombination) in all resonance lines (1s!np) up to the photoelectric edge, demonstrating the importance of photoexcitation as well. We introduce a simple model of a cone of plasma irradiated by the nuclear continuum; the line emission we observe along our line of sight perpendicular to the cone is produced through recombination/radiative cas- cade following photoionization and radiative decay following photoexcitation. A remarkably good fit is obtained to the H-like and He-like ionic line series, with inferred radial ionic column densities consistent with recent observations of warm absorbers in Seyfert 1 galaxies. Previous Chandra imaging revealed a large (extending out to � 500 pc) ionization cone containing most of the X-ray flux, implying that the warm absorber in NGC 1068 is a large-scale outflow. To explain the ionic column densities, a broad, flat distribu- tion in the logarithm of the ionization parameter (� ¼ LX=ner 2 ) is necessary, spanning log � ¼ 0-3. This sug- gests either radially stratified ionization zones, the existence of a broad density distribution (spanning a few orders of magnitude) at each radius, or some combination of both. Subject headings: galaxies: individual (NGC 1068) — galaxies: Seyfert — line: formation — X-rays: galaxies

Characterizing the complex absorber in NGC 4151

We present a detailed analysis of the complex absorption apparent in the 2-6 keV X-ray spectrum of the bright nearby Seyfert galaxy NGC 4151. We first utilize the large bandpass and medium spectral resolution afforded by BeppoSAX data to construct a 1-100 keV spectral template, which assumes the absorption arises in both warm (i.e. partially photoionized) and cold gas present in the line of sight to the active nucleus of the source. Application of this spectral model to an ASCA "long-look" observation of NGC 4151 reveals a partial correlation between the underlying continuum flux and the ionization state of the warm absorber. Such a correlation is an intrinsic property of a warm absorber and argues strongly in favour of this interpretation for the complex absorbing column over alternative partial covering models. The inferred relatively low density for the warm gas, implies an equilibration timescale for the dominant ions of the same order or longer than the timescale of the continuum variability. It follows that the warm component will invariably be observed in a non-equilibrium ionization state. We also find that (i) the reported hardening of the spectrum of NGC 4151 as the continuum level falls may be simply due to the presence of an underlying (hard and relatively constant) Compton-reflection component and (ii) the iron Ka line has a relatively narrow Gaussian profile and a line flux that remains constant over both short (days) and long (months to years) timescales - a relativistically broadened iron Ka feature was not required in our modelling.

On the Column Density of AGN Outflows: The Case of NGC 5548

We reanalyze the Hubble Space Telescope high-resolution spectroscopic data of the intrinsic absorber in NGC 5548 and find that the C IV absorption column density is at least 4 times larger than previously determined. This increase arises from accounting for the kinematical nature of the absorber and from our conclusion that the outflow does not cover the narrow emission line region in this object. The improved column density determination begins to bridge the gap between the high column densities measured in the X-ray and the low ones previously inferred from the UV lines. Combined with our findings for outflows in high-luminosity quasars, these results suggest that traditional techniques for measuring column densities—equivalent width, curve of growth, and Gaussian modeling—are of limited value when applied to UV absorption associated with active galactic nucleus outflows.

The Inner Light-Year of the Nearest Seyfert 1 Nucleus in NGC 4395

The NRAO Very Long Baseline Array (VLBA) was used at 1.4 GHz to image the inner 550 mas (23 lt-yr) of the nearest known Seyfert 1 nucleus, in the Sm galaxy NGC 4395. One continuum source was detected, with flux density 530 ± 130 μJy, diameter d 2.0 × 106 K. The spectral power P of the VLBA source is intermediate between those of Sagittarius A and Sagittarius A* in the Galactic center. For the VLBA source in NGC 4395, the constraints on Tb, d, and P are consistent with an origin from a black hole but exclude an origin from a compact starburst or a supernova remnant like Cassiopeia A. Moreover, the spectral powers of NGC 4395 at 1.4 and 4.9 GHz appear to be too low and too constant to allow analogy with SN 1988Z, a suggested prototype for models of compact supernova remnants. The variable and warm X-ray absorber in NGC 4395 has a free-free optical depth much larger than unity at 1.4 GHz and, therefore, cannot fully cover the VLBA source.

The recent history of the X-ray absorber in NGC 3516

We present two BeppoSAX observations of the bright Seyfert 1 galaxy NGC 3516, performed four months apart (late 1996–early 1997). The earlier spectrum is considerably weaker and harder in the whole 0.1–50 keV energy range. In addition, the warm absorber oxygen features, which were already observed with ROSAT and ASCA, are much less pronounced. The most straightforward explanation is that in 1996 November NGC 3516 was being seen through a substantial (NH≃1022 cm−2) column of cold material. This is the first confirmation with modern instrumentation that NGC 3516 indeed undergoes phases of strong cold X-ray absorption. We speculate that these intervals may be connected to the onset of the remarkably variable UV absorption system, making NGC 3516 the best known example of a low-luminosity broad absorption line (BAL) quasar. The absorbing matter could be provided by clouds ablated from the rim of the circumnuclear molecular torus, seen at a rather high inclination angle.

The X‐Ray Warm Absorber in NGC 3516

The Seyfert 1 galaxy, NGC3516 has been the subject of many absorption line studies at both ultraviolet and X-ray wavelengths. In the UV, strong, broad, variable associated metal line absorption with velocity width $\sim 2000$ \kms\ is thought to originate in gas with nh larger than about $10^{19}$ cm$^{-2}$ lying between 0.01 and 9 pc from the central active nucleus. The Ginga X-ray data are consistent with several possibilities: a warm absorber and a cold absorber combined either with partial covering or an unusually strong reflection spectrum. We present ROSAT observations of NGC3516 which show a strong detection of a warm absorber dominated by a blend of OVII/OVIII edges at $\sim$ 0.8 keV with nh $\sim 7\times 10^{21}$ cm$^{-2}$ and U: 8--12. We argue that NGC3516 contains an outflowing `XUV' absorber showing the presence of X-ray absorption edges, that are consistent with the presence of broad absorption lines in the old IUE spectra and their disappearance in the new UV observations. Our dynamical model suggests that the OVII absorption edge will continue to weaken compared to the OVIII edge, an easily testable prediction with future missions like AXAF. Eventually the source would be transparent to the X-rays unless a new absorption system is produced.

ASCA Observations of the Composite Warm Absorber in NGC 3516

We obtained X-ray spectra of the Seyfert 1 galaxy NGC~3516 in March 1995 using ASCA. Simultaneous far-UV observations were obtained with HUT on the Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law of energy index 0.78. The low energy absorbing column is significantly less than previously seen. Prominent O~vii and O~viii absorption edges are visible, but, consistent with the much lower total absorbing column, no Fe K absorption edge is detectable. A weak, narrow Fe~K$\alpha$ emission line from cold material is present as well as a broad Fe~K$\alpha$ line. These features are similar to those reported in other Seyfert 1 galaxies. A single warm absorber model provides only an imperfect description of the low energy absorption. In addition to a highly ionized absorber with ionization parameter $U = 1.66$ and a total column density of $1.4 \times 10^{22}~\rm cm^{-2}$, adding a lower ionization absorber with $U = 0.32$ and a total column of $6.9 \times 10^{21}~\rm cm^{-2}$ significantly improves the fit. The contribution of resonant line scattering to our warm absorber models limits the Doppler parameter to $< 160~\rm km~s^{-1}$ at 90\% confidence. Turbulence at the sound speed of the photoionized gas provides the best fit. None of the warm absorber models fit to the X-ray spectrum can match the observed equivalent widths of all the UV absorption lines. Accounting for the X-ray and UV absorption simultaneously requires an absorbing region with a broad range of ionization parameters and column densities.